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WBBSE Chapter 7 The Microbial World Long Answer Questions

Question 1. What are microbes? Where they are found? What are the major classes of microbes?
Answer:

Microbes:

The term microbe is short for microorganisms, which means small organisms visible only under microscopes. A microbe is any living thing that is too tiny to be seen with the naked eye.

Microbes are the oldest form of life on earth. They may live as individuals or cluster together in communities. Microbes live in the water you drink, the food you eat, and the air you breathe.

Right now, billions of microbes are swimming in your belly and mouth, and crawling on your skin Don’t worry; over 95% of microbes are good for you.

Microbes include bacteria, viruses, fungi, algae, and protozoa. These single-cell organisms are invisible to the eye, but they can be seen with microscopes.

Classification of Microbes

The term microbe is short for microorganism, which means small organisms. To help people understand the different types of microbes, they are grouped or classified in various ways.

Microbes are very diverse and represent all the great kingdoms of life. In fact, in terms of numbers, most of the diversity of life on earth is represented by microbes.

Here is an outline of the major groups of microorganisms:

1. Viruses
2. Bacteria (Monera)
3. Algae (Plantae)
4. Fungi
5. Protozoa (Protista)

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Viruses

A virus is acellular and composed basically of a tiny bundle of genetic material (either DNA or RNA) carried in a shell called the viral coat.

They have no boundary wall, cytoplasm, or nucleus. Thousands of different viruses, which come in many shapes, are parasites and disease-causing agents.

Viruses are found on or in just about every material and environment on Earth from soil to water to air. Viruses do not show most of the characteristics of living things outside the living host.

But if they come into contact with a suitable plant, animal, or bacterial cell, they spring into action. They infect and take over the cell like pirates hijacking a ship.

Viruses exist to reproduce only. To do that, they have to take over suitable host cells. The new viral genes then come together and assemble into whole new viruses.

The new viruses are either released from the host cell without destroying the cell or eventually build up to a large enough number that they burst the host cell.

 

WBBSE Class 8 Microbial World Long Answer Questions

Question 2. What are bacteria? Classify them according to the structure.
Answer:

Bacteria:

Bacteria consist of only one cell, but they are a very complex group of living things. Unlike viruses, bacteria feed, move, and respire as well as reproduce on their own.

 

 

Some bacteria can live in temperatures above the boiling point and in cold below the freezing point. There are thousands of species of bacteria.

Bacteria are classified into 5 groups according to their basic shapes: spherical (cocci), rod (bacilli), spiral (spirilla), comma (vibrios), or corkscrew (spirochaetes). They can exist as single cells, in pairs, chains, or clusters.

Some bacteria can make their own food from sunlight, just like plants. Also like plants, they give off oxygen. Other bacteria absorb food from the material they live on or in. They have no membrane-bound cell organelles.

A single teaspoon of soil contains more than a billion (1,000,000,000) bacteria. Bacteria reproduce by binary fission. In this process the bacterium, which is a single cell, divides into two identical daughter cells. Binary fission begins when the DNA of the bacterium divides into two (replicates).

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Differences between bacteria and viruses:

Because bacteria and viruses cause many of the diseases we’re familiar with, people often confuse these two microbes. But viruses are entirely different from bacteria.

For one thing, they differ greatly in size. The biggest viruses are only as large as the tiniest bacteria. Another difference is their structure. Bacteria are complex compared to viruses.

A typical bacterium has a rigid cell wall and a thin, rubbery cell membrane surrounding the fluid, or cytoplasm, inside the cell.

A bacterium contains all of the genetic information needed to make copies of itself—its DNA—in a structure called a chromosome. In addition, it may have extra loose bits of DNA called plasmids floating in the cytoplasm.

Bacteria also have ribosomes, necessary for copying DNA so bacteria can reproduce. Some have threadlike structures called flagella that they use to move.

 

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Question 3. What are protozoa? Write about harmful protozoa. What is Achaea?
Answer:

Protozoa:

Protozoa are single-celled organisms having one or more nuclei. They come in many different shapes and sizes ranging from an Amoeba which can change its shape to its fixed shape and complex structure.

They live in a wide variety of moist habitats including freshwater, marine environments, and soil. They can live freely or in a colony.

Protozoa mainly feed on bacteria, but they also eat other protozoa, and sometimes fungi. Some protozoa absorb food through their cell tissues.

 

 

Others, surround food and engulf it. Others have openings similar to mouth pores into which they sweep food.
Protozoa can be classified into three general groups based on their shape. One group is the Ciliates, which are generally the largest protozoa. The second group is the Amoebae.

The nutrients from living or dead organic matter that they grow on. They absorb simple, easily dissolved nutrients, such as sugars, third group is the Flagellates, which are usually the smallest of the protozoa.

Most protozoa do us no harm. But there are a few that cause diseases. One type of amoeba can live in human intestines. It feeds on red blood cells and causes a disease known as dysentery.

Another species of protozoa can sicken hundreds of thousands of people when it gets into the tap water. Perhaps the best-known deadly protozoa cause malaria, a terrible disease that leads to about 800,000 deaths each year worldwide.

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Question 4. What is parasitism? Write about parasitic microbes.
Answer:

Parasitic:

They derive their nutrition from the plants and animals on which they grow. Certain enzymes are produced by them which decompose or kill the protoplasm of the host cells.

The interrelationship between the parasitic microbe and the host is called parasitism. Such effects of the parasites on the host become visible to the naked eye as disease symptoms.

Many well-known diseases of human beings like typhoid, tetanus, tuberculosis, pneumonia, and many others are due to parasitic bacteria.

Microbes are also known as pathogenic bacteria. Some bacteria grow well only in the presence of oxygen, while others grow well in absence of oxygen. The former is known as aerobes and the latter are anaerobes.

Question 5. What are saprophytes? Write a note about saprophytic microbes.
Answer:

Saprophytic:

They grow on dead and decaying plants and animals, dung,’ rotten wood, stagnant water, and many other decaying substances rich in organic matter.

Certain enzymes secreted by the bacteria decompose the complex organic substances of the substrate, converting them into simpler ammonium compounds.

They cause decay and therefore are also known as putrefying bacteria. The souring of milk, the manufacture of cheese, the preparation of butter from milk, and vinegar from sugarcane juice, are various processes completed by the action of certain specific saprophytic bacteria.

Zygomonas ferments glucose-producing alcohol, lactic acid, and carbon dioxide, and plays a significant role in the wine industry.

Acetobacter oxidizes organic compounds to organic acids such as lactic acid thus having a significant role in the vinegar industry. Lactobacillus converts sugars into lactic acid. Canned food is spoiled by some Bacillus and Clostridium bacteria.

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Question 6. What is symbiosis? Explain the term with examples from microbes.
Answer:

Symbiotic:

Symbiosis is the phenomenon by which two organisms maintain a relationship with each other to be mutually benefitted. In symbiotic mode, organisms develop a special relationship with certain other organisms to obtain nourishment.

Organisms involved in this type of relationship are called symbionts. Rhizobium bacteria is a striking example of this type.

They occur in root nodules of leguminous plants and help in fixing the free nitrogen of the atmosphere in the soil for the plants which in return provides carbohydrates and protection to the bacteria.

They are also called nitrogen-fixing bacteria and add to the fertility of the soil. Azotobacter and Clostridium are other examples of nitrogen-fixing bacteria.

They are found in chalky soil and obtain energy from the carbohydrates present in the soil. The energy so obtained is used in fixing atmospheric nitrogen into amino acids in the soil which react with the calcium salts, forming nitrites and thereafter nitrates.

Lichens form a very good example of a symbiotic relationship. Lichens are formed by the symbiotic relationship between algae and fungi sharing a single colony.

Fungus provides water, nutrients, and shelter in the form of the network formed by mycelium. Algae synthesize the food for the entire colony through the process of photosynthesis.

Question 7. Write about various microbial habitats with suitable examples.
Answer:

Microbial Habitat:

Microbes live in almost every nook and corner you can think of, from 20 miles beneath the Earth’s surface to 20 miles overhead. They live at temperatures less than -20 degrees Celsius to temperatures hotter than the boiling point.

Microbes thrive on a huge range of food including oil and toxic wastes. Every time you walk on the ground you step on billions of microbes.

Microbes live in the soil, on rocks, inside roots, buried under miles of earth, in compost piles, and toxic waste all over the earth’s surface.

Microbes are found in boiling hot springs and on frozen snow fields. Most animals interact with microbes in important ways, and all animals, as well as all plants and fungi, depend on microbes for their survival.

Microbes live in their digestive systems, in their mouths, and on their skin. Microbes are important for the good health of animals.

Animals provide microbes with food and shelter. That is why animals are called “hosts” to microbes. For example, cows depend on microbes for their food.

Cows and other animals called ruminants have special stomachs called rumens which are host to billions of microbes that help these animals survive.

Also, we depend on microbes to clean up our environment. Without microbes to get rid of all the dung from animals, the world would be a really smelly place. Microbes make this world a cleaner place.

Long Answer Format for Class 8 Microbial World

Question 8. What is lichen? Mention some economic importance of lichens.
Answer:

Lichen:

A lichen is a composite organism that emerges from algae or cyanobacteria (or both) living among filaments of a fungus in a mutually beneficial (symbiotic) relationship.

The fungus benefits from the symbiotic relationship because algae or cyanobacteria produce food by photosynthesis. The algae or cyanobacteria benefit by being protected from the environment by the filaments of the fungus,

1. which also gathers moisture and nutrients from the environment, and (usually) provides an anchor to it.
2. Economic importance of Lichens:
3. Lichens are used in the tanning of leather.
4. Important dyes are prepared from lichens for use in woolen and silken clothes.
5. Lichens like Evernia is used in cosmetics and perfumes.
6. Lichens are indicators of pollution
7. Lichens play important role in soil formation.

Question 9. Write the full form of DOTS. What is food poisoning? Mention the names of two bacteria that cause food poisoning. What are the symptoms of food poisoning?
Answer:

DOTS:

DOTS stands for Directly Observed Treatment, short course.

The disease caused due to the presence of a large number of microorganisms (like bacteria and fungi) in the food or due to the presence of toxic substances in food formed by the action of microorganisms, is called food poisoning.

Bacteria	Food poisoning
Clostridium botulinum	Botulism
Salmonella Typhimurium	Salmonellosis

 

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Question 10. Mention two types of yeast that are used as food. Name two microbial vitamins along with their sources. Name a disease-causing microorganism in animals.
Answer:

Two types of yeast that are used as food:

Yeast type	Organism used	Purpose
Baker’s Yeast	Saccharomyces cerevisiae	Used to leaven bread
Food Yeast	Torulopsis utilize	Used as a dietary supplement in deficient diets

 

Microbial vitamin	Source
Vit. B12	Bacillus coagulans
Riboflavin	Ashbya gossypii

Anthrax un animals is caused by bacillus anthracis

WBBSE Chapter 6 The Structure Of Living Organisms Long Answer Type Questions

Question 1. Name the characteristics of living things. What is the law of biogenesis?
Answer:

The Seven Characteristics of Living Things

1. Feeding: All living organisms need to take substances from their environment to obtain energy, grow and stay healthy.

2. Movement: All living organisms show the movement of one kind or another. All living organisms have internal movement, which means that they can move substances from one part of their body to another.

Some living organisms show external movement as well—they can move from place to place by walking, flying or swimming.

3. Breathing or Respiration: All living things exchange gases with their environment. Animals take in oxygen and breathe out carbon dioxide.

4. Excretion: Excretion is the removal of waste from the body. If this waste is allowed to remain in the body it could be poisonous.

Humans produce liquid waste called urine. We also excrete waste when we breathe out. All living things need to remove waste from their bodies.

Growth: When living things feed they gain energy. Some of this energy is used for growth. Living things become larger and more complicated as they grow.

Sensitivity: Living things react to changes around them. We react to touch, light, heat, cold and sound, as do other living things.

Reproduction: All living things produce young. Humans make babies, cats produce kittens and pigeons lay eggs. Plants also reproduce. Many make seeds which can germinate and grow into new plants.

Living beings reproduce to form young ones that resemble their parents in the long run. So it is ‘like from like’- referred to as Answer: the law of biogenesis.

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Question 2. What is a cell? Who coined the term? How it can be seen?
Answer:

Differences Among Living And Non-Living

An attempt should now be made to distinguish living things from lifeless nonliving or inanimate objects. There is no difficulty in recognising a coconut palm or guinea pig as living objects and rocks in the field or the sand grains in the seashore as non-living bodies.

It may be mentioned here that viruses are an intermediate stage between living and nonliving forms of matter.
The principal points of difference between living and nonliving objects are described in tabular form below:

	Living		Non-living
1	Each kind of plant and animal has a definite form and size, which may vary within very narrow limits in different individuals of the same kind.	1	Non-living objects, such as masses of clouds or collections of stones have neither a definite size nor any precise form
2	A living body is organized of cells, tissues and organs with the division of labour.	2	No such organisation exists
3	Life is an external manifestation of metabolic activities like nutrition, respiration, secretion, circulation, excretion etc.	3	None of the metabolic activities is detected in nonliving objects.
4	The living body increases in bulk by wedging in new particles in between already existing cellular matter.	4	Growth may occur occasionally by deposition of particles only on the outer surface of the body
5	A living body can reproduce its own kind and thus perpetuate its race.	5	There is no power to reproduce its own kind
6	A living body has a definite life cycle.	6	No life cycle is observed. The period of duration is infinite and there is no death.

 

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Question 3. Write briefly about different types of microscopes.
Answer:

Different types of microscopes:

How a Cell can be seen:

Cells got their name from an Englishman named Robert Hooke in the year 1665. He first saw and named “cells” while he was experimenting with a new instrument we now call a “microscope.

“For his experiment, he cut very thin slices from cork. He looked at these slices under a microscope. He saw tiny box-like shapes. These tiny boxes reminded him of the plain small rooms that monks lived in called “cells”. The smallest objects that the unaided human eye can see are about 0.1 mm long.

A magnifying glass can help you to see them more clearly, but they will still look tiny. Cells are not visible under ordinary magnifying glasses.

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Smaller cells are easily visible under a light microscope. Light microscopes use a system of lenses to magnify an image. The power of a light microscope is limited by the wavelength of visible light, which is about 500 nm. The most powerful light microscopes can resolve bacteria but not viruses.

Light microscopes (optical microscopes) that are commonly used in schools are of two types – compound microscopes and simple or stereo microscopes (also known as dissecting or binocular microscopes).

Left: Stereo microscope; Center: Compound microscope with a binocular head; Right: Compound microscope with a monocular head is greater resolving power than light microscopes, so we can use them to see even more detail than is visible under a light microscope.

 

 

To see anything smaller than 500 nm, you will need an electron microscope Electron microscopes to shoot a high-voltage beam of electrons onto or through an object, which deflects and absorbs some of the electrons.

Resolution is still limited by the wavelength of the electron beam, but this wavelength is much smaller than that of visible light. The most powerful electron microscopes can resolve molecules and even individual atoms.

Some of the specialized types of electron microscopes are – Transmission Electron Microscope (TEM), Scanning Electron Microscope (SEM), Reflection Electron Microscope (REM), and Scanning Transmission Electron Microscope (STEM).

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Question 4. Write, with diagrams, the special features of RBC and nerve cells.
Answer:

Cell-The Unit of an Organism

Your body, as a whole, is one organism. However, many, many parts make up that whole. First, you notice the entire body. Next, you see that the entire body is made up of parts and organs, and each of those organs is made up of a variety of tissues.

And if, as a pathologist does, you examine a magnified sample of one of the human body’s tissues under a microscope, millions of cells become visible.

Yet you can turn up the magnification for an even closer look, Cells contain molecules that are made up of even smaller components called atoms.

Atoms, molecules, cells, tissues, organs, and organ systems are the body’s building blocks. ‘Schleiden and Schwann together proposed the cell theory in 1839.

Cell theory states that “all plants and animals are composed of cells and cellular products.” Thus, the cell is the basic unit of life.

1. Salient points of cell theory:
2. All living things are composed of cells and their products.
3. All cells arise from pre-existing cells.
4. AII cells are basically alike in chemical composition and metabolic activities.
5. The function of an organism as a whole is the outcome of the activities and interaction of the constituent cells of which the organism is built.

Therefore, all living beings are made up of cells which are the smallest structural and functional unit of the body. Some of them are made up of only one cell and others have many cells.

All living organisms are made up of individual and identifiable cells, whose number, together with their size and type, ultimately defines the structure and functions of an organism.

While the total cell number of lower organisms is often known, it has not yet been defined in higher organisms. In particular, the reported total cell number of a human being is about 37 trillion (one trillion =1,000,000,000,000).

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Question 5. Write briefly about the levels of structural organization of organisms. Does the shape of cells remain the same always?
Answer:

Specialised Cells

The table below shows examples of some specialised animal and plant cells, with their functions and special features.

No A cell changes its shape when it undergoes cell division. Human RBC can change its shape when it moves through arteries, veins and capillaries. A cancer-affected cell also undergoes a change in shape of the cell.

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Question 6. What are tissues? Name the four principal animal tissues. Write the differences between plant and animal tissues.
Answer:

Levels of organisation:

The Organisation of Cells, Tissues, Organs, Systems and the Organism

From the least complex to the most complex, the organisms are made up of—

1. Atom – The smallest piece of matter that still has physical and chemical properties of matter.
2. Molecule – The smallest piece of a compound that still has the physical and chemical properties of that substance (e.g., water). The molecule is composed of atoms.
3. Cell – The smallest piece of life (some arguments remain about viruses). It is the basic unit of all living beings.
4. Tissue – Collection of cells that work together.
5. Organ – Collection of tissues that work together.
6. Organ System – Collection of organs that work together.
7. Organism – Collection of organ systems that makes up a single life form.

Two or more kinds of tissues together form each of our organs, like the heart or lungs. Many organs together form an organ system, like the digestive or respiratory system.

All the organ systems together form an organism, such as a person, dog or blue whale. The body’s organisation goes from cells to tissues to organs to organ systems to a whole working organism. In unicellular (single-celled) organisms, the single cell performs all life functions.

It functions independently. However, multicellular (many-celled) organisms have various levels of organization within them. Individual cells may perform specific functions and also work together for the good of the entire organism.

The cells become dependent on one another. Multicellular organisms have the following 5 levels of organization ranging from simplest to most complex:

 

Level-1 Cells

Are the basic unit of structure and function in living things. May serve a specific function within the organism.
Examples – are blood cells, nerve cells, bone cells, etc.

 

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Level-2 Tissues

Made up of cells that are similar in structure and function and which work together to perform a specific activity.
Examples – are blood, nervous, bone, etc. Humans have 4 basic tissues: connective, epithelial, muscle, and nervous.

 

 

Level-3 Organs

Made up of tissues that work together to perform a specific activity. Examples – heart, brain, skin, etc.

 

 

Level-4 Organ Systems

Groups of two or more tissues that work together to perform a specific function for the organism.
The Human body has 11 organ systems – circulatory, digestive, endocrine, excretory (urinary), immune (lymphatic), integumentary, muscular, nervous, reproductive, respiratory, and skeletal.

 

WBBSE Chapter 6 Living Organisms Study Guide

Level -5 Organisms

Entire living things can carry out all basic life processes. Meaning they can take in materials, release energy from food, release wastes, grow, respond to the environment, and reproduce.

Usually made up of organ systems, but an organism may be made up of only one cell such as bacteria or protists.
Examples – are bacteria, Amoeba, mushrooms, sunflowers, and humans.

 

 

The levels of organization in the correct order are:
cells ⇒ tissues⇒  organs ⇒ organ systems ⇒ organisms

Levels of the body from smallest to largest: Atoms, molecules, cells, tissues, organs, and organ systems.

 

 

Name the associated processes: gaseous exchange and energy production increase

Question 7. Which cell organelle is primarily responsible for maintaining the shape of plant cells? Jow is a bacterial cell different from an onion peel cell?
Answer:

Processes:

respiration reproduction The shape of a plant cell depends on the volume of water inside the cell. Intake or removal of water from vacuoles depends on the volume of water a plant cell can absorb from the soil. Therefore, vacuoles play important role in the formation of plant bodies.

A bacterial cell is prokaryotic in nature while an onion peel cell is eukaryotic in nature having well-developed cellular organisation.

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Question 8. Draw the longitudinal section of mitochondria. What are grana?
Answer:

Inside chloroplastids, the thylakoids are arranged in stacks like piles of coins called grana. Chlorophyll pigments are present in thylakoids.

Question 9. Describe with a diagram the structures and functions of any one animal cell organelle and any one plant cell organelle. Which kind of plastid is more common in: the roots of plants, Flowers and fruits
Answer:

Cell organelles and their structure and function

An organelle is any specialized structure based on the analogy that cells have their cell, especially one for which a specific organelle is just as complex as a multicellular function can be assigned.

As we mentioned organisms have their livers, kidneys, and stomachs earlier, the name means “little organ” and in other organs.

The Three Main Components of any Plant or Animal Cell:

1. Plasma Membrane / Cell Membrane

Structure – It is a bilipid membraneous layer containing proteins and lipids. There are two outer layers of protein and a middle layer of phospholipid. It is called a unit membrane and is semifluid and dynamic in nature.

Function – The cell membrane separates the cell from its external environment, and is selectively permeable (controls what gets in and out). It protects the cell and provides stability.

Proteins are found embedded within the plasma membrane, with some extending all the way through in order to transport materials.
Carbohydrates are attached to proteins and lipids on the outer lipid layer.

2. Cytoplasm

Structure – It is a colourless, semisolid and jelly-like substance composed of main water and found between the cell membrane and nucleus.

The cytoplasm makes up most of the “body” of a cell and is constantly streaming. The transparent fluid part of the cytoplasm is called cytosol.

Function – Organelles are found here and substances like salts may be dissolved in the cytoplasm. It helps in the exchange of materials between cell organelles and is the active site for metabolic processes like sugar, protein and fatty acid synthesis.

3. Nucleus

Structure – The largest organelle in the cell. It is dark and round and is surrounded by a double membrane called the nuclear envelope/ membrane.

In spots, the nuclear envelope fuses to form pores which are selectively permeable. The nucleus contains genetic information (DNA) on special strands called chromosomes.

The colourless dense sap present inside the nucleus is called the nucleoplasm. One or more round bodies present in the nucleoplasm are called nucleoli. A network of dark-staining fibre in the nucleus is called chromatin.

Function – The nucleus is the “control centre” of the cell, for cell metabolism and reproduction. It regulates the cell cycle and is the storehouse of genes.

2. The Organelles found in both Plant and Animal cells:

1. “ER” or Endoplasmic Reticulum
The Endoplasmic Reticulum is a network of membranous canals filled with fluid. They carry materials throughout the cell. The ER is the “transport system” of the cell.

There are two types of ER: rough ER (RER) and smooth ER. (SER)Rough Endoplasmic Reticulum is lined with ribosomes and is rough in appearance and smooth endoplasmic reticulum contains no ribosomes and is smooth in appearance.

ER gives support to the cytoplasm. It forms an intracellular transport system. SER plays a crucial role in detoxifying many poisons and drugs. It also helps in the formation of proteins and lipids.

2. Ribosomes

Ribosomes are small particles which are found individually in the cytoplasm and also line the membranes of the rough endoplasmic reticulum. Ribosomes produce protein. They could be thought of as “factories” in the cell.

3. Golgi Body / Apparatus

Golgi bodies are stacks of flattened membranous sacs, (they look like pancakes). The Golgi Body temporarily stores protein which can then leave the cell via vesicles pinching off from the Golgi.

In plants, the Golgi apparatus is scattered in the cytoplasm and found in the diffused form. These are known as Dictyosomes.
Secretion is the main function of the Golgi complex. It mainly performs the function of packaging.

The material synthesized near the ER is packaged and dispatched to various targets inside and outside the cell through the Golgi apparatus. It is directly involved in the formation of lysosomes.

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4. Lysosomes

Lysosomes are small sac-like structures surrounded by a single membrane and containing strong digestive enzymes which when released can break down worn-out organelles or food.

The lysosome is also known as a suicide sac. This help to keep the cell clean by digesting any foreign material and worn-out cell organelles.

5. Mitochondria (Singular = Mitochondrion)

The mitochondria are round “tube-like” organelles that are surrounded by a double membrane, with the inner membrane being highly folded.

The mitochondria are often referred to as the “powerhouse” of the cell. The mitochondria release food energy from food molecules to be used by the cell.

This process is called respiration. Some cells ( muscle cells) require more energy than other cells and so would have many more mitochondria.

6. Vacuoles

Vacuoles are fluid-filled organelles enclosed by a membrane. They can store materials such as food, water, sugar, minerals and waste products.

3. Animal Cell Organelles not found in Plant Cells

1. Cilia and Flagella

Both cilia and flagella are hair-like organelles which extend from the surface of many animal cells. The structure is identical in both, except that flagella are longer and whip-like and cilia are shorter.

There are usually only a few flagella on a cell, while cilia may cover the entire surface of a cell. The function of cilia and flagella include locomotion for one-celled organisms and moving substances over cell surfaces in multi-celled organisms.

4. Organelles and other features found only in Plant Cells

1. Cell Wall

The cell wall is a rigid organelle composed of cellulose lying just outside the cell membrane. The cell wall gives the plant cell its box-like shape. It also protects the cell. The cell wall contains pores which allow materials to pass to and from the cell membrane.

2. Plastids

Plastids are double membrane bound organelles. It is in plastids that plants make and store food. Plastids are found in the cytoplasm and there are two main types:

Leucoplasts – Colourless organelles which store starch or other plant nutrients, (for example – starch stored in a potato).

Chromoplasts – Contain different coloured pigments. The most important type of chromoplast is the chloroplast, which contains the green pigment chlorophyll. This is important in the process of photosynthesis.

3. Central Vacuole

The central vacuole is a large fluid-filled vacuole found in plants.

Question 10. Mention the name of an organism whose body structure does not conform to cell theory What are cytoplasmic organelles? What are A, B and C ? Name the layer which lies outside of the cell membrane in a plant cell.
Answer:

Cells make up A; A makeup B; B make up C; C makes up an organism. The body structure of a virus does not conform to the cell theory.

Some specific living components found in the cytoplasmic matrix of all eukaryotic cells and which are concerned with functions of the cell as a whole are called cytoplasmic organelles,

cells A ⇒B ⇒C ⇒Organism (Tissues) (Organs) (Organ system) Cell wall lies outside the cell membrane in a plant cell.

Chapter 5 Analysis Of Natural Phenomena Long Answer Type Questions

Question 1. Describe in short how lightning occurs during a thunderstorm.
Answer:

Thunderstorm 

A thunderstorm is a type of storm characterized by the presence of lightning and thunder accompanied by strong winds and heavy rain.

Thunderstorms result from the rapid upward movement of warm moist air. As the warm, moist air moves upward (called updraft of air), it cools, condenses, and forms cumulonimbus clouds that can gain heights of 12 km or more.

The upward movement of air in a storm cloud has been measured as more than 80 kilometres per hour. ‘ As the moist air reaches its dew point at the upper atmosphere, it converts to water droplets and tiny ice particles.

These particles begin to fall a long way through clouds and collide with other particles and thus become larger. Downdrafts of air are created by the falling water droplets because they don’t just drag other droplets down with them as they fall, they drag cooler air with them as well. The combined warm updraft and cool downdraft create a storm cell.

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Question 2. Write about the characteristics of epidemic diseases.
Answer:

Characteristics Of Epidemic Diseases

1. All epidemics have the following common characteristics:
2. An unexpected number of cases of a particular disease occur at a particular point in time affecting a large segment of the population.
3. Generally confined to a definite population or geographical area.
4. Usually have a common source of infection,
5. Epidemics generally tend to follow a pattern and repeat periodically when the conditions are favourable again.

Types Of Epidemic Diseases

Epidemics generally follow a pattern depending on the geographical and environmental conditions, the distribution and characteristics of the host population, and their cultural behaviour.

If there is no intervention or change in these conditions, those epidemics tend to repeat themselves. Therefore, knowledge about various types of epidemics and the conditions under which they occur can help in managing them.

WBBSE Class 8 Analysis of Natural Phenomena Long Answers

Question 3. Write briefly about cholera.
Answer:

Cholera

Cholera is an acute intestinal infection caused by the

ingestion of food or water contaminated with the bacterium Vibrio cholera.

It has a short incubation period and produces an enterotoxin that causes a copious, painless, watery diarrhoea that can quickly lead to severe dehydration and death if treatment is not promptly given.

Vomiting also occurs in most patients. Most persons infected with V. cholerae do not become ill, although the bacterium is present in their faeces for 7-14 days.

When illness does occur, about 80-90% of episodes are of mild or moderate severity and are difficult to distinguish clinically from other types of acute diarrhoea.

Less than 20% of ill persons develop typical cholera with signs of moderate or severe dehydration.

Question 4. Write the cause and symptoms of kala-azar.
Answer:

Kala-azar

Kala-azar is the second largest parasitic killer in the world – only Malaria is more deadly. It is also known as Dumdum fever.
Kala-azar is caused by bites from sandflies – which carry the Leishmania donovani parasite, a type of protozoa, responsible for the disease.

If blood containing Leishmania donovani parasites is drawn from an animal or human, the next person to receive a bite will then become infected.

Initially, Leishmania parasites cause skin sores or ulcers at the site of sand fly bites. If the disease progresses, it attacks the immune system.

Kala-azar presents after two to eight months, with more generalised symptoms including prolonged fever and weakness. Indian scientist Upendranath Brahmachari invented the medicine of Kala-azar.

Chapter 5 Natural Phenomena Detailed Questions WBBSE

Question 5. Write about an epidemic due to a non-infectious disease.
Answer:

Epidemic due to a non-infectious disease

Epidemics of non-infectious disease are often caused by exposure to industrial products, intermediates or byproducts, either in the workplace or as a result of the contamination of a wider environment.

The Global Status Report on Non-communicable Diseases 2010 is the first report on the worldwide epidemic of cardiovascular diseases, cancer, diabetes and chronic respiratory diseases, along with their risk factors and determinants.

Non-communicable diseases killed tens of millions of people in 2008, and a large proportion of these deaths occurred before the age of 60, so during the most productive period of life. The magnitude of these diseases continues to rise, especially in low- and middle-income countries.

Question 6. How can a common disease turn into an epidemic? What do you mean by zoonotic disease?
Answer:
The epidemic is due to the sudden rise of cases usually resulting from a new infectious agent or a change in an existing agent,

For example:

1. An agent moving between host populations, for example, moving from animals to humans (zoonotic disease).
2. A genetic change or mutation occurs in the infectious agent, eg. bacteria, viruses, fungi etc.
3. Introduction of the new pathogen to a host population
4. Initially, scattered or isolated incidences of small clusters of the disease occur in humans
5. In the next step human to human transmission occurs at a rate that causes outbreaks in communities.

Zoonotic diseases are caused by germs that spread between animals and people. Examples include Covid-19, Plague, Rabies, Zoonotic influenza etc. Zoonotic diseases are caused by germs that spread between animals and people. Examples include Covid-19, Plague, Rabies, Zoonotic influenza etc.

WBBSE Class 8 Science Long Answer Format

Question 7. Name the causative agents of cholera and tuberculosis.
Answer:

Some Common Infectious Diseases Induced Epidemics

WBBSE Solutions For Class 8 School Science Long Answer Type Questions	WBBSE Solutions For Class 8 School Science Short Answer Type Questions
WBBSE Solutions For Class 8 School Science Very Short Answer Type Questions	WBBSE Solutions For Class 8 School Science Review Questions
WBBSE Solutions For Class 8 School Science Solved Numerical Problems	WBBSE Solutions For Class 8 School Science Experiments Questions
WBBSE Solutions For Class 8 Maths	WBBSE Class 8 History Notes
WBBSE Class 8 History Multiple Choice Questions	WBBSE Solutions For Class 8 History
WBBSE Solutions For Class 8 Geography

 

1. Cholera

Cholera is an acute intestinal infection caused by the

ingestion of food or water contaminated with the bacterium Vibrio cholera.

It has a short incubation period and produces an enterotoxin that causes a copious, painless, watery diarrhoea that can quickly lead to severe dehydration and death if treatment is not promptly given.

Vomiting also occurs in most patients. Most persons infected with V. cholerae do not become ill, although the bacterium is present in their faeces for 7-14 days.

When illness does occur, about 80-90% of episodes are of mild or moderate severity and are difficult to distinguish clinically from other types of acute diarrhoea.

Less than 20% of ill persons develop typical cholera with signs of moderate or severe dehydration.

2. Malaria

Malaria is caused by a protozoan parasite called Plasmodium, which is transmitted via the bites of infected female Anopheles mosquitoes.

In the human body, the parasites multiply in the liver and then infect red blood cells. Symptoms of malaria include fever, headache, and vomiting, and usually appear between 10 and 15 days after the mosquito bite.

1. If not treated, malaria can quickly become life-threatening by disrupting the blood supply to vital organs.
2. Key interventions to control malaria include:
3. prompt and effective treatment with artemisinin-based combination therapies,
4. use of insecticidal nets and
5. indoor spraying with insecticide.

3. Dengue

Dengue is transmitted by the bite of a mosquito. A type of mosquito called Aedes Egypt carries the germ of this disease. It is a febrile illness that affects infants, young children and adults with symptoms appearing 3-14 days after the infective bite.

Dengue is not transmitted directly from person to person and symptoms range from mild fever to incapacitating high fever, with severe headache, pain behind the eyes, muscle and joint pain, rash and lowering of platelet count to an alarming level.

It causes the oozing of blood in the skin due to the lowering of WBC. There is no vaccine or any specific medicine to treat dengue. People who have dengue fever should take rest, drink plenty of fluids and reduce the fever using paracetamol or see a doctor. It reduces the immunity power.

4. Plague

Plague is a bacterial disease, caused by Yersinia pestis, which primarily affects wild rodents such as rats. It is spread from one rodent to another by fleas (a type of insect).

Humans bitten by an infected flea usually develop a bubonic form of plague, which is characterized by a bubo, i.e. a swelling of the lymph node draining the flea bite site.

A type of fly called Xenopsylla cheopis carries the bacteria from the body of a rat afflicted with plague. If the bacteria reach the lungs, the patient develops pneumonia (pneumonic plague), which is then transmissible from person to person through infected droplets spread by coughing.

Initial symptoms of bubonic plague appear 7-10 days after infection. In 1987, Waldemar Haffkine invented the vaccine for the plague in Mumbai.

5. Smallpox

Smallpox is a disease caused by the Variola major virus. Some experts say that over the centuries it has killed more people than all other infectious diseases combined. Worldwide immunization stopped the spread of smallpox three decades ago The last case was reported in 1977.

Smallpox spreads very easily from person to person. Symptoms are flu-like. They include high fever, fatigue, headache, backache, and a rash with flat red sores.

There is no treatment. Fluids and medicines for pain or fever can help control symptoms. Most people recover, but some can die. Those who do recover may have severe scars. Edward Jenner, in 1976 used the cow-Pox virus in the human body to develop resistance against smallpox.

Analysis of Natural Phenomena Chapter 5 Solutions

6. Kala-azar

Kala-azar is the second largest parasitic killer in the world – only Malaria is more deadly. It is also known as Dumdum fever.
Kala-azar is caused by bites from sandflies – which carry the Leishmania donovani parasite, a type of protozoa, responsible for the disease.

If blood containing Leishmania donovani parasites is drawn from an animal or human, the next person to receive a bite will then become infected.

Initially, Leishmania parasites cause skin sores or ulcers at the site of sand fly bites. If the disease progresses, it attacks the immune system.

Kala-azar presents after two to eight months, with more generalised symptoms including prolonged fever and weakness. Indian scientist Upendranath Brahmachari invented the medicine of Kala-azar.

7. Diarrhoea

Diarrhoea is the passage of loose or liquid stools 3 or more times per day, or more frequently than is normal for the individual.

Diarrhoea means ‘to flow/ It is usually a symptom of gastrointestinal infection, which can be caused by a variety of bacterial, viral and parasitic organisms.

Infection is spread through contaminated food or drinking water, or from person to person as a result of poor hygiene.

Severe diarrhoea leads to fluid loss, and may be life-threatening, particularly in young children and people who are malnourished or have impaired immunity. A quick intake of ORS (Oral Rehydration Solution) is an effective remedy for diarrhoea.

8. Tuberculosis

Tuberculosis, or TB, is an infectious bacterial disease caused by Mycobacterium tuberculosis, which most commonly affects the lungs.

It is transmitted from person to person via droplets from the throat and lungs of people with active respiratory disease.
In healthy people, infection with Mycobacterium tuberculosis often causes no symptoms, since the person’s immune system acts to “wall off” the bacteria.

The symptoms of active TB of the lung are coughing, sometimes with sputum or blood, chest pains, weakness, weight loss, fever and night sweats. It is possible to control the disease by proper treatment through DOTS or Directly Observed Treatment, a short course.

9. Hepatitis

Hepatitis is an inflammation of the liver, most commonly caused by a viral infection. There are five main hepatitis viruses, referred to as types A, B, C, D and E.

These five types are of greatest concern because of the burden of illness and death they cause and the potential for outbreaks and epidemic spread.

In particular, types B and C lead to chronic disease in hundreds of millions of people and together, are the most common cause of liver cirrhosis and cancer.

Hepatitis A and E are typically caused by the ingestion of contaminated food or water. Hepatitis B, C and D usually occur as a result of contact with infected body fluids.

Common modes of transmission for these viruses include receipt of contaminated blood or blood products, invasive medical procedures using contaminated equipment and hepatitis B transmission from mother to baby at birth, from family member to child, and also by sexual contact.

Acute infection may occur with limited or no symptoms or may include symptoms such as jaundice (yellowing of the skin and eyes), dark urine, extreme fatigue, nausea, vomiting and abdominal pain.

10. Influenza/Flu

Influenza is a viral infection that affects mainly the nose, throat, bronchi and occasionally, lungs. Infection usually lasts for about a week and is characterized by sudden onset of high fever, aching muscles, headache and severe malaise, non-productive cough, sore throat and rhinitis.

The virus is transmitted easily from person to person via droplets and small particles produced when infected people cough or sneeze.

Influenza tends to spread rapidly in seasonal epidemics. Most infected people recover within one to two weeks without requiring medical treatment.

However, in the very young, the elderly and those with other serious medical conditions, the infection can lead to severe complications of the underlying condition, pneumonia and death.

11. AIDS

The Human Immunodeficiency Virus (HIV) is a retrovirus that infects cells of the immune system, destroying or impairing their function.

As the infection progresses, the immune system becomes weaker, and the person becomes more susceptible to infections. The most advanced stage of HIV infection is Acquired Immuno Deficiency 9/ndrome (AIDS).

It can take 10-15 years for an HIV-infected person to develop AIDS; antiretroviral drugs can slow down the process even further.
HIV is transmitted through unprotected sexual relations, transfusion of contaminated blood, sharing of contaminated needles and between a mother and her infant during pregnancy, childbirth and breastfeeding.

Natural Phenomena WBBSE Class 8 Comprehensive Answers

12. COVID-2019

Coronaviruses are a large family of viruses that cause illnesses ranging from the common cold to Middle East Respiratory Syndrome (MERS-CoV) and Severe Acute Respiratory Syndrome (SARS- CoV).

A novel coronavirus (nCoV) is a new strain that has not been previously identified in humans. Coronaviruses are zoonotic. SARS-CoV was transmitted from civet cats to humans and MERS- CoV from dromedary camels to humans with infected body fluids.

Common modes of transmission for these viruses include receipt of contaminated blood or blood products, invasive medical procedures using contaminated equipment and hepatitis B transmission from mother to baby at birth, from family member to child, and also by sexual contact.

Acute infection may occur with limited or no Common signs of infections including respiratory symptoms, fever, cough, shortness of breath and breathing difficulties.

In severe cases, the infection can cause pneumonia, severe acute respiratory syndromes, kidney failure and death. Standard recommendations to prevent infection spread include regular hand washing, covering mouth and nose when coughing and sneezing, avoiding close contact with people or social gatherings etc.

Epidemic due to a non-infectious disease

Epidemics of non-infectious disease are often caused by exposure to industrial products, intermediates or byproducts, either in the workplace or as a result of the contamination of a wider environment.

The Global Status Report on Non-communicable Diseases 2010 is the first report on the worldwide epidemic of cardiovascular diseases, cancer, diabetes and chronic respiratory diseases, along with their risk factors and determinants.

Non-communicable diseases killed tens of millions of people in 2008, and a large proportion of these deaths occurred before the age of 60, so during the most productive period of life. The magnitude of these diseases continues to rise, especially in low- and middle-income countries.

WBBSE Chapter 4 Occurrence Of Carbon And Its Compounds In Nature Long Answer Questions

Question 1. Mention the similarities between diamond and graphite.
Answer:

Comparison between diamond and graphite, Similarities between diamond and graphite:

 

Diamond	Graphite
1. Has crystalline structure.	1. Has crystalline structure.
2. Chemically very less active.	2. Chemically not very active.
3. Burns at high temperatures (800°C-850°C) in	3. Burns at high temperatures (700°C) in oxygen
oxygen to produce CO2	to produce CO2.
4. Cannot absorb any gas.	4. Cannot absorb any gas.
5. Good conductor of heat	5. Good conductor of heat.

 

Read And Learn More WBBSE Solutions For Class 8 School Science Long Answer Type Questions

Question 2. Mention the dissimilarities between diamond, and graphite.
Answer:

Dissimilarities between diamond and graphite:

 

Diamond	Graphite
1. Hardest natural element.	1. Soft and slippery element.
2. Colourless and transparent.	2. Blackish grey and opaque.
3. Non-conductor of electricity.	3. Good conductor of electricity.
4. Cannot put marks on the paper.	4. Can put a mark on the paper.

 

WBBSE Class 8 Carbon Compounds Long Answer Questions

Question 3. Write a short note on fullerene.
Answer:

Fullerene:

Fullerene – a new allotrope of carbon was first characterized in 1985 in the laboratory by Smalley and Kroto. This is a hollow, closed cage (polyhedral) cluster of 60 or 70 carbon atoms.

Its structure is based on polyhedra formed by fusing pentagons and hexagons-which is very much similar to geodesic domes used in architecture.

Fullerene is named after American architect R. Buckminster Fullerene – the inventor of the geodesic dome. It has been subjected to extensive research since its discovery and is a promising candidate for use in electronics and medicines. It can be used to produce novel enclosure compounds by trapping metal ions within the C60cage.

Question 4. Describe an experiment to show the adsorption property of charcoal.
Answer:

An Experiment To Show The Adsorption Property Of Charcoal:

The adsorption property of charcoal can be easily shown by following the experiment

Apparatus and chemicals needed: A bottle with a lid, some ink (or some colored solution), some finely crushed charcoal powder, and water.

Experiment: Let us first dissolve some ink (or some colored solution) in a small volume of water taken in a bottle with a lid.

Now pour some amount of finely crushed charcoal powder in it and close the lid. Then shake the bottle well for some time and allow it to settle. Now the solution containing the charcoal dust is filtered.

Observation: It is found that the intensity of the color of the solution after filtration has diminished considerably.

Inference: This indicates that a significant fraction of the molecules responsible for coloration has been “adsorbed” on the charcoal.

So, by filtration, when the solid charcoal powder is separated, the intensity of the color in the filtrate decreases.

Occurrence of Carbon Chapter 4 WBBSE Solutions

Question 5. Mention the uses of different amorphous allotropes of carbon.
Answer:

The Uses Of Different Amorphous Allotropes Of Carbon:

The uses of different amorphous allotropes of carbon are briefly discussed below.

1. Coke can be used as fuel during metal extraction and as a reducing agent in a redox reaction.
2. Coal is primarily used as fuel.

Lamp black is used as a pigment and used for making printing ink for use in the printing press. Gas carbon is used to make electrodes for batteries, arc lamps, or electrolytic cells.

Charcoal has a remarkable property of adsorption. It can adsorb impurities from water and hence can purify water. Specially prepared charcoal, known as activated charcoal, can adsorb large amounts of gas on its surface so it is used to prepare a gas mask.

Wood charcoal can be used as fuel in households and for preparing explosives for canons. Sugar charcoal is used in the laboratory as the purest form of carbon.

Bone charcoal is used for the purification of sugar and for the preparation of ivory black color (used by painters).

Question 6. Discuss briefly the consequence of the greenhouse effect.
Solution:

The increasing concentration of several greenhouse gases particularly CO₂ is causing the greenhouse effect (or enhanced greenhouse effect).

This will lead to Global Warming, As a direct consequence, the polar ice caps (in which more than 90% of the earth’s total drinking water is stored) will melt.

So the water level in the oceans will rise and several coastal cities (having a habitat of millions of people) will submerge. The ecosystem will be adversely affected.

Increasing temperature will enhance the growth of mosquitoes which in turn, will spread mosquito-borne diseases. Biodiversity will be hampered and several parts of the world may face drought-like conditions for a prolonged time.

In short, the effect of the greenhouse effect (or enhanced greenhouse effect) will bring devastation.

Question 7. Write a short note on biofuel.
Answer:

Biofuel:

Biofuel is a fuel that is derived from biological materials. Hydrocarbon fuel that is produced from organic matter (living or once-living material) in a short period of time (days, weeks, or even months) can be considered biofuel.

Ethanol, bio-diesel, and methanol are the three most important examples of biofuels. In India, bio-fuels are produced from oil obtained from the seeds of Jatropha plants.

The Jatropha oil can be used directly in diesel generators or diesel engines. It can also be used as a diesel additive.

Since Jatropha can be cultivated in less fertile and dry lands, its cultivation is economically beneficial from the perspective of our country.

WBBSE Class 8 Science Detailed Answers

Question 8. Write a short note on tidal energy as an alternative to fossil fuel.
Answer:

Tidal energy

Tidal energy is a form of hydropower that converts the energy of tides into more usable forms of energy, such as electricity.

Tidal energy is produced by the surge of ocean waters during the rise and fall of tides. High tide and ebb always occur twice a day.

Unlike wind, tides are predictable and stable. Usually, turbines are placed in tidal streams (a fast-flowing body of water created by tides).

The kinetic energy of the tidal stream is utilized to rotate turbines which in turn produce electricity. Tidal energy is more powerful and effective than wind energy.

Since, our country has very long coastlines, this form of renewable energy can be effectively utilized as an alternative form of energy.

Question 9. Describe an experiment to show that CO₂ is heavier than air.
Answer:

Experiment 

Let us take a balloon, a rubber band, sodium bicarbonate and vinegar, and a glass bottle. First, solid sodium bicarbonate and vinegar are taken together in the glass bottle and the balloon is fixed at the mouth of the bottle.

Due to the reaction between vinegar and sodium bicarbonate, CO₂ gas is produced. This gas will inflate the balloon. Now the balloon is taken out and its mouth is tied tightly with a cord, and then it is released.

It is found that the balloon is not floating. Rather it just lies on the floor. This is because CO₂ is heavier than air and that is why it did not float but settled on the floor.

Long Answer Format for Class 8 Science

Question 10. Describe an experiment to show that CO₂ is soluble in water.
Answer:

Experiment

A Wolfe’s bottle is used to produce CO₂ gas and the gas formed is passed through a gas jar filled with water.

After allowing CO₂ gas to flow through the water for sufficient time, a blue litmus paper and a red litmus paper are dipped in that water.

The blue litmus paper turns red, indicating that the solution is acidic. Water becomes acidic because CO₂ dissolves in water to produce carbolic acid (H2C03). H2C03 is a weak acid and it turns blue litmus paper red.

⇒ \(\mathrm{CO}_2+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{H}_2 \mathrm{CO}_3\)

 

The solubility of CO₂ in water is increased by applying pressure. By applying pressure, CO₂ is dissolved in soft drinks.

When the bottle is opened, the pressure is released, the solubility of CO₂ in water decreases and bubbles of CO₂ evolved from the bottle.

Question 11. Discuss briefly the oxidizing property of carbon dioxide.
Answer:

Oxidizing property of CO₂:

In powder or iron dust, the metals are oxidized to metal oxides, and CO₂ is reduced to carbon or carbon monoxide When CO₂ gas is passed over red hot carbon, C is oxidized to CO, and CO₂ is reduced to CO.

 

WBBSE Chapter 4 Carbon Compounds Study Guide

Question 12. What will happen if CO₂ gas is bubbled through clear lime water? What will happen if more CO₂ is passed through that same solution? What will happen if that solution is further boiled?
Answer:

CO₂ is an acidic oxide. It reacts with a base to form carbonate compounds. For example, some lime water is taken in a test tube and CO₂ gas is bubbled through it.

Lime water turns milky due to the formation of calcium carbonate (CaCO₂ ) which is insoluble and remains suspended in the solution.

\(\mathrm{Ca}(\mathrm{OH})_2+\mathrm{CO}_2 \rightarrow \mathrm{CaCO}_3+\mathrm{H}_2 \mathrm{O}\)

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WBBSE Solutions For Class 8 Maths	WBBSE Class 8 History Notes
WBBSE Class 8 History Multiple Choice Questions	WBBSE Solutions For Class 8 History
WBBSE Solutions For Class 8 Geography

 

If more CO₂ is passed through the milky water, calcium bicarbonate [CafHCOjJ is formed, which is soluble in water. So the solution becomes clear.

\(\mathrm{Ca}(\mathrm{OH})_2+\mathrm{CO}_2+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{Ca}\left(\mathrm{HCO}_3\right)_2\)

 

If the clear solution is boiled, it again turns milky, since the bicarbonate dissociates to form CaCO₃ again.

\(\mathrm{Ca}\left(\mathrm{HCO}_3\right)_2 \rightarrow \mathrm{CaCO}_3+\mathrm{CO}_2+\mathrm{H}_2 \mathrm{O}\)

 

Question 13. Write a short note on the greenhouse effect. What do you mean by global warming?
Answer:

Greenhouse Effect:

During the daytime, sunlight falls on the earth through invisible radiation, called infrared radiation, which is primarily responsible for the sensation of heat.

In the night the absorbed heat is radiated back to space by the earth’s surface.

A part of this low-energy infrared radiation is absorbed by the gases present in the atmosphere such as carbon dioxide, water vapor, methane, nitrous oxide, ozone, and some other man-made chemicals (such as chlorofluorocarbons).

This mechanism keeps the earth warm. This is known as the greenhouse effect and the gases present in the atmosphere which absorbs the radiated heat by the earth are called greenhouse gases. This is absolutely essential for the existence of living beings on earth.

Global warming is the increase in the global mean temperature consequent to the disturbance

The balance between the sun’s heat radiated towards the earth and the terrestrial heat re-radiated back to space caused by increased accumulation of greenhouse gases in the atmosphere.

Since 1975 the global temperature has undergone an increase at a rate of roughly 0.15-0.2X per decade.

WBBSE Long Answer Questions for Carbon Compounds

Question 14. Write a short note on cellulose. What is a monomer?
Answer:

Cellulose:

Cellulose is a polymeric carbohydrate that is biodegradable. It is the main constituent of the fiber of cotton and straw.

These polymeric carbohydrates are decomposed in nature by microorganisms such as fungi and bacteria.

Cellulose-based polymers are being developed to prepare biodegradable polymers with desired chemical and mechanical properties, which are non-toxic and can be naturally decomposed.

The small repeating molecules which join together to form a polymer are called monomers. The monomers may all be of the same compound or of two different compounds.

The polymer cellulose has only one type of monomers whereas nylon has two types of monomers arranged alternatively.

Question 15. What do you mean by an alternative source of energy? On what factors does the calorific value of fuels depend?
Answer:

Alternative Source Of Energy:

Those sources of energy which are not based on the burning of fossil fuels or fission of nuclear fuels and are non-polluting are called the alternative source of energy.

These are:

1. Solar energy
2. Hydel energy
3. Geothermal energy
4. Biomass energy etc.

The various factors that affect the calorific value of fuel are:

1. A fixed percentage of carbon indicates the organic matter that does not undergo volatilization during combustion.
2. The moisture content of the fuel
3. Percentage of volatile matter, which expresses the organic and inorganic matter obtainable from the thermal decomposition of the fuel.

WBBSE Class 8 Science Important Long Answers

Question 16. Describe the carbon cycle through a word diagram.
Answer: 

Carbon Cycle through a word diagram:

Chapter 3 Some Common Gases Long Answer Questions

Question 1. Describe burette and pipettes. Why pipettes and burettes are used in the laboratory?
Answer:

Filter Paper

Filter paper is used to separate solid, insoluble particles from a liquid. It is a thick, porous, circular paper. Filter papers having different pore sizes are available.

The filter paper is chosen based on the size of the insoluble, solid particles which are to be separated from the liquid.

After getting some idea about some common laboratory equipment and apparatus, we can now discuss two common gases which are very come to know about their physical and chemical usage.

One of them is oxygen and the other is Properties, their sources, and their preparation of hydrogen. During this brief discussion, we will procedures and their uses.

Read And Learn More WBBSE Solutions For Class 8 School Science Long Answer Type Questions

Question 2. Discuss briefly the laboratory method of preparation of oxygen from potassium chlorate.
Answer:

Laboratory Preparation Of Oxygen From Potassium Chlorate

Oxygen is usually prepared in the laboratory by heating carefully a mixture of potassium chlorate (KCIO₃) and manganese dioxide (MnO₂).

Four parts of solid potassium chlorate are intimately mixed with one part of solid manganese dioxide and taken in a hard glass test tube.

The test tube is fitted in such a way that it is tilted downwards. A delivery tube is fixed at the mouth of the test tube with the help of the bore of the cork.

The other end of the delivery tube is Potassium introduced into the gas jar filled with water. chlorate + The test tube is then heated strongly by a dioxide Bunsen burner.

Potassium chlorate melts and decomposes, evolving oxygen. The gas is collected in the gas jar by the downward displacement of water.

⇒ \(2 \mathrm{KClO}_3+\left[\mathrm{MnO}_2\right] \stackrel{\text { heat }(\Delta)}{\longrightarrow} 2 \mathrm{KCl}+3 \mathrm{O}_2+\left[\mathrm{MnO}_2\right]\)

1. The gas is collected by the downward displacement of water, because,
2. The solubility of oxygen in water is low

Oxygen is almost as heavy as air, so it cannot be collected by the downward displacement of air

In this reaction, MnO₂ acts as a catalyst. If KCIO₃ is heated alone, oxygen is produced at a temperature higher than 610°C.

In presence of a small amount of MnO₂, KCIO₃ decomposes at about 250°C to produce oxygen.

Actually, when KCIO₃ is heated alone, it melts at 357°C and rapidly gives off oxygen at 380°C.

But the mass becomes pasty as the reaction proceeds due to the formation of potassium perchlorate (KCIO₄ )whose melting point is 610°C.

⇒ \(4 \mathrm{KClO}_3 \rightarrow 3 \mathrm{KClO}_4+\mathrm{KCl}\)

When heated above 610°C, it decomposes to produce oxygen and a residue of KCI is left.

⇒ \(\mathrm{KClO}_4 \rightarrow \mathrm{KCl}+2 \mathrm{O}_2\)

In presence of a little amount of MnO₂, KCIO₃ smoothly decomposes at about 250°C to produce oxygen without the formation of KCIO₄ in the intermediate stage.

 

West Bengal Class 8 Science Gases Solutions

Thus MnO₂ accelerates the reaction and acts as a true catalyst. Moreover, both KCIO₃ and KCIO₄ are explosive substances. Any probable danger of explosion in the act of heating them to high temperatures is avoided by performing the reaction at lower temperatures with the help of a catalyst.

Precaution

1. KCIO₃ and MnO₂ should be mixed intimately
2. MnO₂ must not be contaminated with charcoal or antimony sulphide
3. The hard glass test tube must be tilted downwards
4. Heating should be done slowly and should continue from the front to the back side of the test tube.

Question 3. Discuss a laboratory method for the preparation of oxygen at room temperature.
Answer:

Preparation of Oxygen from Sodium Peroxide at room temperature

Oxygen is produced easily when water is added to solid sodium peroxide. No heating is required for this process.

⇒ \(2 \mathrm{Na}_2 \mathrm{O}_2+2 \mathrm{H}_2 \mathrm{O} \rightarrow 4 \mathrm{NaOH}+\mathrm{O}_2\)

Materials and Apparatus Required: Solid sodium peroxide (Na₂O₂), distilled water, a conical flask, a cork with two holes in it, a dropping funnel, a bent delivery tube, and a gas jar.

Experiment: Solid Na₂O₂  is taken in the conical flask and the mouth of the conical flask is fitted with a cork. Through one of the holes, a dropping funnel is attached and through another hole, one end of the bent delivery tube is inserted.

The other end of the bent delivery tube is introduced into the gas jar filled with water. Now water is added

Observation: Oxygen gas is evolved. The gas dropwise to solid Na₂O₂ through the dropping is collected in the gas jar by downward funnel displacement of water.

Common Gases Chapter 3 WBBSE

Question 4. How oxygen can be produced by the electrolysis of water?
Answer:

Preparation of Oxygen by Electrolysis of Water

Electrolysis of water acidified with dilute sulphuric acid can produce hydrogen at the cathode and oxygen gas at the anode. A platinum electrode is used as an anode and cathode in a rectangular tank.

High voltage is passed through the tank to carry out the electrolysis. Here, hydrogen gas is obtained as a by-product.

Manufacture of Oxygen by Fractional Distillation of air Industrially oxygen is produced in bulk quantity by a process known as a fractional distillation of liquid air.

Air is composed of nitrogen and oxygen, in which oxygen forms about 21% by volume. The two gases can be separated from one another by liquefaction of air followed by fractional distillation.

Removal of water vapour, CO₂ & dust particles: Air is first freed from water vapour and carbon dioxide by passing them over fused calcium chloride and slaked lime, respectively. Dust particles are removed from the air by passing them through an electric precipitator.

WBBSE Solutions For Class 8 School Science Long Answer Type Questions	WBBSE Solutions For Class 8 School Science Short Answer Type Questions
WBBSE Solutions For Class 8 School Science Very Short Answer Type Questions	WBBSE Solutions For Class 8 School Science Review Questions
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WBBSE Solutions For Class 8 Maths	WBBSE Class 8 History Notes
WBBSE Class 8 History Multiple Choice Questions	WBBSE Solutions For Class 8 History
WBBSE Solutions For Class 8 Geography

 

Question 5. Discuss briefly the physical properties of oxygen.
Answer:

Physical Properties of Oxygen

1. Oxygen is a colourless, odourless and tasteless gas.
2. It is slightly heavier than air. The density of oxygen at normal temperature and pressure is 1.428 grams per litre.
3. It condenses to a pale blue liquid, which freezes to a blue solid if cooled in liquid hydrogen. The freezing point of liquid oxygen is – 218°C and the boiling point of liquid oxygen is – 183°C.
4. Oxygen is slightly soluble in water. The solubility of oxygen at 0°C and 1 atmospheric pressure is 1438 mg/lit. The ‘ dissolved oxygen sustains the life of aquatic plants and animals.
5. The respiration of aquatic animals is dependent on the dissolved oxygen in the water. Since oxygen is more soluble in water than nitrogen, water is richer in oxygen than ordinary air.

Oxygen has three naturally occurring isotopes, \({ }_8^{16} \mathrm{O},{ }_8^{17} \mathrm{O} \text { and }{ }_8^{18} \mathrm{O}\) But the natural abundance of the last two is very low.

Isotope	168 O	178O	188O
Natural abundance	0.99763%	0.00037%	0.002%

Oxygen exhibits allotropy. Its allotropic modification is ozone (03).

Question 6. Discuss briefly the reaction of oxygen with non-metals.
Answer:

The reaction of oxygen with non-metals:

Oxygen reacts with non-metals such as carbon, sulphur, phosphorous, etc. to produce oxides. Generally, most oxides of non-metals are acidic. Their aqueous solution produces acid. A few examples are given below.

When a piece of glowing charcoal is introduced in a jar of oxygen, charcoal burns more brightly throwing sparks. The product of this reaction is carbon dioxide.

⇒ \(\mathrm{C}+\mathrm{O}_2 \rightarrow \mathrm{CO}_2\)

When a small quantity of sulphur is heated in a flame and then introduced in a jar filled with oxygen, it is observed that the burning takes place brilliantly producing a blue flame and sulphur dioxide (SO₂) is produced.

\(\mathrm{S}+\mathrm{O}_2 \rightarrow \mathrm{SO}_2\)

When a piece of white phosphorous is introduced in a jar of oxygen, it burns brightly with white flames and forms white fumes of phosphorous pentoxide (P2O5) which solidifies on cooling.

⇒ \(4 \mathrm{P}+5 \mathrm{O}_2 \rightarrow 2 \mathrm{P}_2 \mathrm{O}_5\)

WBBSE Class 8 Gases Practice Questions

Question 7. Oxides of non-metals are generally acidic—Why? Generally, most oxides of non-metals are acidic. Their aqueous solution produces acid.
Answer:

Oxides of non-metals are generally acidic:

For example, CO₂ SO₂ and P₂O₅ are three oxides of non-metal. When they are dissolved in water, they form acids (e.g. carbonic acid, sulphurous acid and phosphoric acid, respectively) and turn blue litmus paper red, indicating that their solution is acidic.

⇒ \(\begin{array}{ll}
\mathrm{C}+\mathrm{O}_2 \rightarrow \mathrm{CO}_2 ; & \mathrm{CO}_2+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{H}_2 \mathrm{CO}_3 \\
\mathrm{~S}+\mathrm{O}_2 \rightarrow \mathrm{SO}_2 ; & \mathrm{SO}_2+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{H}_2 \mathrm{SO}_3 \\
4 \mathrm{P}+5 \mathrm{O}_2 \rightarrow 2 \mathrm{P}_2 \mathrm{O}_5 ; & \mathrm{P}_2 \mathrm{O}_5+3 \mathrm{H}_2 \mathrm{O} \rightarrow 2 \mathrm{H}_3 \mathrm{PO}_4
\end{array}\)

Question 8. Discuss briefly the reaction of oxygen with metals.
Answer:

The reaction of oxygen with metals:

Some metals burn in oxygen on heating producing metal oxides. Metal oxides are mostly basic oxides. Some examples are given below.

When a piece of hot, dry sodium is introduced in a jar of oxygen, the metal burns spontaneously producing a golden yellow flame and forming sodium oxide (Na₂O).

⇒ \(4 \mathrm{Na}+\mathrm{O}_2 \rightarrow 2 \mathrm{Na}_2 \mathrm{O}\)

When a burning magnesium ribbon is introduced in a gas jar filled with oxygen, it burns brightly producing blinding white light. The white powdery substance left after burning is magnesium oxide (MgO).

⇒ \(2 \mathrm{Mg}+\mathrm{O}_2 \rightarrow 2 \mathrm{MgO}\)

In a similar way, potassium, calcium, etc. react with oxygen to form basic oxides

\(\begin{gathered}
2 \mathrm{Ca}+\mathrm{O}_2 \rightarrow 2 \mathrm{CaO} \\
4 \mathrm{~K}+\mathrm{O}_2 \rightarrow 2 \mathrm{~K}_2 \mathrm{O}
\end{gathered}\)

Question 9. Oxides of metals are generally basic—Explain.
Answer:

Oxides of metals are generally basic:

Metal oxides are mostly basic oxides. Some basic oxides form hydroxides when they react with water. Hydroxides of some metals such as sodium, magnesium, calcium, potassium etc.

Are soluble in water and turn red litmus paper blue, indicating that the aqueous solution of those metal oxides is basic. Some examples are given below.

⇒ \(\begin{array}{ll}
\mathrm{C}+\mathrm{O}_2 \rightarrow \mathrm{CO}_2 ; & \mathrm{CO}_2+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{H}_2 \mathrm{CO}_3 \\
\mathrm{~S}+\mathrm{O}_2 \rightarrow \mathrm{SO}_2 ; & \mathrm{SO}_2+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{H}_2 \mathrm{SO}_3 \\
4 \mathrm{P}+5 \mathrm{O}_2 \rightarrow 2 \mathrm{P}_2 \mathrm{O}_5 ; & \mathrm{P}_2 \mathrm{O}_5+3 \mathrm{H}_2 \mathrm{O} \rightarrow 2 \mathrm{H}_3 \mathrm{PO}_4
\end{array}\)

Question 10. Give two examples of amphoteric oxides. Why are they so-called? Explain.
Answer:

Examples of amphoteric oxides:

Aluminium oxide (Al₂O₃) and zinc oxide (ZnO) are two examples of amphoteric oxide. They are called amphoteric oxides because they have the properties of acidic and basic oxides.

They undergo a neutralization reaction with both acids and bases. They act as weakly basic oxides towards a strong acid and as weakly acidic oxides towards a strong base.

1. For example, the aluminium metal reacts with oxygen to form aluminium oxide (Al2O3).

⇒ \(4 \mathrm{Al}+3 \mathrm{O}_2 \rightarrow 2 \mathrm{Al}_2 \mathrm{O}_3\)

It reacts with an acid to form aluminium chloride (salt) and water.

⇒ \(\mathrm{Al}_2 \mathrm{O}_3+6 \mathrm{HCl} \rightarrow 2 \mathrm{AlCl}_3+3 \mathrm{H}_2 \mathrm{O}\)

It reacts with a base (such as sodium hydroxide) to produce sodium aluminate and water.

⇒ \(\mathrm{Al}_2 \mathrm{O}_3+2 \mathrm{NaOH} \rightarrow 2 \mathrm{NaAlO}_2+\mathrm{H}_2 \mathrm{O}\)

2. Zinc oxide (ZnO) is an amphoteric oxide. Zinc metal reacts with oxygen to form zinc oxide (ZnO).

⇒ \(2 \mathrm{Zn}+\mathrm{O}_2 \rightarrow 2 \mathrm{ZnO}\)

It reacts with hydrochloric acid to form zinc chloride (salt) and water.

⇒ \(\mathrm{ZnO}+2 \mathrm{HCl} \rightarrow \mathrm{ZnCl}_2+\mathrm{H}_2 \mathrm{O}\)

It reacts with sodium hydroxide to produce sodium zincate and water.

⇒ \(2 \mathrm{ZnO}+4 \mathrm{NaOH} \rightarrow 2 \mathrm{Na}_2 \mathrm{ZnO}_2+2 \mathrm{H}_2 \mathrm{O}\)

West Bengal Board Class 8 Science Revision

Question 11. Stannic oxide and lead monoxide are amphoteric oxides—Explain.
Answer:

Stannic oxide and lead monoxide are amphoteric oxides:

Stannic oxide and lead monoxide are called amphoteric oxide because they have properties of the acidic and basic oxide.

They undergo a neutralization reaction with both acids and bases.

They act as weakly basic oxides towards a strong acid and as weakly acidic oxides towards a strong base.

Stannic oxide (SnO₂) is prepared by burning tin at white heat in the air. It dissolves in concentrated H₂SO₄ to produce stannic sulphate (which is unstable). On fusion with sodium hydroxide, it forms sodium stannate, which is soluble in water.

⇒ \(\mathrm{Sn}+\mathrm{O}_2 \rightarrow \mathrm{SnO}_2\)

Reaction with acid: \(\mathrm{SnO}_2+2 \mathrm{H}_2 \mathrm{SO}_4 \rightarrow \mathrm{Sn}\left(\mathrm{SO}_4\right)_2+2 \mathrm{H}_2 \mathrm{O}\)

Reaction with base: \(\mathrm{SnO}_2+2 \mathrm{NaOH} \rightarrow \mathrm{Na}_2 \mathrm{SnO}_3+\mathrm{H}_2 \mathrm{O}\)

Lead monoxide is obtained by heating lead in the air. It reacts with HNO₃ forming lead nitrate. It dissolves in a hot sodium hydroxide solution forming sodium plumbite.

⇒ \(2 \mathrm{~Pb}+\mathrm{O}_2 \rightarrow 2 \mathrm{PbO}\)

Reaction with acid: \(\mathrm{PbO}+2 \mathrm{HNO}_3 \rightarrow \mathrm{Pb}\left(\mathrm{NO}_3\right)_2+\mathrm{H}_2 \mathrm{O}\)

Reaction with base: \(\mathrm{PbO}+2 \mathrm{NaOH} \rightarrow \mathrm{Na}_2 \mathrm{PbO}_2+\mathrm{H}_2 \mathrm{O}\)

Question 12. Discuss briefly the uses of oxygen in industries.
Answer:

The uses of oxygen in industries:

Oxygen is a very important element in the chemical industry. It is used to remove impurities from crude iron and pure steel Is produced.

Impurities present in crude Iron enhance rusting of iron.

During the preparation of H₂SO₄, oxygen Is utilized. SO₂ Is oxidized to SO₃ by reaction with oxygen which is then converted to HSO₂ In subsequent steps.

H₂SO₄ is an essential Component in car batteries, and storage cells and is used for making paints and fertilizers and for refining petroleum and metals like zinc and copper.

Oxygen is used during the industrial production of HNO₃ by the Ostwald process. HNO₃ is essential for producing fertilizers like ammonium nitrate (NH₄NO₃) and for preparing explosives.

For welding and cutting of metals, oxy-hydrogen flame and oxy-acetylene flame are produced in which temperature as high as approximately 3000°C is achieved.

These flames are produced by the exothermic reactions between oxygen and hydrogen and between oxygen and acetylene.

Question 13. Show with an experiment that hydrogen is lighter than air.
Answer:

An experiment that hydrogen is lighter than air:

With the help of an experiment, it can be shown that hydrogen is lighter than air.

Apparatus and chemicals required: Two gas jars with lids – one filled with hydrogen gas and the other filled with air, a taper.

Experiment: Two gas jars marked A and B are taken. One of the gas Jars A is filled with hydrogen gas and is covered by a lid.

The open mouth of another gas jar B is held upside down over gas Jar A and the lid Is then slowly removed. Now a burning taper is introduced Inside the gas jar B.

Observation: A “pop” sound Is heard. The taper extinguishes but the gas burns with a bluish flame.

Inference: This confirms that the gas in gas Jar B is hydrogen. Hydrogen was initially in the gas Jar A.

When the gas jar filled with air is Inverted over the gas jar filled with hydrogen, hydrogen is lighter than air, moves upwards and is collected in gas jar B by downward displacement of air.

Chapter 3 Common Gases Important Questions

Question 14. Discuss briefly the reaction of hydrogen with the following non-metals: chlorine, nitrogen and sulphur.
Answer:

Preparation of Oxygen from Sodium Peroxide at room temperature

Oxygen is produced easily when water is added to solid sodium peroxide. No heating is required for this process.

⇒ \(2 \mathrm{Na}_2 \mathrm{O}_2+2 \mathrm{H}_2 \mathrm{O} \rightarrow 4 \mathrm{NaOH}+\mathrm{O}_2\)

Materials and Apparatus Required: Solid sodium peroxide (Na₂O₂), distilled water, a conical flask, a cork with two holes in it, a dropping funnel, a bent delivery tube, and a gas jar.

Experiment: Solid Na₂O₂ is taken in the conical flask and the mouth of the conical flask is fitted with a cork.

Through one of the holes, a dropping funnel is attached and through another hole, one end of the bent delivery tube is inserted.

The other end of the bent delivery tube is introduced into the gas jar filled with water. Now water is added

Observation: Oxygen gas is evolved. The gas dropwise to solid Na₂O₂ through the dropping is collected in the gas jar by downward funnel displacement of water.

Class 8 Science Gases Study Material

Question 15. What precautions should be taken during the laboratory preparation of oxygen from potassium chlorate?
Answer:

Laboratory Preparation Of Oxygen From Potassium Chlorate

Oxygen is usually prepared in the laboratory by heating carefully a mixture of potassium chlorate (KCIO₃) and manganese dioxide (MnO₂).

Four parts of solid potassium chlorate are intimately mixed with one part of solid manganese dioxide and taken in a hard glass test tube.

The test tube is fitted in such a way that it is tilted downwards. A delivery tube is fixed at the mouth of the test tube with the help of the bore of the cork.

The other end of the delivery tube is Potassium introduced into the gas jar filled with water. chlorate + The test tube is then heated strongly by a dioxide Bunsen burner.

Potassium chlorate melts and decomposes, evolving oxygen. The gas is collected in the gas jar by the downward displacement of water.

⇒ \(2 \mathrm{KClO}_3+\left[\mathrm{MnO}_2\right] \stackrel{\text { heat }(\Delta)}{\longrightarrow} 2 \mathrm{KCl}+3 \mathrm{O}_2+\left[\mathrm{MnO}_2\right]\)

The gas is collected by the downward displacement of water, because,

The solubility of oxygen in water is low

Oxygen is almost as heavy as air, so it cannot be collected by the downward displacement of air

In this reaction, MnO₂ acts as a catalyst. If KCIO₃ is heated alone, oxygen is produced at a temperature higher than 610°C.

In presence of a little amount of MnO₂, KCIO₃ decomposes at about 250°C to produce oxygen.

Actually, when KCIO₃ is heated alone, it melts at 357°C and rapidly gives off oxygen at 380°C. But the mass becomes pasty as the reaction proceeds due to the formation of potassium perchlorate (KCIO₄ )whose melting point is 610°C.

⇒ \(4 \mathrm{KClO}_3 \rightarrow 3 \mathrm{KClO}_4+\mathrm{KCl}\)

When heated above 610°C, it decomposes to produce oxygen and a residue of KCI is left.

⇒ \(\mathrm{KClO}_4 \rightarrow \mathrm{KCl}+2 \mathrm{O}_2\)

In presence of a little amount of MnO₂, KCIO₃ smoothly decomposes at about 250°C to produce oxygen without the formation of KCIO₄ in the intermediate stage.

 

 

Thus MnO₂ accelerates the reaction and acts as a true catalyst. Moreover, both KCIO₃ and KCIO₄  are explosive substances. Any probable danger of explosion in the act of heating them to high temperatures is avoided by performing the reaction at lower temperatures with the help of a catalyst.

Precaution

1. KCl03 and MnO₂ should be mixed intimately
2. MnO₂ must not be contaminated with charcoal or antimony sulphide
3. The hard glass test tube must be tilted downwards
4. Heating should be done slowly and should continue from the front to the back side of the test tube.

WBBSE Class 8 Environment and Science Notes

Question 16. A soft white metal A reacts with water to form a compound B and a colourless gas C. When C is passed over heated copper oxide, water and a red-brown coloured element D are formed. Identify A, B, C, and D and write the reactions.
Answer:

Given:

A soft white metal A reacts with water to form a compound B and a colourless gas C.

When C is passed over heated copper oxide, water and a red-brown coloured element D are formed.

A is sodium. Sodium reacts with water to form sodium oxide (Na₂O:B) and hydrogen (H₂:C) 2Na + H₂O = Na₂O + H₂O , When hydrogen is passed over heated cupric oxide (CuO), water and metallic copper (Cu :D) are formed.

⇒ \(\mathrm{CuO}+\mathrm{H}_2=\mathrm{Cu}+\mathrm{H}_2 \mathrm{O} \text {. }\)

WBBSE Chapter 2 Element Compound And Chemical Reaction Chemical Effects Of Electricity Long Answer Questions

Question 1. Describe briefly the process of electrolysis of molten sodium chloride, using suitable electrodes.
Answer:

The process of electrolysis of molten sodium chloride, using suitable electrodes:

When electricity is passed through the molten or fused sodium chloride through graphite anode and iron cathode, NaCI is electrolyzed.

Na⁺ ions move towards the cathode and negatively charged Cl^– ions move towards the anode. At the cathode, Na⁺ ions take up electrons and are converted to sodium metal.

At the anode, Cl^– ions give up electrons and are ultimately converted to chlorine gas (Cl₂) Electrons are taken up by Na⁺ ions at the cathode.

So reduction occurs at the cathode. Oxidation occurs at the anode because the electron is given up by the Cl₂ ion. So sodium metal is produced at the cathode and chlorine gas is produced at the anode.

Electrode reaction: At cathode (Reduction) : \(\mathrm{Na}^{+}+e \rightarrow \mathrm{Na}\)

At anode (Oxidation): \(2 \mathrm{Cl}^{-}-2 e \rightarrow \mathrm{Cl}_2 (gas)\)

Read And Learn More WBBSE Solutions For Class 8 School Science Long Answer Type Questions

WBBSE Class 8 Chemical effects of electricity long answer questions

Question 2. Describe briefly the process of electrolysis of acidified water, using suitable electrodes.
Answer:

Electrolysis Of Sodium Chloride:

Let us now discuss what happens when fused sodium chloride is electrolyzed using the above setup,

We know that Aien NaCI is dissolved in water, it is completely dissociated into Na⁺ and Cl^– ions and they move freely within the solution. In molten or fused states also Na⁺ and ions are produced which move freely within the molten state.

⇒ \(\mathrm{NaCl} \rightleftharpoons \mathrm{Na}^{+}+\mathrm{Cl}^{-}\)

When electricity is passed through the molten or fused sodium chloride through graphite anode and iron cathode,

Na⁺ ions (which are positively charged) move towards the cathode (connected to the negative terminal of the battery) and negatively charged Cl^– ions move towards the anode (connected to the positive terminal of the battery).

At the cathode, Na⁺ ions take up electrons and are converted to sodium metal. At the anode, Cl^– ions give up their “extra” electrons and are ultimately converted to chlorine gas (Cl₂).

Electrons are taken up by Na⁺ ions at the cathode. So reduction occurs at the cathode while oxidation occurs at the anode because the electron is given up by the Cl^– ion. Hence sodium metal is produced at the cathode and chlorine gas is produced at the anode.

The electrolysis of molten or fused sodium chloride can be summarized in the following table.

 

Electrolysis of molten or fused sodium chloride
Electrolyte	Molten or fused sodium chloride (NaCI)
Electrode	Anode: graphite Cathode: Iron
Electrode reaction :
At the cathode (Reduction):	\(\mathrm{Na}^{+}+\mathrm{e} \rightarrow \mathrm{Na}\)
At anode (Oxidation):	\(2 \mathrm{Cl}^{-}-2 \mathrm{e} \rightarrow \mathrm{Cl}_2 \text { (gas }\)

 

Long answer type questions on chemical reactions for Class 8

Question 3. The phenomenon of electrolysis is actually a phenomenon of oxidation reduction-explain with a suitable example. Or “During electrolysis, reduction occurs at the cathode and oxidation occurs at anode”. Explain with a suitable example.
Answer:

Let us describe it with the help of electrolysis of molten sodium chloride.

When electricity is passed through the molten or fused sodium chloride through graphite anode and iron cathode, positively Na⁺ ions solution of the chloride salts of No, Mg or Ca is electrolyzed, and the metals cannot be extracted.

moves towards the cathode and negatively charged Cl^– ions move towards the anode. At the cathode, Na⁺ ions take up electrons and are converted to sodium metal.

At the anode, Cl^– ions give up the electron and are ultimately converted to chlorine gas (Cl₂). So sodium metal is produced at the cathode and chlorine gas is produced at the anode.

Electrons are taken up by Na⁺ ions at the cathode. So reduction occurs at the cathode. Oxidation occurs at the anode because the electron is given up by the Cl^– ion.

Electrode reaction:At cathode (Reduction) :  \(\mathrm{Na}^{+}+e \rightarrow \mathrm{Na}\)

At anode (Oxidation) : \(2 \mathrm{Cl}-2 e \rightarrow \mathrm{Cl}, (gas)\)

Question 4. Discuss the electro-refining of metal with a suitable example.
Answer:

Purification or electro-refining of metals:

Electrolysis is used in the purification of impure metals that are extracted from their ores. In this process:

A thick rod of impure metal is made the anode. It is connected to the positive terminal of the battery.

A thin strip of pure metal is made of the cathode. It is connected to the negative terminal of the battery.

A water-soluble salt of the metal to be purified is taken as an electrolyte.

On passing an electric current, the metal dissolves from the impure anode and goes into the electrolyte solution. The metal present in dissolved form in the electrolyte gets deposited on the cathode in the pure form.

The impurities are left behind in the electrolyte solution. Metals like copper, zinc and aluminium etc are purified by electrolysis.

Purification of copper may be taken up as a typical example to discuss electro-refining. Copper is obtained from copper ores which contain various impurities.

Hence when copper is extracted from the ores, it contains various impurities. The metallic copper with impurities is not suitable for various uses such as in electrical appliances such as copper wire.

So, impure copper is to be purified by electrolysis. The impure metal in the form of thick blocks is used as an anode and the pure metal in the form of a thin sheet is used as a cathode.

An acidified copper sulphate solution is taken as an electrolyte. On carrying out electrolysis, copper atoms dissolve out as Cu²⁺ ions from the anode and are deposited at the cathode as metallic copper.

So cathode now consists of pure copper. As the process continues more and more copper dissolves from the anode and is deposited as pure copper on the anode. This process is known as the electro-refining of metals.

Electro-refining of copper
Electrolyte	Acidified copper sulphate solution (CuSO4)\(\left[15 \% \mathrm{CuSO}_4+5 \% \mathrm{H}_2 \mathrm{SO}_4\right]\)
Electrode	Anode: copper bar containing impurities Cathode: a thin sheet of pure copper
Electrode reaction :
At anode (oxidation) :	\(\mathrm{Cu} \rightarrow \mathrm{Cu}^{2+}+2 e (pure copper comes into) solution as \left(\mathrm{Cu}_2+\right. ion )\)
At the cathode (reduction):	\(\mathrm{Cu}^{2+}+2 e \rightarrow \mathrm{Cu} (copper metal is deposited on the cathode)\)

 

WBBSE Chapter 2 electricity and chemical reactions detailed answers

Question 5. Describe with a suitable example, the extraction of a metal from its halide salt.
Answer:

The extraction of a metal from its halide salt:

This can be illustrated by the extraction of calcium metal from calcium chloride. In this process, molten calcium chloride is taken in a graphite container.

The walls of the graphite container act as an anode while an iron electrode is immersed partially in the molten salt and acts as a cathode. Calcium metal is formed at the cathode and chlorine gas is liberated at the anode.

Extraction of Calcium by Electrolysis of molten or fused calcium chloride
Electrolyte	Molten or fused magnesium chloride (CaCl2)
Electrode	Anode: graphite Cathode: iron
Electrode reaction:
At the cathode (Reduction) :	\(\mathrm{Ca}^{2+}+2 e \rightarrow \mathrm{Ca}\)
At anode (Oxidation) :	\(2 \mathrm{Cl}-2 e \rightarrow \mathrm{Cl}_2 \text { (gas) }\)

 

(In the industrial process, calcium chloride is mixed with calcium fluoride. This lowers the melting point of calcium chloride. The molten electrolyte is heated at 700°C and at this temperature electrolysis is carried out.)

Question 6. Describe briefly the process of electroplating copper on iron nails.
Answer:

The process of electroplating copper on iron nails:

During the electroplating of copper on iron nails, the pure copper metal is used as anode and the iron nails (i.e., the objects which are to be electroplated) are used as cathode.

An acidified copper sulphate solution is taken as an electrolyte. On carrying out electrolysis, copper atoms gradually dissolve out as Cu²⁺ ions from the anode and are deposited at the cathode as metallic copper.

As the process continues more and more copper dissolves from the anode and is deposited as pure copper on the iron nails (which are used as the anode). So after some time, the iron nails are coated with a thin layer of copper.

Electro-refining of copper on iron nails
Electrolyte	Acidified copper sulphate solution (CuS04)
Electrode	Anode: pure copper metal Cathode: iron nails
Electrode reaction:
At anode (oxidation) :	\(\mathrm{Cu} \rightarrow \mathrm{Cu}^{2+}+2 e (pure copper comes into) solution as \left(\mathrm{Cu}_2+\right. ion )\)
At the cathode (reduction):	\(\mathrm{Cu}^{2+}+2 e \rightarrow \mathrm{Cu} (copper metal is deposited on the cathode)\)

 

In-depth explanations of chemical effects of electricity for Class 8

Question 7. Describe briefly the electroplating of different metals on various objects familiar to us.
Answer:

The electroplating of different metals on various objects familiar to us:

The various objects familiar to us can be electroplated by various metals for various purposes such as to protect from rusting, to improve the external appearance, to enhance commercial value, etc.

In the case of electroplating, the metal which is to be electroplated is used as an anode and the article on which the coating is given is used as the cathode, and a suitable salt solution is used as an electrolyte.

During electroplating, the pure metal from the anode is gradually dissolved as metal ions into the solution and the metal ions are deposited on the cathode material in the form of a thin film.

Various instances have been presented in tabular form below:

Objects which is to electroplated	The metal which has to be electroplated on the objects	The material used as cathode	The material used as anode	Electrolyte
Iron pipe	Zinc	Iron pipe	zinc	An aqueous solution of zinc chloride
The handle of the bicycle made of iron	Chromium	The handle of the bicycle made of iron	Pure sheet of chromium	An aqueous solution of chromic sulphate and chromic acid
Tap made of brass	Chromium	Tap made of brass	Pure sheet of chromium	The aqueous solution of chromic sulphate and chromic acid

 

WBBSE Class 8 Science practice long answer questions on chemical reactions

Question 8. What are the chemical changes associated with electrolysis?
Answer:

The chemical changes associated with electrolysis:

The products obtained at electrodes vary as the electrodes used in electrolysis support. The chemical changes brought about by electrolysis are-

1. A bubble of gases may be formed on the electrodes
2. deposits of metals may occur on electrodes

changes in the colour of the solution may occur Electrolysis of the same electrolyte with different electrodes produce different products.

When an aqueous solution of CuSO₄ is electrolysed with Cu electrodes, SO^2- ions, in preference to OH^– ions accumulate at the anode and Cu²⁺ ions will be discharged at the cathode.

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WBBSE Class 8 History Multiple Choice Questions	WBBSE Solutions For Class 8 History
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But, if platinum electrodes are used in this electrolysis, OH^– ions, in preference to SO₄ ions will be discharged at the anode and Cu²⁺ ions will be discharged at the cathode.

Similarly, if an aqueous solution of NaCI is electrolysed with platinum electrodes, H⁺ ions, in preference to Na⁺ ions are discharged at the cathode.

But, if a mercury cathode is used in this electrolysis, Na⁺ ions, in preference to H⁺ ions will be discharged at the cathode.

Chapter 2 Element Compound And Chemical Reaction Chemical Reaction Long Answer Questions

Question 1. What is meant by a positive catalyst and a negative catalyst? Give a suitable example.
Answer:

Positive catalyst And a Negative catalyst:

The substance which can increase the rate of a chemical reaction is called a positive catalyst.
When potassium chlorate is heated at 630°C, oxygen is produced very slowly. But when potassium chlorate is mixed with manganese dioxide (in 1: a 4 ratio i.e., 1 part MnO₂ with 4 part KClO₃), then oxygen is produced at a much lower temperature (240°C). So, in this reaction, MnO₂ acts as a positive catalyst and increases the rate of the reaction.

⇒ \(2 \mathrm{KClO}_3 \rightarrow 2 \mathrm{KCl}+3 \mathrm{O}_2\)

The substance which decreases the rate of a chemical reaction is called a negative catalyst Hydrogen peroxide dissociates slowly even at room temperature to produce oxygen and water.

But when a little phosphoric acid ( H₃PO₄) is added to H₂O₂, it slows down the rate of decomposition of H₂O₂. So H₃PO₄ acts as a negative catalyst in this reaction.

⇒ \(\mathrm{H}_2 \mathrm{O}_2 \rightarrow \mathrm{H}_2 \mathrm{O}+\mathrm{O}_2\)

Question 2. Mention the characteristic properties of a catalyst
Answer:

Catalyst:

Many chemical reactions occur very slowly. But the addition of a minute quantity of another substance can speed up the reaction.

The substances which can increase the rate of a chemical reaction are called catalysts. There are certain catalysts which slow down the rate of a particular chemical reaction.

They are termed negative catalysts. The chemical reaction which involves the use of a catalyst is called a catalytic reaction.

A catalyst is a substance which is present in small quantities and increases the rate of a chemical reaction without itself undergoing any permanent change. [ A negative catalyst however decreases the rate of the chemical reaction.] Examples Of Some Catalytic Reactions

 

WBBSE Class 8 Chemical effects of electricity long answer questions

Characteristics of a catalyst

1. At a particular temperature, the addition of a catalyst increases the rate of a chemical reaction.
2. A catalyst cannot initiate a reaction. This means that a catalyst cannot “start” a reaction which otherwise does not occur. It only influences the rate of a reaction.
3. A catalyst participates in the reaction it catalyzes but is regenerated at the end of the reaction.
4. A catalyst present in a very small amount is able to influence the rate of a reaction significantly.
5. There is no universal catalyst which can enhance the rate of all the reactions. A suitable catalyst for a specific reaction must be found only by proper experimentation.

How Does A Catalyst Work

For various reactions, the mechanism by which a specific catalyst works is known. But this discussion is beyond the scope of this book.

At this point, we can only say that a catalyst helps a reaction to occur by a different route, which requires relatively less energy.

So more reactants can participate in a reaction, producing more products. Let us consider a situation where a catalyst is in the solid state within a reaction mixture which is in a liquid state (or gaseous state).

Generally in such cases, the solid catalyst provides a surface where at least one of the reactants can “seat” and only those reactants which are “seated” on the solid surface can react with other reactants which are also seated or which are not seated but remain very close to the solid surface.

So we realize that work more the surface area of a solid catalyst, the more molecules it can accommodate on its surface and the more wilt the rate of the reaction.

If a solid catalyst is crushed to powder, the surface area of the solid substances increases substantially and thus more reactants get the opportunity to react.

(The process by which the reactant molecules “sit” on the solid surface is called “adsorption”.) If the physical state of the catalyst is different from that of the reactants, then the catalyst is called a heterogeneous catalyst.

For example, in the production of ammonia from the reaction between nitrogen gas and hydrogen gas, a solid powder of iron is used as a heterogeneous catalyst. For a heterogeneously catalyzed reaction, adsorption always precedes the chemical reaction.

Read And Learn More WBBSE Solutions For Class 8 School Science Long Answer Type Questions

Organic catalysts: Enzymes

Chemical reactions are constantly occurring within our bodies too. Formation or breaking of chemical bonds rarely happens on their own within biological systems.

Enzymes are biological molecules that act as catalysts and help complex reactions occur. The enzymes are basically proteins, but they, ey may be associated with non-protein substances (known as coenzymes or prosthetic groups) that are essential for the action of the enzyme.

The term “enzyme” meaning “from yeast” was coined by German physiologist Wilhelm Kuhne in 1876. Eduard Buchner showed that fermentation, previously believed to depend on a mysterious “life force” contained only in living organisms, could be achieved by extracts from yeast (which is not “living”).

Enzymes are highly specific catalysts in the sense that a particular enzyme acts on only a particular reactant (generally called “substrate”) to complete a specific task.

Experiments have revealed the catalytic activity of an enzyme Some hands-on experiments can be done to observe the effect of an enzyme on a chemical reaction.

Urease	Catalyses hydrolysis of urea \(\begin{aligned} & \left(\mathrm{NH}_2 \mathrm{CONH}_2+\mathrm{H}_2 \mathrm{O} \rightarrow\right. \\ & \left.2 \mathrm{NH}_3+2 \mathrm{CO}_2\right) \end{aligned}\)
Invertase	Converts surcose to glucose and fructose
Pepsin (Found in our Stomach)	Converts proteins into smaller amino acids
Trypsin (Found in our intestine)	Smaller Amino acids
Catalase	decompose Hydrogen peroxide (H2O) to water and oxygen  \(2 \mathrm{H}_2 \mathrm{O}_2 \rightarrow 2 \mathrm{H}_2 \mathrm{O}+\mathrm{O}_2\)

 

Long answer questions on elements and compounds for Class 8

1. Let us mix equal volumes of hydrogen peroxide and water in a test tube and a small piece of fresh potato is added. The enzyme, catalase, is found in the fresh potato.

Almost immediately after the addition, bubbles of oxygen start evolving from the reaction mixture. We have already mentioned that catalase decomposes hydrogen peroxide into oxygen and water. The reaction can be repeated with fish or goat liver, which also contains the enzyme catalase.

Enzymes are indispensable for living things because all the biological reactions occurring within living things are catalyzed by enzymes.

Starting from digestion of food to synthesis is due to a relatively small region of the protein molecule present in the enzyme. This region is commonly referred to as an “active centre

2. Let us dissolve some urea in a small volume of water taken in a beaker and to it some amount of powdered ahar daal is added.

After 10 – 15 minutes, if we smell the reaction mixture, a faint pungent smell of ammonia is obtained. Since ammonia is a weak base, so if we add a few drops of phenolphthalein, the reaction mixture turns pink. Arhar daal and watermelon seeds contain the enzyme urease.

Urease catalyses the hydrolysis of urea. Microbes in urinals decompose urea present in urine and that is why we get the smell of ammonia in the urinal.

proteins, hormones, DNA etc., and enzymes are everywhere. Each enzyme performs only a particular action on a particular substrate. Enzymes are indispensable for living things because all the biological reactions occurring within living things are catalyzed by enzymes.

Starting from digestion of food to synthesis is due to a relatively small region of the protein molecule present in the enzyme.

This region is proteins, hormones, DNA etc., enzymes are everywhere. Each enzyme performs only a particular action on a particular substrate.

WBBSE Solutions For Class 8 School Science Long Answer Type Questions	WBBSE Solutions For Class 8 School Science Short Answer Type Questions
WBBSE Solutions For Class 8 School Science Very Short Answer Type Questions	WBBSE Solutions For Class 8 School Science Review Questions
WBBSE Solutions For Class 8 School Science Solved Numerical Problems	WBBSE Solutions For Class 8 School Science Experiments Questions
WBBSE Solutions For Class 8 Maths	WBBSE Class 8 History Notes
WBBSE Class 8 History Multiple Choice Questions	WBBSE Solutions For Class 8 History
WBBSE Solutions For Class 8 Geography

 

Question 3. What is an exothermic and endothermic reaction? Give examples.
Answer:

Exothermic And Endothermic Reaction:

Chemical reactions which proceed with the evolution of heat energy are called exothermic reactions. Reactants → Products + heat (Q) Burning of coal is an example of an exothermic reaction. Here, the carbon present in coal burns in oxygen to produce carbon dioxide with the evolution of large amounts of heat.

⇒ \(\mathrm{C}+\mathrm{O}_2 \rightarrow \mathrm{CO}_2+\text { heat (Q) }\)

Chemical reactions which proceed with the absorption of heat energy are called endothermic reactions.
Reactants + heat (Q) →Products Making quicklime from limestone is an example of an endothermic reaction, where heat is absorbed.

⇒ \(\mathrm{CaCO}_3 \rightarrow \mathrm{CaO}+\mathrm{CO}_2-\text { heat (Q) }\)

Question 4. Give one example of each of the exothermic physical processes and endothermic physical processes.
Answer:

When ammonium chloride (NH₄CI) is dissolved in water, then heat is absorbed and the solution becomes cold. This is an example of an endothermic physical process.

When sulphuric acid ( H₂SO₄) is slowly added dropwise to water, it becomes soluble and a large amount of heat is liberated and the solution becomes very hot. This is an example of an exothermic physical process.

Question 5. Describe a simple experiment to show that water and oxygen, both are necessary for the rusting of iron.
Answer:

Rusting:

Rusting of iron is a common form of corrosion in which the metal is eaten up gradually due to oxidation of the metal by the action of air, moisture or a chemical (such as acid) on its surface.

When metallic iron is exposed to moist air (i.e., in presence of oxygen and water), it is converted to hydrated iron oxide (Fe₂O₃.n H₂O) [where n is the number of water molecules].

This is brittle and the mechanical strength of metallic iron is absent in this hydrated iron oxide. In other words, metal is “degraded”.

This is an example of an oxidation reaction, where metallic iron is oxidized to hydrated iron oxide.

⇒ \(4 \mathrm{Fe}+3 \mathrm{O}_2+2 \mathrm{nH}_2 \mathrm{O} \rightarrow 2 \mathrm{Fe}_2 \mathrm{O}_3 \cdot \mathrm{nH}_2 \mathrm{O}\)

Iron oxygen water hydrated iron oxide or rust.

The number of water molecules (n) in the rust varies, it is not fixed. Rusting involves unwanted oxidation of iron which occurs in nature on its own. It is a continuous process.

This single class of reaction is responsible for the destruction of various materials, instruments and infrastructures made of iron.

Everything made of iron and which is exposed to moist air is vulnerable to corrosion. Every year, crores of rupees are required for round-the-clock maintenance of costly instruments and infrastructures.

Corrosion control can be achieved by recognizing and understanding the corrosion mechanism, by using corrosion-resistant materials and designs and by using protective systems and devices and treatments.

For example, it is found that the more the exposed area of metallic iron is too moist air, the more the extent of rusting. Hence, if the exposed surface of iron is coated with some paints (such as coal tar or some synthetic paints) which prevent direct contact between iron and moist air, the rate of corrosion can be significantly reduced.

Probably this is the cheapest way to prevent corrosion Another more sophisticated way to prevent rusting is to coat an iron surface with another metal.

For example, metallic zinc can be electroplated on the iron surface to prevent rusting. The process of coating the surface of any substance with metallic zinc is known as galvanization.

We have mentioned that during rusting metallic iron is converted to hydrated iron oxide. Alternatively, we can say that Fe is converted to Fe²⁺ ion.

But we also know that oxidation and reduction occur simultaneously. So, if Fe-2e → Fe²⁺ is the oxidation reaction, which one is the reduction reaction? In fact, in an acidic solution, there can occur two types of reduction reactions, as follows:

⇒ \(\begin{aligned}
& 2 \mathrm{H}^{+} \text {(aqueous) }+2 \mathrm{e} \rightarrow \mathrm{H}_2 \text { (gas) } \\
& 4 \mathrm{H}^{+} \text {(aqueous) }+\mathrm{O}_2 \text { (gas) }+4 \mathrm{e} \rightarrow 2 \mathrm{H}_2 \mathrm{O} \\
& \text { (liquid) }
\end{aligned}\)

A simple experiment can be performed to show that water and oxygen, both are necessary for the rusting of iron.

Experiment: Let us take three beakers. In the first beaker, some iron nails are placed and it is left open in the air for a few days.

In the second beaker, some normal water is taken. A few iron nails are immersed in it, and then some molten wax is poured into the beaker in such a way, so as to create a layer of wax on the surface of the water, which prevents the passage of air through it into the water.

In the third beaker, instead of normal water some amount of boiled water is taken and the iron nails are immersed in it. In this case, also, the surface of the water is covered with a layer of wax,

so that water does not come in direct contact with the air outside. The second and third beakers are also left undisturbed for a few days.

Observation: After some days, it will be found that the nails in the first beaker are rusted. Some rusting takes place in the nails kept in the second beaker. But no rusting takes place in the nails kept in the third beaker.

Inference: This is because the nails are in direct contact with moisture and oxygen present in the air in the first beaker.

So rusting occurs. In the second beaker, nails are in direct contact with water and oxygen (which remains dissolved in water under ordinary temperature and pressure).

So here also rusting of the nails occurs. But in the third beaker, boiled water is taken. When water is properly boiled, the dissolved oxygen is driven out.

So, in absence of any oxygen rusting does not occur, although the nails are in direct contact with water.

This conclusively proves that for rusting to occur, the presence of both water and oxygen is necessary.

In-depth explanations of chemical reactions and electricity for Class 8

Question 6. What is oxidation? Give a suitable example.
Answer:

Oxidation:

Oxidation is a chemical reaction, which involves the addition of oxygen or addition of chlorine or the elimination of hydrogen.

1. Addition of oxygen: Burning of sulphur in air or oxygen produces sulphur dioxide. Here, oxygen combines with sulphur.

So, it is an example of an oxidation reaction. \(\mathrm{S}+\mathrm{O}_2 \rightarrow \mathrm{SO}_2\)

2. Addition of chlorine: Chlorine combines with iron to produce ferric chloride. So this is an oxidation reaction.

3. Elimination of hydrogen: When hydrogen sulphide ( H₂S) gas is passed through chlorine water, some amount of sulphur is precipitated and hydrogen chloride (HCI) is produced. Here hydrogen is eliminated from hydrogen sulphide. So this is an example of oxidation.

Question 7. What is reduction? Give a suitable example. Reduction is a chemical reaction which involves, the elimination of oxygen or elimination of chlorine or the addition of hydrogen.
Answer:

1. Elimination of oxygen:

When hydrogen gas is passed over hot, black, cupric oxide, reddish-brown metallic copper is formed as residue. Here, oxygen is eliminated from CuO. This process is a reduction.

 

 

2. Elimination of chlorine: When HCI gas is irradiated with light, it is dissociated into hydrogen and chlorine gas. Here, chlorine is eliminated from HCI. So it is an example of reduction.

 

3. Addition of hydrogen: When hydrogen sulphide ( H₂S) gas is passed through chlorine waterborne amount of sulphur is precipitated and hydrogen chloride (HCI) is produced. So hydrogen is eliminated from H₂S, so reduction has occurred in this case.

 

WBBSE Class 8 Science practice long answer questions on elements and compounds

Question 8. What is the electronic theory of oxidation and reduction? Explain with suitable examples.
Answer:

Electronic theory of oxidation and reduction:

Oxidation is defined, as, a chemical reaction involving the loss of electrons, from an atom or ion and reduction, is a chemical reaction involving the gain of electrons by an atom or ion.

The substance which accepts electron(s) is reduced. It is called the oxidising agent or oxidant. The substance that loses an electron(s) is itself oxidised. That is known as a reducing agent or reductant.

Let us consider the following chemical reaction:

⇒ \(\mathrm{CuSO}_4+\mathrm{Fe} \rightarrow \mathrm{FeSO}_4+\mathrm{Cu}\)

In copper sulphate, copper exists as a cupric ion (Cu²⁺) whereas, in ferrous sulphate, iron is present as a ferrous ion (Fe²⁺). Hence, the above reaction can be alternatively represented as follows:

⇒ \(\mathrm{Cu}^{2+}+\mathrm{Fe} \rightarrow \mathrm{Cu}+\mathrm{Fe}^{2+}\)

Here, uncharged metallic iron (Fe) loses two electrons and becomes positively charged Fe²⁺ and these two electrons are gained by positively charged Cu²⁺ and are converted into uncharged metallic copper (Cu).

⇒ \(\mathrm{Fe}-2 \mathrm{e} \rightarrow \mathrm{Fe}^{2+}\)

⇒ \(\mathrm{Cu}^{2+}+2 \mathrm{e} \rightarrow \mathrm{Cu}\)

So, as per the electronic theory, Fe is oxidized and Cu²⁺ is reduced.

⇒ \(\mathrm{Fe}-2 \mathrm{e} \rightarrow \mathrm{Fe}^{2+}\) Loss of two electrons by Fe atom and formation of Fe²⁺ This is an oxidation reaction and Fe is the reducing agent.

⇒ \(\mathrm{Cu}^{2+}+2 \mathrm{e} \rightarrow \mathrm{Cu}\) Gain of two electrons by Cu²⁺ atom and formation of Cu.

This is a reduction reaction and From the above discussion we find that an oxidising agent (or oxidant) accepts electrons) and is reduced (i.e. converted to reduced species) while a reducing agent (or reductant) loses an electron(s) and is oxidised (i.e., converted to oxidised species)

 

Question 9. Show that oxidation and reduction take place simultaneously.
Answer:

Let us consider the following reaction.

We know that in the case of reduction removal of oxygen takes place and in the case of oxidation addition of oxygen takes place.

Here, oxygen is removed from cupric oxide, the reduction has occurred and oxygen is added to hydrogen to form water, so oxidation has occurred. It is also clear that both oxidation and reduction have occurred simultaneously.

Understanding chemical effects of electricity long answers for Class 8

Question 10. Using the electronic theory of oxidation and reduction, show with a suitable example that oxidation and reduction take place simultaneously.
Answer:

When some pieces of metallic zinc are dropped in an aqueous solution of copper sulphate, then after some time it is found that reddish-brown patches of metallic copper are deposited on the silver or grey-coloured pieces of zinc.

Further analysis confirms the presence of Zn²⁺ ions in the aqueous solution. The overall chemical reaction is,  Alternatively, we can write, \(\mathrm{CuSO}_4+\mathrm{Zn} \rightarrow \mathrm{ZnSO}_4+\mathrm{Cu}\) Here, Zn has lost two electrons, so it is oxidised and Cu²⁺ has gained two electrons, and it is reduced.

So, we find that in this chemical reaction, both oxidation and reduction have occurred simultaneously.

 

 

Question 11. For the following reactions, predict with an explanation, which substance has been oxidised and which substance has been reduced.

⇒ \(2 \mathrm{KI}+\mathrm{Cl}_2 \rightarrow 2 \mathrm{KCl}+\mathrm{I}_2\)

⇒ \(\mathrm{Fe}_2 \mathrm{O}_3+3 \mathrm{CO} \rightarrow 2 \mathrm{Fe}+3 \mathrm{CO}_2\)

⇒\(\mathrm{ZnO}+\mathrm{C} \rightarrow \mathrm{Zn}+\mathrm{CO}\)

⇒ \(2 \mathrm{SO}_2+\mathrm{O}_2 \rightarrow 2 \mathrm{SO}_3\)

⇒ \(\mathrm{FeO}+\mathrm{H}_2 \rightarrow \mathrm{Fe}+\mathrm{H}_2 \mathrm{O}\)

⇒ \(\mathrm{C}_2 \mathrm{H}_4+\mathrm{H}_2 \rightarrow \mathrm{C}_2 \mathrm{H}_6\)

⇒ \(2 \mathrm{FeCl}_3+\mathrm{H}_2 \mathrm{~S} \rightarrow 2 \mathrm{FeCl}_2+2 \mathrm{HCl}+\mathrm{S}\)

Answer:

Examples of long answer questions on chemical effects of electricity for Class 8

Question 12. For the following reactions, predict which substance has been oxidised and which substance has been reduced. Also, identify the oxidant and reductant.

⇒ \(\mathrm{H}_2 \mathrm{~S}+2 \mathrm{FeCl}_3 \rightarrow 2 \mathrm{HCl}+\mathrm{S}+2 \mathrm{FeCl}_2\)

⇒ \(2 \mathrm{Na}+\mathrm{H}_2 \rightarrow 2 \mathrm{NaH}\)

⇒ \(\mathrm{Zn}+\mathrm{CuSO}_4 \rightarrow \mathrm{ZnSO}_4+\mathrm{Cu}\)

⇒ \(\mathrm{CH}_4+2 \mathrm{O}_2 \rightarrow \mathrm{CO}_2+2 \mathrm{H}_2 \mathrm{O}\)

⇒ \(\mathrm{FeO}+\mathrm{CO} \rightarrow \mathrm{Fe}+\mathrm{CO}_2\)

Answer:

 

Question 13. Prove by an example that oxidation and reduction are two complementary processes of electron loss and electron gain in a reaction.
Answer:

In an oxidation reaction, electrons are given up by an atom or ion and are thereby oxidised. These electrons can not escape out of the reaction medium or accumulate in it, but they should be consumed up or gained by some other atoms or ions that remain in the reaction medium. These latter atoms or ions will thereby be reduced.

An atom of magnesium is changed into an Mg2+ ion by the loss of two electrons: Mg- 2e →Mg²⁺. Again, an atom of oxygen is changed into an oxide ion (O_2-) by the gain of 2 electrons: O + 2e→O₂^–. So in this case, the magnesium atom is oxidised and the oxygen atom is reduced.

As a result, the first process is oxidation and the second one is reduction.

WBBSE Chapter 2 exercises long answer solutions on chemical effects

Question 14. Water is added to a test tube containing dry baking soda and dry crystals of tartaric acid. Again kerosene oil is added to another similar test tube containing an equal quantity of each baking soda and dry crystals of tartaric acid. What would be your observations in these two cases and why?
Answer:

We know that “like dissolves like”. Baking soda is not soluble in organic solvents like kerosene. The second test tube will show no reaction.

When water is added to the first test tube, an aqueous solution of baking soda and an aqueous solution of tartaric acid react together to evolve bubbles of carbon dioxide.

Since baking soda is soluble in water, hence the aqueous solution of the salt allows sufficient contact with the aqueous solution of another reactant for the chemical reaction to occur.

The presence of water as a solvent produces ions and these ions react between themselves to make the chemical reaction happen.

Chapter 2 Element Compound And Chemical Reaction Structure Of Matter Long Answer Type Questions

Question 1. What do you mean by plasma?
Answer:

Plasma:

Plasma is one of the fourth fundamental states of matter. Plasma may be produced by heating a gas to an extremely high temperature.

Due to the heating of gas at very high temperatures, vigorous collisions between atoms and molecules take place as a result of which electrons are ripped off yielding electrons and ions.

So, plasma is an electrically conducting medium. Like the gaseous state, plasma does not have a definite shape or a definite volume.

Plasma is the most abundant form of matter in the universe, most of which is present in intergalactic regions and in stars including the sun.

Question 2. Indicate two similarities and two dissimilarities between isotopes.
Answer:

Two similarities between the isotopes:

1. The number of protons in the nucleus of isotopes is the same.
2. The number of extranuclear electrons in the isotopes is also the same.
3. Two dissimilarities between the isotopes:
4. The number of neutrons in the nucleus of isotopes is different.
5. Some of the physical properties of isotopes are different.

WBBSE Class 8 Structure of Matter long answer questions

Question 3. Compare solid, liquid, and gas in connection with molecular motion.
Answer:

Comparing solid, liquid, and gas in connection with molecular motion

In a gas, the average distance between the molecules is quite large compared to liquids and solids and the molecules move at very large speeds and move randomly.

During this random, chaotic movement, they collide with each other and also with the walls of the container in which they are kept. Apart from moving from one place to another (i.e., translational motion) they can rotate and vibrate.

Compared to a gaseous state, in the liquid state, the molecules are closer to one another. The force of attraction between the molecules is greater compared to that in a gaseous state. The liquid molecules can still move but their movements are somewhat restricted.

They can rotate, vibrate and move over a small distance. The attraction between the atoms or molecules in solids is very high compared to liquids and gases. As a result, the position of atoms or molecules remains fixed with respect to one another.

So atoms or molecules remain in a relatively ordered state. The atoms or molecules within a solid have no mobility (i.e., cannot move from one place to another) and cannot rotate. Each atom or molecule can only vibrate about its mean position.

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Question 4. Identify isotope, isobar, and isotone in the following atoms: \({ }_6^{13} \mathrm{C},{ }_{17}^{37} \mathrm{Cl},{ }_{20}^{40} \mathrm{Ca},{ }_6^{14} \mathrm{C},{ }_7^{14} \mathrm{~N}\)
^Answer:

¹³₆Ca and ³⁷₁₇ Cl are isotopes {having the same atomic number (6) but a different mass number (13 and 14 respectively)}.
44 C and are isobar {having the same mass
number (14) but different atomic numbers (6 and 7 respectively)}.

⁴⁰₂₀Ca and ³⁷₁₇Cl are isotones {having a different atomic number (20 and 17. respectively), and different mass numbers (40 and 37, respectively) but the same number of neutrons (20)}.

Question 5. Write the differences between isotope and isobar.
Answer:

The Differences between isotope and isobar:

	Isotope		Isobar
1.	Isotopes have the same atomic number but different mass numbers.	1.	Isobars have the same mass number but a different atomic number.
2.	The chemical properties of isotopes are mostly similar.	2.	The chemical properties of isobars are different.

 

Long answer questions on elements and compounds for Class 8

Question 6. What do you mean by radical? Give two examples.
Answer:

Radical:

Sometimes, a group of atoms consisting of different elements collectively behave as a single entity during a chemical reaction.

Such a group of atoms is known as a radical. Since a radical behaves like a single entity during a chemical reaction, it has a definite valency. For example ammonium ion (NH4-) and nitrate ion (NO3)-

Question 7. Write the formula of the following: Aluminium Sodium sulfide, Cuprous chloride, Ferric fluoride, Potassium chloride, Calcium oxide, oxide, Zinc oxide.
Answer:

Question 8. Write the formula of the following: Mercuric Aluminium sulfate, Ferric sulfate, oxide, Stannous chloride, Mercurous chloride, Magnesium carbonate, and Cupric nitrate.
Answer:

Question 9. Write the formula of the following: Calcium sulfate, Calcium sulfite, Aluminium bicarbonate, Sodium hydroxide, Ammonium bicarbonate, and Ferrous nitrate.
Answer:

WBBSE Chapter 2 structure of matter detailed answers

Question 10. Mention the characteristics of an ionic compound.
Answer:

The general characteristics of an ionic compound can be summarized as follows :

1. In the case of ionic compounds, molecules do not exist. In solid-state, cations and anions are properly arranged in a definite pattern.
2. In ionic compounds, the total charges on cations will be the same as the total charges on anions.
3. Ionic compounds are generally soluble in water.
4. An aqueous solution of ionic compounds which are soluble in water conducts electricity.
5. The melting point and boiling point of ionic compounds are totally high.

Question 11. Mention the cation(s), Marion(s), formula, and Requirement for maintaining charge neutrality and calculation for a net charge for the following compounds: sodium chloride, potassium fluoride, zinc oxide, calcium oxide, calcium chloride, aluminum oxide, sodium sulfide
Answer:

 

Question 12. Mention the cation(s), anion(s), formula, and Requirement for maintaining charge neutrality and calculation for a net charge for the following compounds: ferrous chloride, ferric chloride, cuprous oxide, cupric oxide, mercurous chloride, mercuric chloride, ferrous oxide, ferric oxide, stannous chloride, stannic chloride.
Answer:

In-depth explanations of the structure of matter for Class 8

Question 13. Mention the cation(s), anion(s), formula, and Requirement for maintaining charge neutrality and calculation for a net charge for the following compounds: ferrous sulfate, aluminum sulfate, calcium bicarbonate, calcium phosphate, ammonium sulfate, cupric nitrate, magnesium carbonate, sodium hydroxide, aluminum nitrate, calcium sulfite, ammonium nitrate, ferric sulfate, magnesium hydroxide, potassium carbonate.
Answer: 

 

Question 14. Mention the characteristics of covalent compounds.

Answer:

The characteristics of covalent compounds are the following:

1. In covalent compounds, molecules exist.
2. The melting point and boiling point of covalent compounds are usually low.
3. Covalent compounds are generally soluble in organic solvents and usually insoluble in water.
4. These compounds usually exist as gas or liquid under normal temperatures and pressure.

Question 15. The atomic number of an element (Z) is 12 more than that of nitrogen. If a mass number of the atom is 39, find out the number of protons, neutrons, and electrons in the atom. Identify the element Z.
Answer:

Given:

The atomic number of an element (Z) is 12 more than that of nitrogen.

If a mass number of the atom is 39.

The atomic number of nitrogen = 7.

∴ An atomic number of element 2=7 + 12 = 19. The mass number of element Z = 39.

∴ Number of neutrons = 39-19 = 20. Hence, an atom of Z has 19 protons, 20 neutrons, and 19 extra nuclear electrons.

Z is potassium having atomic number 19 and mass number 39.

WBBSE Class 8 Science practice long answer questions on elements and compounds

WBBSE Solutions For Class 8 School Science Long Answer Type Questions	WBBSE Solutions For Class 8 School Science Short Answer Type Questions
WBBSE Solutions For Class 8 School Science Very Short Answer Type Questions	WBBSE Solutions For Class 8 School Science Review Questions
WBBSE Solutions For Class 8 School Science Solved Numerical Problems	WBBSE Solutions For Class 8 School Science Experiments Questions
WBBSE Solutions For Class 8 Maths	WBBSE Class 8 History Notes
WBBSE Class 8 History Multiple Choice Questions	WBBSE Solutions For Class 8 History
WBBSE Solutions For Class 8 Geography

 

Question 16. The electronic configuration of an atom of an element (Symbol M) is [ K(2), L(8), M(8), N(2)]. The number of neutrons is 20. Find out the atomic number, mass number, and valency of the element. What is the formula of the oxide of M?
Answer:

Given:

The electronic configuration of an atom of an element (Symbol M) is [ K(2), L(8), M(8), N(2)].

The number of neutrons is 20.

Total number of electrons = 2 + 8 + 8 + 2 = 20.

∴Atomic number, Z = 20

Mass number, A = 20 + 20 = 40.

Since the valence shell (N shell) contains 2 electrons, hence valency of the element (M) shall be +2.
The formula of oxide:

 

Examples of long answer questions on structure of matter for Class 8

Question 17. The formula of an oxide of a metal M is M203. Write down the formula of its chloride and nitrate.
The valencies of two elements A and B are 3 and 5 respectively. If B is solid among them what will be the formula of the compound formed when A reacts with B?
Answer:

Given:

The formula of an oxide of a metal M is M203.

The valencies of two elements A and B are 3 and 5 respectively.

From the formula of oxide (M₂O₃) it is clear that the valency of metal M is 3.

∴ Metal chloride = MCI₃ and Metal nitrate = M(NO₃)₂ Formula of compound

 

Chapter 1 Physical Environment Long Answer Questions

Question 1. What are the differences between a real image and a virtual image?
Answer:

Differences Between a real image and a virtual image

Real image		Virtual image
1	Real images are formed when reflected rays or refracted rays actually meet at the same points.	1	Virtual images are formed when the reflected or the refracted rays appear to diverge from other points.
2	It can be cast on a screen.	2	It cannot be cast on a screen.
3	By single reflection or refraction, it is always inverted.	3	It is always erect.
4	It may be magnified or diminished or equal to the size of an object.	4	Images formed by a plane mirror are equal to the size of the object. Those formed by lenses or curved mirrors may be equal, larger or smaller in size in comparison to the object.

 

Question 2. Describe the formation of a real image with the help of a magnifying glass. What is the angle, in degrees, between a plane mirror and the straight line joining the object and its?
Answer:

Formation Of A Real Image With The Help Of A Magnifying Glass:-

Let us take a magnifying glass and hold it in the sunlight above a piece of white paper, placed on the floor. We can see a round shape of light on the paper.

It is nothing but the image of the sun, which can be cast on the paper, which is the “screen” here and the image is a real image of the sun because,

after refraction, the rays of light coming from the sun actually meet at some point. The formation of a real image of magnifying glass has been The angle is 90°.

WBBSE Class 8 Light long answer questions

Question 3. What are the characteristics of images formed by a plane mirror? If a diverging beam of light is incident on a plane mirror, is it reflected as a converging, parallel, or diverging beam?
Answer:

Characteristics Of Images Formed By A Plane Mirror

Formation of multiple images by plane mirrors

Let us now take two plane mirrors and place them vertically; on a white sheet of paper in such a way that the angle between them is 90°,

If we now place an object (say an eraser) between the mirrors, we can see multiple images. We can see three images when the angle between the two mirrors is 90°.

If the angle between the mirrors is 45°, then the number of images formed is 7. So, to generalize our observation,

we can say that if the angle between the two plane mirrors is x°, then the number of images formed will be equal to (360°/x°-1), if 360°/x° is an even integer and (360°/x°),360°/x° is an odd integer

Read And Learn More WBBSE Solutions For Class 8 School Science Long Answer Type Questions

So, when two plane mirrors are placed vertically facing each other and if an object is placed between them then a large number of images are formed.

Diverging beam

Long answer type questions on light for Class 8

Question 4. Describe the construction of the periscope. Mention one disadvantage of periscope.
Answer:

Construction Of The Periscope:-

It is a simple long, tubular instrument with which a viewer can see different objects from the other side of a barrier that extends high above his or her head and are out of the direct line of sight.

It consists of a long rectangular box made of wood or metal. Two plane mirrors M₁ and M₂ (or in some cases two prisms) are fixed inside the box, one at the top and the other at the lower end of the box such that the mirrors face each other.

Each mirror makes a 45° angle with the axis of the periscope box. Rays of light coming from a distant object are incident on the mirror Mr. The rays get reflected by M₁ and are incident on mirror M₂.

The mirror M₂ then reflects the reflected rays of light towards the eyes of the observer. The observer thus sees any object from the other side of a high barrier.

Soldiers use it to observe the movements of enemies keeping themselves hidden in trenches. In submarines, a periscope is used to watch the movements of the enemy vessel on the surface of the water, while remaining submerged in the water.

Sports lovers, unable to get entry into the galleries of a playground, take the help of a periscope to watch games from outside the barriers of the playground.

The disadvantage of periscope: The final image is not bright due to successive reflections in a periscope.

Question 5. What are the differences between simple reflection and total internal reflection?
Answer:

Differences Between Simple Reflection And Total Internal Reflection

	Simple Reflection		Total Internal Reflection
1	Simple reflection occurs when light from any medium is incident on a reflector.	1	In the case of total internal reflection, the ray of light must pass from an optically denser medium to an optically rarer medium.
2	Reflection occurs for any value of the angle of incidence.	2	In this case, the angle of incidence in the optically denser medium must be greater than the characteristic critical angle of the two media.
3	In the case of simple reflection, a part of the light is absorbed by the reflector, another part is refracted and the remaining is reflected.	3	In this case, the incident light totally reflects back from the surface of separation of the two media and no part of the incident light is either absorbed by the medium or refracted.
4	For simple reflection, a reflector is required.	4	In the case of total internal reflection, the surface of separation between the two media itself acts as a reflector.

 

WBBSE Chapter 1 light detailed answers

Question 6. Explain with a suitable the formation of mirages in the desert during day time.
Answer:

Total Internal Reflection

When a ray of light tends to travel from an optically denser medium to an optically rarer medium, then if the angle of incidence (∠DON1 exceeds the critical angle (°C), the incident light totally reflects back along OR to the first medium (optically denser medium)

Refraction of light does not take place in this case. Such a phenomenon is called total internal reflection.

The term “total” is used because of the incident media. The points at which total internal light totally reflects back into the same denser reflection takes place look very bright,

as the medium from the surface of separation of the two incident lights reflects totally from these points.

The conditions required for the total internal reflection to take place are :

1. light rays should travel from the denser to the rarer medium.
2. The angle of incidence should be greater than the critical angle for the pair of media involved
3. Phenomena related to total internal reflection

1. Brightness of diamonds or gems

Usually, diamonds and other gems are constituted of materials of high refractive index, the critical angle of each of which with respect to air is thus very small.

For example, the critical angle of a diamond is only 24.5°. Also, diamonds or any other gem is cut in such a way that, light can get into it through all surfaces but can emerge from very few surfaces.

This is because, the diamond or the gemstone is cut in such a way, that the rays within the “body” trying to come out are incident on most of the surfaces at an angle exceeding the critical angle.

After undergoing several total internal reflections, the light rays are incident on a small number of surfaces at angles less than the critical angle and emerge from those surfaces only.

Hence, the emergent light is very intense, and that is why a diamond or a gem looks very bright.

 

2. A crack in the glass of a window pane looks shiny

Some air is present in the gap of a crack in the glass. So, light rays pass through the denser medium (i.e. glass)

when tend to pass through the rarer medium (i.e. air), and total internal reflection occurs at some point in the crack. Hence those points of the crack look shiny.

3. An empty test tube dipped in the water looks shiny

An empty test tube is dipped in water in an inclined way. Light rays passing through water outside the tube tend to pass through air present in the empty test tube.

Thus light passes from an optically denser medium to an optically rarer medium. At some points on the surface of the test tube, rays of light are incident at angles exceeding the critical angle of water to air.

At those points total internal reflection takes place and so the empty portion of the test tube looks bright when viewed from above vertically.

 

 

4. Drop Of Water On the Arum Leaf Seems Glittering

This is because when a ray of light travels from inside the water droplet to the air, the angle of incidence exceeds the critical angle of the two media (i.e. water and air).

So, total internal reflection occurs at the surface of the separation between water and air. When the emerging ray of light reaches to viewer’s eye, the viewer finds the area glittering.

5. Mirage in the desert

In deserts, during day time, the sand bed becomes extremely hot. So the air just above it is also heated and the density of air decreases.

With increasing altitude, the successive layers of the air have gradually increasing density. In absence of any flow of air, this is maintained for a long time.

Let us consider a light ray coming from point “A” on top of a tree in the desert moving downwards

As the density of air decreases downwards, and as the ray of light moves downwards through different layers (of decreasing density), the angle of refraction increases progressively.

At some interface between two layers of air, the angle of incidence is greater than the critical angle, and it suffers total internal reflection and consequently moves upwards.

As it moves upwards from a rarer medium to a denser medium, the ray of light bends towards the normal. When the ray reaches an observer, he or she “secs” a virtual image of A at A7.

In this way, rays coming from different parts of the object (i.e. tree) reach to viewer’s eye after suffering total internal reflection.

Ultimately the viewer sees an inverted, virtual image of the original object in a direction far away from the original position of the object.

Due to variations in temperature, the density of different layers of air changes continuously, and to an observer, the image seems shimmering.

The observer thinks this inverted, shimmering image of the tree is the reflection of the tree formed on the water surface below the tree and the viewer is totally misguided. This optical illusion is called a mirage.

6. Mirage in the cold country

In colder countries, the air in contact with water is denser and with increasing altitude, the density of air decreases.

For our convenience, we can think of different layers of air of decreasing density with increasing altitude.

The rays of light from a boat far away from the jetty, when going in an upward direction, it travels from a denser to a rarer medium.

In each layer, the refracted ray moves away progressively from the normal and the angle of incidence increases gradually.

Ultimately, a point is reached when the angle of incidence becomes greater than the critical angle of the two adjacent layers of air and the incident ray suffers total internal reflection at that particular interface

(or surface of separation) and bends downwards. When it reaches the viewer’s eye, the viewer sees a virtual image of the boat, which is inverted, moving in the sky.

In-depth explanations of light concepts for Class 8

Question 7. What do you mean by looming?
Answer:

Looming:-

The velocity of light in a vacuum is 3×1010 cm/s. The refractive index of glass with respect to air is 1.5. What is the velocity of light in a glass medium?

Looming is an optical phenomenon observed in cold countries due to total internal reflection, wherein objects like ships, which are normally below the horizon appear to be hanging in air.

The refractive index of a glass

= velocity of light in vacuum/velocity of light in the glass
or, 1.5= 3×1010 / velocity of light in the glass

or, 1-5 – velocity of light in the glass

or, the velocity of light in the glass

= (3x 1010)/1.5 = 2 x 1010 cm/s.

Question 8. What is the value of the absolute critical angle of the denser medium as shown in the adjacent ?
Answer:

 

Mention the conditions necessary for total internal reflections to occur.
Answer: Absolute critical angle, 0c = 48°.
Conditions for total internal reflections:

Question 9. When a light ray travels from a rarer to a denser medium, how does the bending of light depend on the refractive index of the material? Why does refraction occur?
What is the angle of incidence if a ray is reflected by a plane mirror back along its original path?
Answer:

The higher the refractive index of the material, the more the light bends as it is refracted. Cause of refraction: This is the case of normal incidence wherein, Zi = Zr = 0°.

Refraction of Light

It is our common experience that when light travels from one medium to the other, it deviates from its original path.

If we dip a pencil obliquely in a beaker containing water and observe it from a particular position as shown in it seems that the pencil is bent at the point of contact between the water surface and air.

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(It is also called the air-water interface). This occurs due to the optical phenomenon known as refraction.

It means that light deviates from its path if one optical medium is changed with another optical medium or the density of the same medium changes because of variations in temperature, pressure, etc.

The path of light remains a straight line path in the second medium but it is inclined at some angle with the original path in the first medium.

The phenomenon, due to which a ray of light deviates from its original path while traveling from one optical medium to another optical medium is called refraction.

1. Refraction of light in glass slab

Let us discuss the phenomenon in some detail. Let an oblique ray of light traveling through air transmit through a glass slab and then come out of the glass slab to air again The ray AO in the first medium is called the incident ray.

If no deviation of path occurs at 0 then the light would have traveled along OA’. But the path of the light deviates at O and light moves through the glass medium along OB.

OB is the refracted ray. Again, at B there is a glass-air interface, and the deviation of the path of the light is observed and in the air medium it is refracted along BC.

BC is called the emergent ray. The angle between the incident ray and normal (i.e. NIST) is the angle of incidence (i.e. ∠AON) and the angle between the refracted ray and the normal at the

2. Finding the refractive index of glass with respect to air

Let us take a white sheet of paper and place it on the surface of a table. Let us now place a glass slab and place it at the middle of the paper and draw its boundary ABCD with a pencil.

Now place two board pins P and Q in an upright position towards the AB side of the glass slab, as

Now looking from the side CD, let us fix two more board pins R and S such that these two pins and the images of pins P and Q as seen through the glass slab are in the same straight line.

The glass slab and the pins are then removed and the position of the pins is marked. Join PQ to meet AB at point O and join SR to meet CD at L.

Draw NM such that it is perpendicular to AB at point

Now with O as the center, let us draw a circle of any radius, intersecting PO at E and OL at G. Draw EF and GH in such a way that EF is perpendicular to NO and GH is perpendicular to MO.

Understanding light concepts long answers for Class 8

So PQ. is the incident ray, 0 is the point of incidence, OL is the refracted ray, ∠EON is the angle of incidence and ∠LOM is the angle of refraction.

The ratio of EF and GH is determined. Now if the angle of incidence is changed, the angle of refraction will also change. But in each case the ratio, EF/GH will remain constant.

[More appropriately it can be said that if the two media are fixed and the color of the light remains unchanged during refraction, then whatever the angle of incidence, the magnitude of EF/GH remains constant.]

The ratio is called the Refractive Index of the second medium (i.e. here it is glass) with respect to the first medium (i.e. here it is air).

When refraction occurs between a vacuum and a certain medium, then the refractive index of the said medium is called Absolute Refractive Index.

The refractive index depends on the nature of the two media and on the color of a ray of light. When a ray of light travels from an optically denser medium to an optically rarer medium, then the magnitude of the refractive index for the light of different colors will be in the order: of red< green < blue < violet.

When a ray of light travels from an optically rarer medium, such as air, to an optically denser medium (say glass), the ray in the denser medium moves towards the normal and the angle of incidence is always greater than the angle of refraction (i.e. Z/ > Zr).

medium, such as glass, to an optically rarer medium (say air), then the ray of light in the rarer medium moves away from the normal and the angle of incidence is always less than the angle of refraction (i.e. Z/ < Zr).

 

 

It should be kept in mind that the density of a medium and its optical density is not the same. For example, oil is optically denser than water, but its natural density (i.e. mass per unit volume) is less than that of water.

Velocity Of Light In Rarer And Denser Medium

A medium in which light moves faster or the velocity of light is higher is optically rarer whereas a medium in which light moves slower is optically denser.

Whether a medium is optically denser or rarer depends upon its absolute refractive index. Light has a constant velocity of 3 x 108 m/s for all colors in a vacuum (or air).

However, if the light travels through any other optical medium, it is slowed down. It is this slowing down of light that is responsible for the phenomenon of refraction.

It has been found experimentally that, Absolute refractive index of a medium Velocity of light in a vacuum “ /Velocity of light in that medium

Laws of Refraction

First Law: The incident ray, the refracted ray, and the normal drawn at the point of incidence on the refracting surface lie on the same plane.

Second Law: For refraction of an obliquely incident ray of light of any given color in a given pair of optical media, the refractive index is constant, irrespective of the magnitudes of the angle of incidence and angle of refraction.

In the beginning, we mentioned that when water is poured into an empty tub, the base of the tub appears to be raised.

This occurs due to refraction. Now we can explain this in a better way. Let us consider a point “0” at the base of the tub.

When water is poured into the tub, the light from “0” travels from water (an optically denser medium) to air (an optically rarer medium).

When the ray of light enters in air, it moves away from the normal drawn surface of separation between the two media.

If the refracted rays are extrapolated linearly backward, they meet at point “O'”, which is positioned higher than “0” So, to a viewer, it seems that the base of the tub has been raised

 

Key long answer questions about light for Class 8

4. Critical Angle

If a ray of light starting from an optically denser medium refracts in a rarer medium, for all oblique incident rays, the angle of refraction is greater than the angle of incidence.

In the adjacent AO is an obliquely incident ray. Its corresponding refracted ray is OA’ and the angle of refraction is ∠NOA’ which is greater than the corresponding angle of incidence, ∠AON’.

If the angle of incidence is gradually increased, the corresponding angle of refraction is also gradually increased.

This continues till for a certain angle of incidence, ∠CON’ (denoted as °C) the corresponding angle of refraction, ∠GON, becomes 90°

That means the refracted ray, OG, grazes along the surface of the separation of the two media. This particular angle of incidence for which the angle of refraction becomes 90° is called the critical angle.

Its value depends on the pair of media as well as the color of the incident light.

Definition: When a ray of light of any given color tends to travel from an optically denser medium to an optically rarer medium, then for a certain angle of incidence the angle of refraction is 90°.

The corresponding angle of incidence is the critical angle of the given pair of media for the given color of light.

It is to be noted that the critical angle for a certain color of light is different in different pairs of media.

The critical angle of glass with respect to air for yellow light is 42°, but that of water with respect to air is 49°. Also, a given pair of media have different critical angles for different colors of light.

 

WBBSE Class 8 Science practice long answer questions on light

Question 10. How many images will be formed when an object is placed between two parallel plane mirrors with their reflecting surfaces facing each other? Why do more distant images appear fainter?
Answer:

Two plane mirrors are mounted at right angles to each other. Let an object be placed in front of them. How many images are formed in all? Draw the ray to illustrate the situation.

Kaleidoscope

This is a kind of funny toy that utilizes the property of formation of multiple images of an object when placed between three or more mirrors.

Suppose, three rectangular pieces of mirrors of the same size are joined together to give it a prism-like appearance

It is joined in such a way that the reflecting surface of each mirror should face inside.

Then one of the open ends is covered by a ground glass of the required size. Some broken, colored glass pieces or colored small objects like beads, etc.

They are placed within it. If this instrument is now aimed at a suitable source of light and is seen from the other open end, we can see some beautiful patterns formed due to multiple image formations by the three mirrors.

If we spin this kaleidoscope slowly, numerous colorful patterns will be created continuously. Formation of images by two mirrors: Ray.