TejaNaga Puram

Chapter 2 Element Compound And Chemical Reaction Nature Of Matter MCQs

Question 1. Carbon monoxide is

1. A basic oxide
2. A neutral oxide
3. An acidic oxide
4. An amphoteric oxide

Answer: 1. A basic oxide

Question 2. Blue vitriol is

1. ZnSO₄.7H₂O
2. CusO₄.5H₂O
3. FesO₄.7H₂O
4. Na₂CO₃. 10 H₂O

Answer: 2. CusO₄.5H₂O

Question 3. Naphthalene and camphor can be identified

1. By their odour
2. By their colour
3. By their taste
4. By their structure

Answer: 1. By their odour

WBBSE Class 8 Nature of Matter review questions

Question 4. Which one of the following is a chemical change?

1. Dissolving common salt in water
2. Rusting
3. Melting of wax
4. Freezing of water

Answer: 2. Rusting

Question 5. One of the characteristic properties of metal is

1. Brittleness
2. Good conductor of heat and electricity
3. Forms acidic oxide
4. dull appearance

Answer: 2. Good conductor of heat and electricity

Question 6. Which one of the following is a non-metal?

1. Tungsten
2. Sulphur
3. Titanium
4. Iron

Answer: 2. Sulphur

Question 7. Which one of the following has magnetic properties?

1. Iron
2. Copper
3. Aluminium
4. Zinc

Answer: 1. Iron

Review questions on elements and compounds for Class 8

Question 8. Which one of the following gases is odourless?

1. Ammonia
2. H₂S
3. Oxygen
4. Ozone

Answer: 3. Oxygen

Question 9. The gas has a smell of rotten eggs is

1. H₂S
2. Nh₃
   
3. Chlorine
4. Ozone

Answer: 1. H₂S

Question 10. When heated, solid iodine

1. Sublimes
2. Melts
3. Boils
4. Ionizes

Answer: 1. Sublimes

Read And Learn More WBBSE Solutions For Class 8 School Science Review Questions

Question 11. The metalloids are

1. Fe, al
2. As, sb
3. Cl, Br
4. Ar, ne

Answer: 2. As, sb

Question 12. The metal which does not produce hydrogen gas when reacts with H₂C is

1. Al
2. Cu
3. Zn
4. Fe

Answer: 2. Cu

Question 13. Which one of the following metals is chemically most inert?

1. Fe
2. Au
3. Cu
4. Na

Answer: 2. Au

Question 14. The substance having a characteristic odour is

1. Silver
2. Water
3. Kerosene
4. None of these

Answer: 3. Kerosene

Question 15. The metal which reacts explosively even with cold water is

1. Sodium
2. Iron
3. Calcium
4. Aluminium

Answer: 1. Sodium

Question 16. Water and glycerin can be distinguished

1. By touching
2. By their odour
3. By their colour
4. Their magnetic properties

Answer: 1. By touching

Question 17. The substance which does not sublime is

1. Naci
2. Nh cl
3. Iodine
4. Camphor

Answer: 1. Naci

Question 18. The order of melting point for the following metals is

1. Fe>au>ai
2. Fe>ai>au
3. Au<fe<ai
4. Al > au > fe

Answer: 1. Fe>au>ai

Nature of matter concepts summary and review questions for Class 8

Question 19. If a very thin sheet of metal is required in an experiment, then which of the following is to be taken?

1. Zn
2. Cu
3. Fe
4. Au

Answer: 4. Au

Question 20. An example of a metal which can be easily cut by a sharp knife is

1. Sodium
2. Iron
3. Titanium
4. Copper

Answer: 1. Sodium

Question 21. Which one is a neutral oxide?

1. Calcium oxide
2. Carbon dioxide
3. Zinc oxide
4. Carbon monoxide

Answer: 4. Carbon monoxide

Question 22. Four test tubes contain four different types of salt solutions. Strips of zinc metal are added to each solution in the four test tubes. In which cases there will be a visible colour change occurring in the test tubes?

1. I & ii
2. I& IV
3. II & III
4. III & IV

Answer: 4. III & IV

WBBSE Class 8 Science practice review questions on elements and compounds

Question 23. An element e reacts with water to form a solution which turns the phenolphthalein solution pink. The element e is most likely to be

1. S
2. Ca
3. C
4. Ag

Answer: 4. Ag

Question 24. Maximum intermolecular forces of attraction exist in

1. Bromine
2. Air
3. Mercury
4. Copper

Answer: 4. Copper

Question 25. Which among the following pairs possesses low melting points?

1. Magnesium, mercury
2. Iron, copper
3. Sodium, potassium
4. Calcium, manganese

Answer: 3. Sodium, potassium

Chapter 2 Element Compound And Chemical Reaction Fill In The Blanks

Question 1. _________ helps in the transport of oxygen Within Muscles.
Answer: Myoglobin

Question 2. Lack of _________ causes diabetes.
Answer: Insulin

Question 3. Concentrated sulphuric acid is slowly added to water. This is an example of _________ change.
Answer: Exothermic

Question 4. Ammonium chloride is dissolved in water. This is an example of_________ change.
Answer: Endothermic

Question 5. _________ is a non-metal but good conductor of electricity.
Answer: Graphite

Question 6. In general, metals are _________ conductors of heat and electricity.
Answer: Good

Question 7. The colour of nitrogen dioxide is _________.
Answer: Brown

Question 8. _________ is a metal which has a low melting point.
Answer: Gallium

Question 9. Rusting occurs due to a reaction between iron and _________ air.
Answer: Moist

Question 10. Oxygen is transported through the blood by _________
Answer: Haemoglobin

Question 11. If solid naphthalene is heated it _________.
Answer: Sublimes

Examples of nature of matter review questions for Class 8

Question 12. _________ is a metal which is liquid at room temperature.
Answer: Mercury

Question 13. _________ is used for preparing pencils.
Answer: Graphite

Question 14. _________ is a metalloid.
Answer: Arsenic

Question 15._________ ion helps in the conduction of nerve impulses from one nerve cell to another.
Answer: Ca2⁺

Question 16. When a piece of zinc is added to dilute Sulphuric acid, _________ gas is produced.
Answer: Hydrogen

Question 17. Carbon dioxide is _________ oxide
Answer: Acidic

Question 18. Magneslurrr oxide is _________ oxide.
Answer: Basic

Question 19. When sulphur is burnt in oxygen _________ is produced.
Answer: Sulphur Dioxide

Question 20. _________ and _________ play a crucial role in maintaining the w&tfer balance in the human body.
Answer: Na⁺ ,K⁺

Question 21. _________ ion plays an important role in the coagulation of blood.
Answer: Ca²⁺

Question 22. _________ ion plays a significant role in the process of maturation of RBC in the bone marrow.
Answer: Cobalt

Question 23. _________ magnesium and phosphorous are responsible for the mechanical strength of our teeth and bones.
Answer: Calcium

Question 24. When distilled water is added to magnesium oxide and then a piece of _________ litmus paper is dipped in that solution, it turns is a metalloid.
Answer: Red, Blue

Question 25. When CO₂ gas is bubbled through water for sometime andrtHerrarptiece of _________ litmus paper is dipped In that solution, it turns.
Answer: Blue, red

Question 26. _________ ion helps in the conduction of nerve impulses from one nerve cell to another. When a piece of zinc is added to dilute
Answer: Zn²⁺

Chapter 2 Element Compound And Chemical Reaction Nature Of Matter Identify As True Or False

Question 1. In general, metals are good conductors of heat and electricity.
Answer: True

Question 2. The intermolecular attractive forces are highest in the case of solids.
Answer: True

Question 3. When heated at 1 atmospheric pressure, solid iodine becomes liquid.
Answer: False

Question 4. The iodine vapour is violet-coloured.
Answer: True

Question 5. When sulphur is burnt in oxygen sulphur dioxide gas is produced.
Answer: True

Question 6. Hydrogen sulphide gas has a sweet smell.
Answer: False

Question 7. The melting point and boiling point of non-metals are generally lower than that of metals.
Answer: True

Question 8. Sodium chloride is soluble in petrol.
Answer: False

Question 9. Cobalt has no magnetic properties.
Answer: False

Question 10. Bromine is a liquid at room temperature.
Answer: True

WBBSE Chapter 2 exercises review solutions on nature of matter

Question 11. The colour of chlorine gas is greenish-yellow. When strongly heated, sugar becomes black. In general, metal oxides are basic and oxides of non-metals are acidic.
Answer: True

Question 12. The melting point of a solid does not depend on pressure.
Answer: True

Question 13. Copper is a good conductor of heat and electricity.
Answer: True

Question 14. Carbon monoxide is a neutral oxide. Graphite and diamond are both good conductors of electricity.
Answer: False

Question 15. The colour of nitrogen dioxide is brown.
Answer: True

Question 16. At room temperature, magnesium metal does not react with water.
Answer: True

Question 17. Erosion of bone joints is initiated due to loss of acid-base equilibrium.
Answer: False

Question 18. The colour of nitrogen dioxide is brown.
Answer: True

Question 19. At room temperature, magnesium metal does not react with water.
Answer: True

Question 20. Erosion of bone joints is initiated due to loss of acid-base equilibrium.
Answer: True

Chapter 2 Element Compound And Chemical Reaction Nature Of Matter Match The Columns

1.

Answer: A-2, B-1,C-4,D-3  

2.

Answer: A-4, B-3,C-2,D-1

3.

Answer: A-3, B-4,C-1,D-2  

4. 

Answer: A-3, B-4,C-1,D-2 

5.

Answer: A-3, B-1,C-2,D-4

Chapter 2 Element Compound and Chemical Reaction Nature Of Matter

Physical State of the Substances

We are surrounded by different substances. All the substances can be sensed by our sense organs. They have mass and occupy some volume. They have inertia.

These substances can be divided into three categories according to their physical states solid, liquid and gas. Below a table is given where the natural physical states of different substances are given.

Read And Learn More WBBSE Notes For Class 8 School Science

The basic building block of matter or substance is known as a molecule. The molecule is the smallest constituent particle of matter which retains all the properties of the matter and has independent existence.

The arrangement of the molecules gives rise to the variation in the physical appearance and behaviour of matter in the form of solid, liquid and gas.

Arrangement of molecules in the different states of matter Molecules is packed in different ways in three different states of matter which leads to variation in parameters like intermolecular space, an intermolecular force of attraction and the movement of the molecules. Let us study them one by one:

1. Intermolecular Space: The space between the constituent molecules of the matter is called intermolecular space.

The molecules of a solid are very closely packed with negligible intermolecular space. This sort of arrangement gives them definite volume, definite shape and high density.

The rigidity (i.e. they can not flow) of solids is also due to the closed packing of the solid molecules. The molecules in a liquid are loosely packed resulting in more intermolecular space than that among the solid molecules.

Hence liquids do not have a definite shape and have lesser density than solids though they possess definite volume. Due to a greater magnitude of intermolecular spaces, liquids are slightly compressible.

They are less rigid and can flow (that is why they are also called fluids). Gas molecules are very loosely packed with the maximum amount of intermolecular spaces among them.

This results in gases having no definite volume and shape, the least density and very high fluidity (viz. diffusion occurs due to high fluidity). Gases are highly compressible also.

2. Intermolecular forces of attraction: The attractive forces that exist between the adjacent molecules of matter are called intermolecular forces of attraction.

This can either be cohesive force or adhesive force depending upon the nature of the participant molecules involved.

The intermolecular forces of attraction are maximum among molecules of solids followed by those in the liquids and it is negligible among molecules of gases.

Hence solid is rigid and they do not diffuse. Some liquids can diffuse spontaneously into others (for example water and alcohol) but others do not diffuse (for example oil and water).

Gas molecules can diffuse spontaneously and rapidly due to the minimal magnitude of intermolecular forces of attraction that exist between them.

A Substance can occur in all states at suitable temperatures and pressures. This means varying the temperature and pressure,

the physical state of a particular substance can be changed Matter can be transformed from one state to another by altering the temperature and pressure and this phenomenon is called interconversion of states of matter.

So, let us now discuss the inter-conversion between the various physical states of a particular substance by changing the temperature.

Melting And Boiling: Let us take some amount of ice in a beaker and a thermometer is dipped in the beaker. Suppose, the thermometer shows the temperature of the ice as 0°C. Now the beaker is slowly warmed.

So the heat is being supplied to the beaker and ice starts melting, but still, the temperature of the system remains at 0°C until all the ice melts.

Once all the ice in the beaker melts, the temperature of the system starts rising. After some time, the water starts boiling and its temperature becomes 100°C.

Until all the water in the beaker is vaporized, the temperature remains the same, that is, remains at 100°C We should note here that during this experiment, the pressure over the system is constant (which is equal to 1 atmospheric pressure).

So, from the above experiment, we find that the melting point of ice is 0°C (at 1 atmospheric pressure) and the boiling point of water is 100°C (at 1 atmospheric pressure).

The melting point and boiling point of a substance can be changed by changing the pressure over it.

Also, from the above experiment we find that by changing temperature, the physical state of a substance can be changed.

The melting point and boiling point of some pure substances are listed below (at 1 atmospheric pressure).

Condensation and freezing: We have just found that when heat is supplied to a solid, it melts. So, if we extract heat from a liquid, it will be transformed into a solid.

Similarly, the exchange of heat can transform a liquid into vapour and vapour into liquid. Condensation is the process which involves the change of matter from gas to liquid on cooling, viz. conversion of steam to water.

The process involving the change of matter from liquid to solid on cooling is called freezing or solidification.

Water changes to the ice at 0°C under normal atmospheric pressure. The temperature of 0°C is called the freezing point of water.

Sublimation: Generally, when heat is supplied directly into the vapour state is called sublimation. The solid obtained on cooling the vapour is called sublimate and the vapour formed is called sublime.

Substances which sublime are iodine, naphthalene, ammonium chloride, etc. The sublimation of iodine can be easily demonstrated with an experiment.

Let us take some amount of solid iodine in an evaporating dish (or a round-bottomed dish) and cover it with a glass funnel, The other end of the funnel is plugged with cotton.

The evaporating dish is then heated slowly. It is found that as the temperature is increased, the solid iodine sublimes (i.e., transformed directly into vapour, without going through the liquid state) and it again collects as solid at the inner surface of the upper and colder part of the funnel.

Actually, the vapour formed due to the sublimation of iodine condenses as a solid when comes in contact with the upper part of the funnel which is comparatively colder.

Elements and Compounds Class 8 summary

The experiment can be repeated with camphor, naphthalene and ammonium chloride. We will observe the same result.
Solid substances have a definite shape, but liquids and gas do not have definite shapes.

Liquids and gases can “flow”; this means they have fluidity. So, liquids and gases are commonly called fluids. When a solid is transformed into a liquid, generally its volume increases.

One important exception is ice. When ice melts, its volume decreases. All liquids, when transformed into vapour, their volume increases significantly.

Different substances have different properties. Generally, each substance has special properties by which it can1 be distinguished from the others. We sometimes term these properties as “characteristic properties” of the substance.

Such Properties Can Be Classified Into Two Categories:

1. Physical properties
2. Chemical properties

Physical properties: The property of a substance, which gives us an idea about the nature and external condition of the substance is called its physical property.

It cannot provide us with an idea about its internal structure (or “molecular-level information”). Examples of physical properties are – physical state, colour, odour, melting point, boiling point, magnetic property, solubility in a particular solvent, etc.

Chemical properties: The property of a substance which decides the tendency and capacity of that substance to react with other substances is called its chemical properties.

For example, the sulphur burns in the air to produce sulphur dioxide (S0₂); zinc reacts with dilute sulphuric acid to produce hydrogen gas.

Identification of Substances by their Physical Properties As we have already stated, the physical properties of substances from others and so preliminary of substances are those characteristics which are identification of a substance becomes easy, exhibited externally.

They distinguish one The physical properties, which help the differentiation of substances, are given below :

Chemical reactions and nature of matter for Class 8

Identification of Substances by their Chemical Properties

The properties that a substance exhibits when it undergoes a complete change of its initial atomic composition and arrangement due to some action with other substances or influenced by heat or electricity are known as chemical properties.

Different substances show different chemical properties when they are treated similarly. A substance can be uniquely identified or distinguished from others with its chemical properties.

To determine its chemical property, the substance is allowed to react with air, water, acid, alkali and some other chemical reagents under various conditions

(such as at different temperatures and pressures) and the change in the composition of the substance and the properties of the products formed under such reaction conditions are analyzed. Some examples are given below.

1. Nitric oxide (NO) gas is colourless. Suppose it is stored in a closed container. When the lid is opened, NO reacts with oxygen present in the air and produces brown fumes of nitrogen dioxide
(NO₂)

⇒ \(2 \mathrm{NO}+\mathrm{O}_2 \rightarrow 2 \mathrm{NO}_2\)

2. If sugar and quicklime are added separately to two glasses of water, then the glass in which quicklime is added becomes hot.

Here, quicklime (Calcium oxide, CaO) reacts with water to form calcium hydroxide [Ca(OH)₂] and during this reaction, heat is produced. (It is an example of an exothermic reaction.

⇒ \(\mathrm{CaO}+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{Ca}(\mathrm{OH})_2+\text { heat }\)

For sugar added to water, no chemical reaction takes place, only the sugar dissolves in water as it is highly soluble in water.

3. Two stainless steel spoons with insulated handles are taken. In one, powdered sugar and in the other common salt are taken.

Both the spoons are then heated over a flame. The sugar first turns brown and then becomes black, but no such visible change is observed for common salt.

In the case of sugar, water is first liberated from it and ultimately only black carbon (charcoal) is left on the spoon. This is called charring.

⇒ \(\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11} \rightarrow 12 \mathrm{C}+11 \mathrm{H}_2 \mathrm{O}\)

4. Heating a substance may cause chemical changes in it and it may be transformed into new substances.

For example, some amount of hydrated cupric nitrate, solid iodine and a magnesium ribbon is separately heated at elevated temperatures. Cupric nitrate forms deep blue hydrated crystals of the composition \(\mathrm{Cu}\left(\mathrm{NO}_3\right)_2, 3 \mathrm{H}_2 \mathrm{O}\)
When hydrated cupric nitrate \(\left[\mathrm{Cu}\left(\mathrm{NO}_3\right)_2 \cdot 3 \mathrm{H}_2 \mathrm{O}\right]\) Is strongly heated, then brown-coloured nitrogen dioxide is produced and a black residue of copper oxide (CuO) is left.

⇒ \(2 \mathrm{Cu}\left(\mathrm{NO}_3\right)_2 \cdot 3 \mathrm{H}_2 \mathrm{O} \stackrel{\Delta}{\longrightarrow} 2 \mathrm{CuO}+4 \mathrm{NO}_2+\mathrm{O}_2\)

Iodine is a deep brown (almost black) crystalline solid with a shining lustre. When solid iodine is heated, it sublimes and is converted into violet-coloured iodine vapour. No chemical change takes place in this case.

⇒ \(\mathrm{I}_2 \text { (solid) } \stackrel{\Delta}{\longrightarrow} \mathrm{I}_2 \text { (vapour) }\)

5. The addition of dilute acids to some substances may cause chemical changes in them, forming new substances which may (or may not) have characteristic colour or odour.

So some substances may be identified by reacting them with dilute acids such as sulphuric acid or hydrochloric acid.

For example, zinc powder, iron powder and solid ferrous sulphide are taken separately in three dry test tubes. Dilute sulphuric acid (H₂S04) is added to each of them.

When dilute sulphuric acid is added to zinc powder, a colourless and odourless gas bubbles out from the mixture.

This gas burns with a blue flame when ignited with a burning stick. This gas is hydrogen (H₂).

⇒ \(2 \mathrm{Mg}+\mathrm{O}_2 \stackrel{\Delta}{\longrightarrow} 2 \mathrm{MgO}\)

When dilute sulphuric acid is added to the iron powder, here also a colourless and odourless gas bubbles out from the solution.

This gas is hydrogen. The solution turns light green due to the formation of water-soluble ferrous sulphate (FeSaO₄).

⇒ \(\mathrm{Fe}+\mathrm{H}_2 \mathrm{SO}_4 \rightarrow \mathrm{FeSO}_4+\mathrm{H}_2 \uparrow\)

When the same acid is added to ferrous sulphide, a gas bubbles out with the smell of a rotten egg. This gas is hydrogen sulphide (H₂S). The solution turns faint green due to the formation of ferrous sulphate.

⇒ \(\mathrm{FeS}+\mathrm{H}_2 \mathrm{SO}_4 \rightarrow \mathrm{FeSO}_4 \text { (faint green) }+\mathrm{H}_2 \mathrm{~S} \uparrow\)

As we have just shown some substances can be identified by their reaction with dilute acids, similarly, some substances can be identified by their reaction with alkaline substances.

For example, common salt and ammonium chloride (NH₄CI) are separately mixed with sodium bicarbonate (NaHCO3) or quicklime or sodium hydroxide (NaOH) using a mortar and a pestle.

In the case of common salt, no observable change occurs. But in the case of ammonium chloride, a gas evolves (vapour).

When magnesium ribbon is strongly heated, it burns brightly producing light and white-coloured magnesium oxide is formed and has a strong, pungent odour. The gas is ammonia (NH₃).

When NH₄CI reacts with NaHCO3:

⇒ \(\mathrm{NH}_4 \mathrm{Cl}+\mathrm{NaHCO}_3\rightarrow\mathrm{NaCl}+\mathrm{NH}_3 \uparrow+\mathrm{H}_2 \mathrm{O}+\mathrm{CO}_2\) reacts with NaOH:

⇒ \(\mathrm{NH}_4 \mathrm{Cl}+\mathrm{NaOH} \rightarrow \mathrm{NaCl}+\mathrm{NH}_3 \uparrow+\mathrm{H}_2 \mathrm{O}\)

When NH4CI reacts with Ca(OH)2:

∴ \(2 \mathrm{NH}_4 \mathrm{Cl}+\mathrm{Ca}(\mathrm{OH})_2 \rightarrow\mathrm{CaCl}_2+2 \mathrm{NH}_3 \uparrow+\mathrm{H}_2 \mathrm{O}\)

So, it is clear now that physical and chemical properties can be utilized to identify a substance.
In fact, both the physical and chemical properties of a particular substance are studied to conclusively identify the substance.

Metals and Non-metals: Characteristics and Uses

There are about 92 naturally occurring elements, out of which about 70 are metals. The rest are categorized as non-metals and metalloids.

Physical properties of metals: The Important Physical Properties Of Metals Are Briefly Given Below

1. Malleability: The property which allows the metals to be hammered into thin sheets is called malleability. Most of the metals are malleable.

Gold and silver are some of the best malleable metals. Aluminium and copper are also highly malleable metals. All these metals can be beaten with a hammer to form very thin sheets called foils.

2. Ductility: The property which allows the metals to be drawn into thin wires is called ductility. Gold is the most ductile metal.

Silver is also among the best ductile metals. Copper and aluminium are also very ductile and can be drawn into thin copper wires and aluminium wires. Thin wires of tungsten metal are used to make filaments of electric bulbs.

3. Good thermal conductivity: Metals are generally good conductors of heat as they allow heat to pass through them easily.

Silver metal is the best conductor of heat. Copper and aluminium are also very good conductors of heat.

4. Good electrical conductivity: Metals are good conductors of electricity as they allow an electric current to pass through them easily.

Silver is the best conductor of electricity followed by copper. Gold, aluminium and tungsten are also good conductors of electricity. Metals are good conductors of, electricity because they contain free electrons.

5. Metallic lustre: The property of metal having a shining surface is called metallic lustre. The shining surface of metals makes them good reflectors of light and useful in making jewellery and decoration pieces. Gold and silver are used in ornaments. Silver is used in mirrors.

6. Hardness: Most metals like iron, copper, aluminium etc are very hard. They can not be cut with a knife.

7. High strength: Metals are strong and they can hold large weights without snapping. For example, iron (in the form of steel) is very strong and due to this, it is used in the construction of bridges, buildings, railway lines, chains etc.

8. Solid at room temperature: Most metals like iron, copper, aluminium, silver and gold etc are solids at room temperature.

9. High melting and boiling points: Metals have high melting and boiling points. Iron has a melting point of 1535°C, copper has a melting point of 1083°C etc.

10. High density: Metals are heavy due to their high density.

11. Sonorousness: Metals are sonorous by virtue of which they are capable of producing a deep or ringing sound.

It is due to this property of sonorousness that metals are used for making bells and strings of musical instruments.

12. Physical properties of non-metals: The physical properties of non-metals are just the opposite of those of the metals:

1. Brittleness: Non-metals are neither malleable nor ductile; non-metals are brittle. Solid non-metals can neither be hammered into thin sheets nor drawn into thin wires.

Non-metals break into pieces when hammered or stretched. Carbon, sulphur and phosphorous are solid non-metals that are. brittle.

However brittleness is only applicable for solid non-metals, it is not applicable for liquid or gaseous non-metals.

2. Non-conductor of heat and electricity: Non-metals do not conduct heat and electricity because they have no free electrons.

3. Dull appearance: Solid non-metals have a dull appearance. For example, sulphur and phosphorus have no lustre and they appear to be dull.

4. Soft: Most of the solid non-metals are quite soft.

5. Low strength: Non-metals are not strong, they are easily broken and they do not have appreciable tensile strength.

Solid, liquid or gas at room temperature: Non-metals can exist in all three physical states: solid, liquid and gaseous.

For example, carbon and sulphur are solid non-metals, bromine is a liquid non-metal whereas hydrogen, oxygen etc are gaseous non-metals at room temperature.

Low melting and boiling points: The melting point of sulphur is only 115°C, which is quite low.

6. Light in weight: Due to their low density, non-metals are light substances.

7. Non-sonorous: Non-metals do not produce sound when hit with an object.

Different Physical Properties Of Metals And Non-Metals Can Be Summarized As Follows:

Understanding nature of matter for Class 8

Metalloids are those elements which have both metallic and non-metallic properties. Examples of metalloids are arsenic, antimony, etc.

Metalloids have a metallic appearance (i.e., shiny) but they are brittle. Chemically they have similarities with non-metals. They generally form alloys with metals.

One should always remember that there are exceptions that exist in the physical properties of both metals and non-metals.
For example:

Generally, metals are solid at or near room temperature. But mercury is a metal which is liquid at normal temperature.

1. Metals are generally hard and they cannot be cut into pieces easily by a knife. But lithium, sodium and potassium – all metals are soft and can be cut with a knife.

2. Carbon (in the form of a diamond) is a non-metal that is very hard. In fact, diamond is the hardest natural substance known.

3. Metal surfaces appear shining and it is called metallic lustre. It reflects light from the polished or freshly cut surface. Non-metals have a dull appearance. But iodine, which is a non-metal, has a metallic lustre.

4. Metals usually have high densities. But there are several exceptions. For example, lithium is a light metal (density = 0.53 g/ cm3); sodium, potassium, calcium, magnesium and aluminium also have low densities. Non-metals have low densities. Diamond, though non-metal, has high density.

Key concepts of elements and compounds for Class 8

5. Metals generally have a very high melting point and boiling point. But gallium (Ga) and caesium (Cs) have low melting points (29.75°C and 26.45°C, respectively).

6. Non-metals are usually gases or volatile solids or liquids. For example, fluorine and chlorine are gases, bromine is a liquid while iodine is a volatile solid (which sublimes when heated).

On the other hand carbon, silicon and boron have high melting and boiling points.

7. Metals are good conductors of electricity and heat, but non-metals are not. Carbon is non-metal. One of its “allotropes” – graphite – is a very good conductor of heat and electricity.

8. Metals possess tenacity and toughness which means they possess resistance to, rupture by a stretching force and twisting respectively.

They are also malleable and ductile. But some metals, such as bismuth and antimony are brittle and can be easily crushed to powder.

It can be proved with an experiment that metals such as iron, copper, aluminium, etc., are good conductors of electricity but non-metals such as charcoal, sulphur etc., are bad conductors of electricity.

Use Of Metals And Non-Metals In Human Life And Environment

The earth was created nearly 4500 million years ago. Initially, it was in a very hot condition. Gradually with the progress of time, it started to cool down.

The molten substances also began to cool down and the earth’s crust was formed. Life originated on earth much later (nearly 3500 million years ago). The first life form was very much simple.

Gradually it became more and more complex. Various species appeared and disappeared on earth with the passage of time. At different ages different species were dominant.

In the modern world, the most important and dominating species is the human being. Among all the species which exist in today’s world, human beings were probably the last species to come.

Yet, human beings tried to adapt themselves to changing situations and have been struggling to improve the condition of their living.

In this process, we, human beings have utilized various natural resources for our own benefit and development.

One of the most important natural resources human beings have been using is the metals such as iron, copper etc.

Various non-metals are also used by us. Every metal and non-metal has its specific uses depending on their physical and chemical properties.

Below are some important materials used in our everyday life and the metals and non-metals present in them have been listed.

Nature of matter examples for Class 8

Our body is made up of several elements (metals and non-metals). This includes metals like sodium, potassium, calcium, iron etc., and non-metals like nitrogen, carbon, hydrogen, oxygen, phosphorous etc.

All these metals or non-metals remain within our body mostly as ions, bounded to other ions, forming complex substances.

They are always undergoing some complex but specific processes which are responsible for the functioning of all the physiological processes due to which we remain “alive”.

Various metals and non-metals are present in our bodies in definite proportion. A large deviation from this may result in severe problems in the human body and may sometimes prove fatal.

Let Us Now Briefly Discuss The Role Of Various Elements In Different Important Physiological Processes.

1. Maintenance Of Water Balance In the Human Body:

Na⁺ and K⁺ are two of the most important ions present in intracellular and extra-cellular body fluids.

Sodium is present in soft tissues, like muscle and nerves, in blood and extracellular fluids and in bones.

Potassium is present mainly in skeletal muscles and approximately 75% of potassium present in our body is found in the skeletal muscles.

The presence of sodium ions in extra-cellular body fluid helps in maintaining the level of water in the intracellular and extracellular regions by a process known as “osmosis”.

When urine is formed, the volume of water in it is regulated by Na⁺ ions, so that the level of water in the blood is maintained.

If we start taking more raw salts through our dietary intake, the concentration of Na⁺ in the extra-cellular fluid increases. This increases the osmotic pressure of the extra-cellular fluid.

To normalize this, water from the intracellular fluid comes out through the cell membrane to the outside, resulting in the absorption of more water by blood.

So, the level of water within the intracellular fluid decreases. As a result blood pressure increases, which in turn can adversely affect the proper functioning of the heart and kidneys.

Similarly, when excess Na is lost from our body (for example, due to diarrhoea or due to profuse sweating), water from extracellular regions moves towards the intracellular region.

Hence, blood pressure decreases to an alarming level and may even cause cardiac arrest.

2. Functioning Of Heart:

Cardiac muscles require calcium to contract and squeeze blood out of the heart and into the arteries.

Calcium flows into the muscle cells and works as a switch that allows cardiac muscles to contract.

At the end of the contraction, calcium flows out of the muscle cells to allow the muscle to relax and expand again.

So the rate of heart contraction increases with increasing concentration of Ca2⁺ and decreases with decreasing concentration of this Ca ion.

K is also crucial to the functioning of the heart. A low level of K⁺ leads to irregular contraction of the heart and abnormal electrocardiogram results.

Actually, the electrocardiogram is a measure of heart function and is related to the force and rate of contraction of cardiac muscles. Too much K⁺ in the body may cause palpitation and disruption of heart rhythm.

3. Acid-Base Equilibrium:

K⁺ions play a significant role to maintain acid-base equilibrium in our body. Acidity (due to the high concentration of H⁺ ions inside the cellular fluid) causes a shift, in the concentration of K⁺ extracellularly.

H+ ions are then exchanged with K⁺ and Na⁺. Unless sufficient K⁺ and Na⁺ are not taken withstood, this causes alkalinity in extracellular fluid.

In the ‘same way, if the concentration of K⁺ and Na⁺ increases in extracellular fluid, then they go inside the cellular fluid and H⁺ ions come outside into the extracellular fluids, causing increased acidity.

Due to loss of acid-base balance, erosion of bone joints is initiated and bone density may be lowered.

4. Formation of teeth and bone:

Several ions like Ca2⁺, Mg2⁺ and phosphorous play a major role in the formation of the skeletal structure of our body.

Besides they are important for the formation of teeth. They give mechanical stability to the teeth and bones. These ions are also present in the enamel, dentine and cementum of teeth.

5. Functioning Of Enzymes:

Enzymes can be regarded as bio-catalyst since they function as catalysts inside the body and they are highly selective (in the sense that each enzyme participates-)h only in a specific reaction involving a specific substance).

All the important physiological processes occur due to the involvement of enzymes. Several metallic and non-metallic species like Ca2⁺, Mg2⁺, Na⁺, Fe2⁺, Fe[ Naphthalene and camphor can be identified and are required for the proper functioning of different enzymes.

6. Coagulation Of Blood:

Whenever our skin tissues are broken, blood comes out. As soon as blood from the wound is exposed to air, the platelets (present in the blood) disintegrate and react with fibrinogen to create fibrin – a mass of tiny threads.

This triggers a whole series of reactions that relies on an adequate concentration of Ca2⁺ and vitamin K. Fibrin eventually hardens quickly to form a coating over the wound and the blood clots.

Without adequate levels of Ca2⁺, blood will take a longer time to clot.

7. Contraction Of Muscle And Conduction Of Nerve Impulse:

Contraction and relaxation of muscles occur due to rapidly changing concentrations of Ca2⁺ inside the muscle cells.

This process is commonly referred to as the calcium cycle. Two proteins play a vital role in muscle contraction.

The contraction of smooth muscles is dependent on myosins and the contraction of the striated muscle depends on actin. Muscles contain myofilaments.

Contraction represents the shortening of myofilaments. Too much or too little Ca2⁺ causes muscular symptoms due to disruption of the calcium cycle.

Mg2⁺, Na⁺ and K⁺ also control the excitability of muscles. Ca2⁺ ion also plays a major role in the conduction of nerve impulses from one nerve cell to the other.

8. Oxidation—Reduction Of Tissues And Cells:

During breathing, we inhale oxygen. This is utilized by various proteins to generate energy in mitochondria present within the cell.

9. Transport, storage and utilization of oxygen: Iron is one of the most important metals n being has 4.2 to 6.1 grams of iron in his body.

Iron is present within our body as an iron-containing protein. These proteins are usually called heme proteins. They are responsible for oxygen transport and the electron transfer process in our body.

WBBSE Class 8 Science notes on nature of matter

It is done by changing the oxidation state of iron (inter-conversion between Besides, they play a major role in iron storage and its transport across the body. Haemoglobin in blood transports oxygen.

Myoglobin – an iron-containing binding protein found in muscle tissues of vertebrates and mammals, are regarded as the primary oxygen-carrying pigment within muscle tissues.

Its higher concentration allows an organism to hold its breath for a longer time. Diving mammals such as whales have muscles with a high abundance of myoglobin.

10. Prevention Of Excess Oxidation And The Process Of Ageing:

The presence of different free radicals is held responsible for the ageing process. Compounds of metals like copper, selenium, magnesium and zinc generally neutralize the activities of free radicals such as superoxide anion. Thus the ageing process is delayed.

11. Formation of hormones:

Type – I diabetes is caused by a lack of insulin production in our body. Zinc plays an important role in the production of insulin in the beta-cells of the pancreas for the vast majority of animal species.

Iodine plays a vital role in the synthesis of thyroxine hormone. Secreted from the thyroid gland, thyroxine hormone controls the oxygen intake by different cells of our body and the production of heat energy.

12. Formation Of Blood :

Cobalt plays a significant role in the process of maturation of red blood corpuscle (RBC) in the bone marrow and the formation of haemoglobin.

13. Formation Of Some Important Compounds In the Human Body:

Phospholipids, glycolipids, nucleic acid, DNA, RNA etc., play the most important role in all biological systems. Several metals.

(such as Fe, Cu, Mn, Se) and several non-metals (such as C, H, N, O, and P, So we can conclude that the role of metals and non-metals are not only important for our everyday life but is also crucial for our physiological and biochemical activities.

As we have learnt that presence of some elements in the proper amounts is mandatory for the proper functioning of our body, similarly, there are some elements too, which may cause damage to several of our body parts like the brain, kidney, liver, heart, lungs, etc.

In today’s world, sometimes we are exposed to such elements unknowingly. Examples of such elements are lead, mercury, arsenic, fluorine, cadmium, aluminium etc.

The source of these elements is various. Below, we have outlined some of the common sources of such elements which are harmful to our bodies.

Chapter 1 Physical Environment Short Answer Type Questions

Question 1. What do you mean by a real image and a virtual image?
Answer:

Real Image And Virtual Image:-

Real images are formed when reflected rays or refracted rays actually meet at some point. Virtual images are formed when the reflected or the refracted rays appear to diverge from some point.

Question 2. What do you mean by the refraction of light?
Answer:

Refraction Of Light:-

When light is traveling from one medium to the other, it deviates from its original path at the surface of separation between the two mediums. The change of direction of the incident light from its original direction is called the refraction of light.

Question 3. What do you mean by absolute refractive index?
Answer:

Absolute Refractive Index:-

When refraction occurs between a vacuum and a certain medium, then the refractive index of the medium is called Absolute Refractive Index. This is the ratio of the velocity of light in a vacuum to the velocity of light in that medium.

Question 4. Can the absolute refractive index of a medium be less than 1 ?
Answer: Absolute refractive index of a medium = velocity of light in vacuum/ velocity of light in the medium. Since the velocity of light is maximum in a vacuum, so absolute refractive index of a medium can never be less than 1.

Question 5. Write the laws of refraction.
Answer:

Laws Of Refraction:-

There are two laws of refraction. They are as follows:

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.

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

Question 6. On which factors does the refractive index depend?
Answer:

Refractive index depends on the nature of two media and on the color of light. For example, 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.

Question 7. Inversion of the image of an object formed by a plane mirror occurs left for right and vice versa. Why does it not occur top for the bottom?
Answer:

The straight line joining any point on an object and its corresponding mirror image always meets the mirror perpendicularly.

If the inversion of an image happens top for the bottom, the straight line, connecting a point of the object with its image would meet the mirror obliquely.

Thus, a plane mirror does not produce inversion of any extended non- sym metric object top for bottom or vice versa.

Short answer questions on light for Class 8

Question 8. Describe appropriately, the refraction of light when a ray of light travels from an optically rarer medium to an optically denser medium.
Answer:

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 (as shown in ). Here ∠AON is the angle of incidence;

∠BON’ is the angle of refraction; AO is the incident ray; OB is the refracted ray and NN’ is normal.

Question 9. Describe appropriately, the refraction of light when a ray of light travels from an optically denser medium to an optically rarer medium.
Answer:

When a ray of light travels from an opt it ally denser 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 (as shown in).

Here Z AON is the angle of incidence; Z BON is the angle of refraction; AO is the incident ray; OB is the refracted ray and NN’ is normal.

Question 10. What do you mean by critical angle for a given pair of media?
Answer:

Critical Angle For A Given Pair Of Media:-

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.

WBBSE Chapter 1 light short answer solutions

Question 11. What do you mean by total internal reflection?
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 (∠DON exceeds the critical angle (6C), the incident light reflects back along OR that is to the first medium (optically denser medium)

Such a phenomenon is called total internal reflection. The term “total” is used because the incident light 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.

Question 12. What are the primary conditions to be fulfilled so that total internal reflection can occur?
Answer:

The primary conditions for total internal reflection to occur are – the ray of light must pass from an optically denser medium to an optically rarer medium and

the angle of incidence in the optically denser medium must be greater than the characteristic critical angle of the two media.

Question 13. Explain why a diamond or a gemstone glitters and looks very bright.
Answer:

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

For example, the critical angle of a diamond is only 24.5°. Also, a diamond 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.

Light concepts summary with short answers for Class 8

Question 14. Explain why a crack in the glass of a window pane looks shiny.
Answer:

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 passing through the denser medium (i.e. glass)

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

Question 15. Explain why an empty test tube dipped in the water looks shiny.
Answer:

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

Question 16. Explain why a drop of water on an arum leaf seems glittering.
Answer:

A 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.

Question 17. A fish of length 8 cm is lying parallel to the surface of the water at a depth of 4cm below the surface. If it is viewed perpendicularly from the top, then how long would it appear? Explain with reasons.
Answer:

A ray incident normally on the surface of separation of two media does not undergo refraction and passes undeviated in the second medium from the first medium.

Hence when viewed down normally from above the surface of the water, the fish would appear 8 cm in length.

Question 18. 1. If the angle of incidence of a ray of light on a plane mirror is 60° then what is the angle of deviation?
2. If the distance between the object and its image in a plane mirror is 120 cm then what is the distance between the object and the mirror?
Answer:

1. <i=<r=60°
Angle of deviation
=180°-(<i+<r)
=60°

2. Distance between object 5 and the plane mirror =120/2=60 cm.

WBBSE Class 8 Science practice short answer questions on light

Question 19. A boy walks at a speed of 2 km/hr toward a mirror. What will be their speed of his? Let the boy be x km away from the mirror at the beginning.
Answer:

Given That

A boy walks at a speed of 2 km/hr toward a mirror.

Therefore, his image was also x km away from the mirror and the distance between the boy and his image was 2x km. The distance of the boy from the mirror after 1 hour = (x-2) km.

∴Distance of his image from the mirror after 1 hour = (x-2) km.

Distance covered by image w.r. to mirror in 1 hour = x – (x-2) = 2 km.

∴Speed of image w.r. to the mirror=2 km/hour. Distance between the boy and his image after 1 hour = 2 (x – 2) km = 2x – 4 km.

Change of Distance of the image w.r. to the boy in 1 hour = 2x – (2x-A) = 4 km.

∴Speed of image w.r. to the boy = 4 km/hour.

Chapter 1 Physical Environment VSAQs

Question 1. What is the critical angle of a diamond with respect to air?
Answer:

The critical angle of a diamond concerning air Is 24.5°

Question 2. What kind of image is formed in a plane mirror?
Answer:

Virtual image is formed in a plane mirror.

Question 3. Which kind of image can be cast on a screen?
Answer:

Real image can be cast on a screen.

WBBSE Class 8 Light very short answer questions

Question 4. If the angle between two plane mirrors is 60° and if an object is placed in between them, how many images will be formed?
Answer:

The number of images that will be formed is 360°
1= 5. So five virtual images will be

Question 5. If two plane mirrors are placed parallel facing each other, and if an object is placed in between them then how many Answer: images will be formed?
Answer:

If the two plane mirrors are placed parallel facing each other, then an infinite number of images will be formed. But except for a finite number of images, most of the images cannot be seen as they would become faint.

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

Question 6. In which medium the velocity of light is maximum?
Answer:

The velocity of light is maximum in a vacuum.

Question 7. On which factors does the refractive index of a medium to another depend? 
Answer:

Refractive index depends on the nature of the two media and on the color of a ray of light.

Question 8. Give one example of total internal reflection which is observed in nature.
Answer:

Mirage, formed during daytime in the desert is an example of total internal reflection observed in nature.

Question 9. If a ray of light is incident on a plane mirror making an angle of 60° with the plane of the mirror, what will be the value of the angle of reflection?
Answer:

If the angle between the plane mirror and the incident ray is 60°, then the angle of incidence is (90° – 60°) = 30°.

Since, from the laws of reflection, we know that the angle of incidence is the same as the angle of reflection, so the angle of reflection, in this case, will be 30°.

If a ray of light is incident on a plane mirror making an angle of 90° with the plane of the mirror, what will be the value of the angle of reflection?

Question 10. If the ray of light is incident on a plane mirror making an angle of 90° with the plane of the mirror, then the  angle of incidence is (90? – 90?)
Answer:

=0°. Since, from the laws of reflection, we know that the angle of incidence is the same as the angle of reflection, the angle of reflection will be 0°.

Very short answer questions on light for Class 8

Question 11. If the distance between an object and a plane mirror is 10 cm, what will be the distance between the object and its virtual image?
Answer:

If the distance between an object and a plane mirror is 10 cm, then the distance between the plane mirror and the virtual image of the object is 10 cm. So the total distance between the object and its virtual image is (10 + 10) cm = 20 cm.

Question 12. An object is placed in front of a plane mirror. If the object is moved 2 cm away from the mirror, how much does the virtual image move from the object?
Answer:

If the object is moved 2 cm away from the mirror, then the distance between the object and the plane mirror is (x + 2) gm.

So the distance between the object and its virtual image will be 2(x + 2) cm.
So the distance by which the virtual image will move away from the object is = 2(x+2) – 2x = 4cm.

Question 13. The critical angle of glass concerning air
Answer:

The critical angle of glass with respect to air

Is 42°. If a ray of light, when passing from glass to air is incident at the glass-air interface with the angle of incidence of 42°, what will be the angle of refraction in the air?

If the critical angle of glass concerning air is 42? then for a light passing from glass to air, the angle of refraction will be 90? in air.

Question 14. Where do we observe superior mirage?
Answer:

Superior mirage is observed in colder countries during winter.

Question 15. When viewed from the top, the depth of water in a water-filled bucket seems to be raised. Which property of light is responsible for this phenomenon?
Answer:

Refraction of light is responsible for this phenomenon.

Question 16. An image of a tree is formed on the surface of the water of a pond. Is it an erect or an inverted image?
Answer:

Inverted real image.

WBBSE Chapter 1 light quick answer solutions

Question 17. Which principle of light is utilized in making periscope?
Answer:

A periscope works on the reflection of light from two plane mirrors arranged parallel to one another.

Question 18. Two plane mirrors are inclined at an angle of 72° to each other with an object placed between them. How many images of the object are formed by successive reflections?
Answer:

360° Number of images = 360°/72°- 5.

Question 19. Find out the odd term :
Answer:

Total internal reflection, critical angle, refraction, reflection.
Refraction.

Light concepts summary with very short answers for Class 8

Question 20. What do you mean by the statement that the critical angle of glass is 42° ?
Answer:

Critical angle of glass is 42°, this means that when a ray of light of any given colour tends to travel from glass to air, then for an angle of incidence of 42° in glass, the angle of refraction in the air is 90°.

Question 21. Sitting in front of a plane mirror when we write with our right hand, it appears in the mirror that we are writing with our left hand. Which phenomenon is responsible for this?
Answer:

Lateral inversion of image.

Chapter-1 Physical Environment Review Questions MCQs

Question 1. In the case of a virtual image formed by a plane mirror, the distance between the object and the mirror is

1. Greater than the distance between the image and the mirror
2. Same as the distance between the image and the mirror
3. Less than the distance between the image and the mirror
4. Twice the distance between the image and the mirror.

Answer: 2. Same as the distance between the image and the mirror

Question 2. The screen used in the cinema hall should be

1. White and smooth
2. White and rough
3. Colored and smooth
4. Black and rough

Answer: 2. White and rough

3. If the distance between the object and the mirror is reduced by 2 cm, the distance between the object and its image will be reduced by

1. 4 Cm
2. 2 Cm
3. Ocm
4. None of these

Answer: 1. 4 Cm

Question 4. The drop of water on an arum leaf glitters due to

1. Reflection of light
2. Total internal reflection of light
3. Refraction of light
4. Absorption of light

Answer: 2. Total internal reflection of light

Read And Learn More WBBSE Solutions For Class 8 School Science Review Questions

Question 5. The image formed by the magnifying glass is

1. Real
2. Virtual
3. Inverted
4. Hazy

Answer: 1. Real

Question 6. The velocity of light in an optically denser medium is

1. Greater than the velocity of light in an optically rarer medium
2. Less than the velocity of light in an optically rarer medium
3. Same as the velocity of light in an optically rarer medium
4. None of these

Answer: 2. Greater than the velocity of light in an optically rarer medium

WBBSE Class 8 Light review questions

Question 7. During total internal reflection, the angle of incidence is

1. Greater than the critical angle
2. Less than the critical angle
3. Same as the critical angle
4. Does not depend upon the critical angle

Answer: 1. Less than the critical angle

Question 8. If the angle between the two plane mirrors is 30°, and if an object is placed in between them, the number of images formed is

1. 12
2. 13
3. 11
4. Infinite

Answer: 3. 11

Question 9. When a ray of light is incident on a plane mirror, the angle of incidence for which the angle between the incident ray and reflected ray will be 90° is

1. 45°
2. 30°
3. 15°
4. 60°

Answer: 1. 45°

Question 10. The letter which will not suffer lateral inversion when placed in front of a plane mirror is

1. M
2. P
3. S
4. B

Answer: 1. M

Question 11. For which color of light, the refractive index will be minimum?

1. Green
2. Red
3. Violet
4. Indigo

Answer: 2. Red

Question 12. The critical angle of the diamond with respect to air is

1. 2.42°
2. 24.2°
3. 2.45°
4. 24.5°

Answer: 2. 24.2°

Question 13. Two boys are standing in front of a plane mirror as shown. When boy looks into the mirror, how far away from him will the image of boy b appear to be?

1. 6M
2. 8M
3. 2M
4. 10M

Answer: 4. 10M

Question 14. The refractive index depends upon

1. Color of a ray of light
2. The critical angle between the two media
3. Nature of the two media
4. Both color of a ray of light and the nature of the two media.

Answer: 4. Both color of a ray of light and the nature of the two media.

Question 15. When a ray of light travels from an optically rarer medium to an optically denser medium, the ray in the denser medium

1. Moves away from the normal
2. Grazes along the surface of separation of the two media
3. Moves towards the normal
4. Suffers total internal reflection

Answer: 3. Moves toward the normal

Chapter-1 Physical Environment Fill in the Blanks

Question 1. A ________image can be cast on screen.
Answer: Real

Question 2. The velocity of light in glass is ________ than the velocity of light in vacuum.
Answer: Less

Question 3. An image of the sun formed by a magnifying glass is a ________ image.
Answer: Real

Question 4. An image formed by a plane mirror is a ________ image.
Answer: Virtual

Question 5. When refraction occurs between ________ and a certain medium, then the refractive index of the medium is called Absolute Refractive Index.
Answer: Vaccum

Question 6. In the case of an image formed by a plane mirror, the distance between the object and the mirror is ________ as the distance between the image and the mirror.
Answer: Same

Question 7. When passing from an optically ________ medium to an optically medium, the refracted ray moves away from the
normal drawn at the surface of separation between the two media.
Answer: Denser, rarer

Question 8. When passing from an optically ________medium to an optically medium, the refracted ray moves closer to the normal drawn at the surface of separation between the two media.
Answer: Greater

Question 9. In the case of total internal reflection, the light must be incident at an angle ________than the critical angle.
Answer: Dense

WBBSE Chapter 1 light review questions and answers

Question 10. In the case of total internal reflection, the light must travel from an optically ________medium to an optically medium.
Answer: Dense, rarer

Question 11. In the periscope, each mirror makes an angle of ________ with the axis of the periscope box.
Answer: 45°

Question 12. In a kaleidoscope, the three mirrors are joined together facing each other in a shape resembling a ________
Answer: Prism

Chapter 1 Physical Environment Identify As ‘True Or ‘False’

Question 1. A virtual image cannot be cast on a screen.
Answer: True

Question 2. The virtual image is formed by a plane mirror.
Answer: True

Question 3. If the angle between two plane mirrors is 45° and if an object is placed between them then 8 images will be formed.
Answer: False

Question 4. A drop of water on the arum leaf glitters due to the total internal reflection of light.
Answer: True

Question 5. In the case of total internal reflection of light, no part of it is absorbed or refracted.
Answer: True

Question 6. The higher the density of a medium higher will be its optical density.
Answer: False

Question 7. When a ray of light passes from an optically denser medium to an optically rarer medium, the refracted light moves away from the normal.
Answer: True

Question 8. The velocity of light is lower in an optically rarer medium.
Answer: False

Question 9. In the case of total internal reflection, the angle of incidence of the ray of light must be smaller than the critical angle.
Answer: False

Question 10. The critical angle of the diamond with respect to air is low.
Answer: True

Question 11. Periscope is used to view a magnified image of an object.
Answer: False

Light concepts summary and review questions for Class 8

Question 12. Lateral inversion occurs when an image is formed by a plane mirror.
Answer: True

Question 13. Kaleidoscope utilizes the property of formation of multiple images by three or more mirrors.
Answer: True

Question 14. Light is a form of energy.
Answer: True

Question 15. Mirage is a virtual image.
Answer: True

Question 16. The refractive index of a medium has no unit.
Answer: True

Chapter 1 Physical Environment Match The Columns

1.

Answer: A-2,B-1,C-4,D-3

2.

Answer: A-3,B-4,C-1,D-2

Chapter 1 Physical Environment Light

Light is a form of energy. We can see objects around us with the help of light, although light itself is invisible.

If a person enters a dark room, he does not see the articles present in the room. When he or she switches on the electric lights, the objects within that room immediately become visible.

Light, emitted from some light source may come directly to our eyes or sometimes it may undergo reflection to reach our eyes after “bouncing off” from a surface(such as a mirror). Then it is called reflected light.

There is another optical phenomenon – known as refraction. In this case, when a ray of light coming from one medium, enters another medium, the direction of the ray of light changes at the surface of the separation of the two media.

We come across different instances of refraction in our daily life. When water is poured into an empty tub, the base of the tub appears to be raised. This occurs due to refraction.

Reflection of Light

1. Let us now discuss the reflection of light.

When we stand in front of a plane mirror, we see our images. The image appears to be “behind” the mirror. But we can immediately understand that light cannot pass through the mirror and go beyond it.

So, if we place a screen behind the mirror, no image will be formed on the screen. Such an image is called a virtual image. Formation of the 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

Read And Learn More WBBSE Notes For Class 8 School Science

2. Formation of the virtual image with the help of a mirror

When an object is placed in front of a plane mirror, its image is formed in the plane mirror. We can locate the image of the object in a plane mirror by using the laws of reflection of light.

Let us consider that a point object “A” is placed in front of a plane mirror, MM’. AB is the incident ray coming from A.

It falls perpendicularly on the mirror at B and is reflected along BA. AC and AD are two other rays of light, which are reflected along CE and DF, respectively.

If AB, EC, and FD are extrapolated behind the mirror, they meet at point A’. So, to a spectator it seems that the rays of light are coming from A’. Here A’ is the virtual image of A.

So, a virtual image is formed by the intersection of the light rays when extrapolated behind the mirror and not by their actual intersection.

The path of light rays forming a virtual image is generally shown by dotted lines or dashed lines in  Real images and the path of the light rays forming them are shown by solid (i.e. continuous) lines.

Hence, there are two types of images – some of which can be cast or produced on screen while others cannot be cast on a screen. The former are called real images and the latter are called virtual images.

The image formed by the magnifying glass is real. Real images are always inverted (i.e. upside down). The image formed by a mirror is a virtual image. These images are always erect (i.e. upright).

3. Formation of the virtual image of an extended object

Considers an extended object AB placed in front of a plane mirror MM’. The light ray AC from point A is incident perpendicularly on the mirror at point C and retraces its path along CA. Another light ray AD from point A of the object is incident on the mirror at point D and gets reflected along DE.

The two reflected rays CA and DE, when extrapolated backward, intersect at point A’. Therefore, A’ is the image of point A.

Similarly, the reflection of the extended object takes place throughout the body of object AB and similar rays are plotted for the bottommost position of object B.

Hence the total image of the object AB is formed as A’B’. It is found that size of A’B’ is exactly equal to the size of AB.

The distance of object AB from the plane mirror is equal to the distance of the image from the mirror.

Physical Environment Light summary

4. Shifting of the object and its virtual image in a plane mirror

When rays of light coming from a certain point, after reflection or refraction, meet at a point, the real image of the first point is formed.

It implies that a real image is formed by the actual intersection of light rays. This is not the case for virtual images.

Let us now take a ruler on the table and a plane mirror is placed at the zero mark of the ruler, perpendicularly with the plane of the table as

Let us now place the tip of a pencil at the “eight” mark on the ruler. This means that the tip of the pencil is 8 cm from the plane mirror.

Since the virtual image of the tip of the pencil is also formed 8 cm “behind” the mirror, so the total distance between the object (i.e. the tip of the pencil) and its virtual image is (8 + 8) cm = 16 cm. away from the plane mirror, its virtual image moves away by a distance of (20 -16) cm = 4 cm away from the object.

Similarly, it can be shown that if we move the object towards the mirror by 3 cm, the distance between the object and its virtual image is decreased by (3+3) cm = 6 cm.

So, it is seen that if we increase or decrease the distance between the mirror and the object, the distance between the object and its virtual image will be increased or decreased by a factor of two.

5. Lateral Inversion of the in-plane mirror

If we stand in front of a plane mirror and look at ourselves, we find that our images are of the same size as ours.

The image is at the same distance behind the mirror as we are in front of the mirror. The image is also erect. But if we move our right hand, the image moves its left hand.

This means that lateral inversion has taken place. Actually, in an image formed by a plane mirror, the left side of the object appears on the right side in the image whereas the appears on the right side in the image whereas the right side of the object appears on the left side of the image.

This change of sides of an object and its mirror image is called lateral inversion. In a lateral inversion, the image, therefore, undergoes a rotation of 180° the vertical axis.

So from the above observation and experimentation, we have learned some aspects of images formed by a plane mirror. They can be summarized as follows.

1. Characteristics of images formed by a plane mirror
2. Images are virtual and erect.
3. Images appear to form as far behind the mirror as the object is in front of the mirror.
4. The size of the image is exactly equal to the size of the object.
5. The image of an asymmetric object is seen inverted laterally.
6. No inversion of the top and bottom of an extended object occurs.

6. 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. In fact, 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

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.

7. Images formed by two parallel plane mirrors

Two plane mirrors M₁, and M₂ are placed parallel to and facing each other. O is a luminous point situated between the mirrors.

From O perpendicular OT is drawn on Mx and perpendicular OV is drawn on M₂ OT and OV is extended both ways. Light rays diverge from O.

We first consider a reflection of the rays from M₁when is the image of O₁ is such that OT = TO₁

Understanding Light for Class 8

Some of these reflected rays a, b fall on M₂ and appear to diverge from O₂ which is the image of Or O₁.O₂ serves as the virtual object in front of M₂ and so O₁= VO₂.

Again O₂ acts as the virtual object ill front of and the image O₃ forms, such that TO₂= TO₃. The same phenomenon repeats and thus a series of images form.

Similarly, starting with the rays first being reflected from the mirror M₂ another series of images form.

Theoretically, an infinite number of images should form, but, as some light is absorbed during each reflection, the images become fainter and fainter.

Thus, a finite but quite large number of images are visible. Plane mirrors can be used to prepare periscopes and kaleidoscopes.

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.

WBBSE Chapter 1 Light exercises

8. 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.

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.

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.

(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.

Light concepts in Physical Environment

1.Refraction of light in glass slab

Let us discuss the phenomenon in some detail. Let an oblique ray of light traveling through air transmits through a glass slab and then comes 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. Z 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.

So PQ. is the incident ray, 0 is the point of incidence, OL is the refracted ray, Z EON is the
the angle of incidence and Z 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.]

WBBSE Class 8 Environmental Science Light

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 had 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.

WBBSE Class 8 science practice on Light

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 Z NOA’ which is greater than the corresponding angle of incidence, Z 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, Z CON’ (denoted as 0C) the corresponding angle of refraction, Z 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.

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 (0C), 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.

Light and its properties for Class 8

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.

WBBSE Chapter 1 Physical Environment Long Answer Questions

Question 1. When water freezes its volume increases. Cite one advantage and one disadvantage of this property. Mention the factors that affect the melting point of substances.
Answer:

Change of State of substance

Substances generally exist in either of the three forms:

1. Solid – for example, wood, iron, gold, aluminum, ice, etc.
2. Liquid – for example, water, alcohol, milk, etc.
3. Gas or vapor – for example, steam, oxygen, nitrogen, etc.

1. Melting and Freezing

These three states are inter-convertible with Melting (or fusion) meaning the transformation of exchange of heat.

For example, the supply of heat to a solid to its liquid state at some fixed solid ice produces liquid water and supplies temperature by absorption of heat energy. The more heat to water produces steam.

the fixed temperature at which melting occurs is Again, on cooling (i.e. by extracting heat) liquid called the melting point of the substance, and if water is formed from steam and extraction of different for different substances, further heat from the water will produce solid ice.

Freezing (or solidification) is the process of Thus the exchange of heat and plays the most transformation of liquid to its solid state at some important role in the interconversion of the three fixed temperatures by extraction of heat energy, states of matter.

The fixed temperature at which the freezing (or solidification) occurs is called the freezing point of the substance.

The melting point and freezing point of metals and crystalline solids are the same under the same pressure. For example, the freezing point of water and melting point of ice at normal pressure (1 atmospheric pressure) is 0°C.

But non-crystalline substances like wax, glass, butter, pitch, etc. melt and freeze at two different temperatures. For example, the butter melts at a temperature between 28°C to 37°C and freezes between 20°C to 25°C.

WBBSE Class 8 Heat long answer questions

2. Change of volume during melting and solidification

Normally, a solid expands in volume on melting,, and liquid contracts on freezing. So, in general, the density of the liquid is less than that of its solid state.

An example is wax. But water is an exception. In the case of water, the density of ice (solid state) is less than that of water (liquid state). So, during freezing, water expands in volume as it becomes solid.

Experiment -1

Let us take two test tubes – one is filled with wax and the other is filled with ice. Both test tubes are heated separately to melt the wax and ice completely.

Now some solid wax is carefully dropped into the test tube containing liquid wax and some solid ice is dropped into the test tube containing water.

It is found that the wax sinks in the molten wax and the ice floats on the water

Inference: This confirms that solid wax has a higher density than molten wax (liquid state). Liquid wax has less density because the volume of wax increases as solid wax melts.

But the density of ice is less than that of water. This is because the volume of water increases as it freezes to ice When water freezes its volume increases. This phenomenon has certain advantages as well as disadvantages.

Disadvantages: As water freezes, its volume expands. Hence in cold countries in the winter season, pipelines used for water supply or water pipes in the radiator of cars are often found to crack and burst.

Sometimes liquid water penetrates within the solid rocks through cracks. In winter, when this water freezes, it expands and puts enormous pressure on the rocks and stones from inside.

As a result, rocks burst into pieces and may cause landslides in the hilly regions.

Advantages: The density of ice is less than that of water, and floats on water. The water underneath these ice blocks supports aquatic life.

Metallic mementos are prepared using certain substances like brass, cast iron, etc. Molten substances are poured into the molds for preparing these mementos.

When freezes, the volume of these substances increases and fits tightly against the molds. As a result, the intricate and delicate designs are properly, imprinted on the solidified material.

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

3. Factors Affecting The Melting Point Of A Substance

The melting point of a solid substance depends on two factors:

1. The pressure applied to the substance
2. Presence of impurities in the substance
3. Effect of pressure on the melting point of a substance

For those substances whose volume increases due to melting, their melting point increases with increasing pressure.

For example, copper, gold, etc. Increased pressure resists the volume increase, thus increasing the melting point.

Melting point of wax increases by approximately 0.04°C due to an increase of pressure by one atmosphere.

For those substances, whose volume decreases due to melting, their melting point decreases with increasing pressure. For example, ice, brass, cast iron, etc.

Increasing pressure helps in melting by decreasing their volume. The melting point of ice decreases by approximately 0.0007°C due to an increase of pressure by one atmosphere.

When two pieces of ice are pressed together for some time and then released, they form a single lump. This is because when pressure is applied, then at the contact area between two pieces of ice, the melting point decreases.

So some ice melt at the contact point. When pressure is released, the melting point again returns to the original value.

So the water formed at the contact area again freezes, forming a single lump of ice. This is called regelation.

Effect Of Presence Of Impurity In The Substance On Melting Point

The presence of impurities or presence of any other substance decreases the melting point of the substance. For example,

1. Melting point of ice is 0°C at normal pressure. But if some salt is added, its melting point becomes much lower than 0°C.

Again, a mixture of metals (called alloys) has a melting point lower than the melting point of any of its constituents.

[Example: The melting point of a Fuse wire used in safety fuse, is made up of lead and tin. The melting point of their mixture is lower than the melting point of either of them.

When excess current flows through this wire, heat is produced and the wire melts easily thereby preventing further passage of electricity through it and thus preventing fire hazards.

A freezing mixture is prepared by mixing two substances. At the existing temperature, one of them should melt requiring heat for this.

This heat is taken from the mixture and as a result, the temperature of the mixture falls. When salt is mixed with ice, its temperature decreases. This mixture is called a freezing mixture.

When ice and common salt are mixed in a 3:1 weight ratio, part of the ice undergoes melting and the heat required for this is taken from the salt mixture as a result of which the temperature becomes approximately – 23°C.

This freezing mixture is frequently used for the preservation of fish, meat, etc. and to carry medicine at low temperatures from one place to another place.

Long answer type questions on heat for Class 8

Question 2. Explain the principle of action of the pressure cooker. – explain why?
Answer:

The severity of burn caused by steam at 100°C is greater than that by water at 100*C

4 pressure cooker consists of a container and the lid is provided with a rubber gasket which does not allow the steam to go out of the container.

In a pressure cooker, the steam formed from boiling water is not allowed to escape. This increases the pressure inside the cooker and so, water boils at a higher temperature.

The time required to cook food is greatly reduced if water is made to boil at a higher temperature than its boiling point. The pressure cooker raises the boiling point of water to nearly 120°C.

1 g of steam at 100°C contains 537 cal of more heat than lg of water at 100°C since the latent heat of vapourization of water at normal pressure is 537 cal/g.

Due to this greater heat content, the severity of a burn is more in the case of steam than that of water at the same temperature.

Question 3. Discuss briefly the characteristic properties of evaporation.
Answer:

Evaporation

It is our common experience that wet clothes get dry when left in the sunlight for some time. This is because of the evaporation of water from wet clothes.

During this process, the water on the surface of the cloth takes up heat from the surrounding and is slowly converted into vapor.

All liquids undergo evaporation but the rate of evaporation of volatile liquids such as spirits and alcohols is very fast.

Definition: Evaporation is a process in which liquid is slowly and gradually converted into vapor. It takes place from the surface of the liquid and it can occur at any temperature and pressure.

Characteristic properties of evaporation

1. More the exposed area of the liquid, more will be the rate of evaporation. For example, a folded sari left in the sun will take more time to dry compared to a sari left for drying in the sun without any fold.

2. Evaporation can occur at any temperature. No particular temperature is needed. But the higher the temperature the faster will be the rate of evaporation.

For example, a wet cloth becomes dry even in the winter season, but the time taken for that will be more than the time required during the summer.

3. If air flows over the liquid, it facilitates the process of evaporation and the rate of evaporation becomes faster.

Wet clothes become dry quickly if air flows over them. They dry up faster in the winter season than in the rainy season (when the moisture content in the air is very high).

4. The rate of evaporation depends on the nature of the liquids. Volatile liquids like spirits, alcohol, etc. evaporate more quickly than water.

Rate of evaporation increases as air pressure decreases.

Question 4. What is dew? Why dew is formed at the dawn?
Answer:

Condensation

This is the reverse process of vaporization. This is the phenomenon by which vapor is transformed
In summer, when we sweat heavily, it feels comfortable if we stand beneath a fan.

This is because the airflow produced by the fan facilitates into liquid. The latent heat of condensation is equal to the latent heat of vapourization. Condensation of water vapor in the air produces clouds, fog, etc. in nature.

1. Cloud: The water that evaporates from sea, ocean, river, lakes, etc. mixes with hot air. Air containing water vapor is lighter than “dry” air which contains much less water vapor.

So, lighter air goes upwards towards the higher altitudes. But with increasing altitude, the air pressure reduces. The air cools down and the water vapour of the air condenses as water droplets on the tiny dust particles floating in the air, forming clouds.

2. Fog: It is often seen during dawn in the winter season. During daytime, water evaporates and water vapor thus formed mixes with the air.

At night, when the temperature falls, the air becomes saturated with water vapor. It then condenses as water droplets on suspended dust and smoke particles, coal dust, etc.

And floats in the air, thus forming fog. In big cities and industrial belts, where the pollution level is significantly high, the air contains more dust and smoke particles, thus increasing the chance of fog formation.

At noon, the fog disappears, because, at higher temperatures, the small water droplets are again converted to water vapor.

3. Dew: In the winter season, dew is found on leaves and grasses in the morning. During the daytime, the surface of the earth and objects near it becomes hot.

But after sunset, the earth gradually cools down by radiating heat. The air in contact with the earth’s surface also cools down and when it becomes sufficiently cold, it becomes saturated with water vapor.

The water vapor then condenses into water droplets on the cold surface of leaves and grasses, which is called dew.

Dew is not formed immediately after sunset but is formed in the later part of the night and at the dawn. This is because it takes time to cool down the earth’s surface temperature so that water vapor can condense.

WBBSE Chapter 1 heat detailed answers

Question 5. How fog is formed? Why does it disappear at noon? While sweating heavily in summer, why do we feel comfortable under a fan?
Answer:

Condensation

This is the reverse process of vaporization. This is the phenomenon by which vapor is transformed
In summer, when we sweat heavily, it feels comfortable if we stand beneath a fan.

This is because the airflow produced by the fan facilitates into liquid. The latent heat of condensation is equal to the latent heat of vapourization. Condensation of water vapor in the air produces clouds, fog, etc. in nature.

1. Cloud: The water that evaporates from sea, ocean, river, lakes, etc. mixes with hot air. Air containing water vapor is lighter than “dry” air which contains much less water vapor.

So, lighter air goes upwards towards the higher altitudes. But with increasing altitude, the air pressure reduces. The air cools down and the water vapour of the air condenses as water droplets on the tiny dust particles floating in the air, forming clouds.

2. Fog: It is often seen during dawn in the winter season. During daytime, water evaporates and water vapor thus formed mixes with the air.

At night, when the temperature falls, the air becomes saturated with water vapor. It then condenses as water droplets on suspended dust and smoke particles, coal dust, etc.

And floats in the air, thus forming fog. In big cities and industrial belts, where the pollution level is significantly high, the air contains more dust and smoke particles, thus increasing the chance of fog formation.

At noon, the fog disappears, because, at higher temperatures, the small water droplets are again converted to water vapor.

3. Dew: In the winter season, dew is found on leaves and grasses in the morning. During the daytime, the surface of the earth and objects near it becomes hot.

But after sunset, the earth gradually cools down by radiating heat. The air in contact with the earth’s surface also cools down and when it becomes sufficiently cold, it becomes saturated with water vapor.

The water vapor then condenses into water droplets on the cold surface of leaves and grasses, which is called dew.

Dew is not formed immediately after sunset but is formed in the later part of the night and at the dawn. This is because it takes time to cool down the earth’s surface temperature so that water vapor can condense.

In-depth explanations of heat concepts for Class 8

Question 6. Describe a demonstrative experiment to show that water is a bad conductor of heat.
Answer:

The flow of Heat: Conduction, Convection, and Radiation

To carry heat from one part of an object to its other part or from one object to another object is called transfer of heat.

Transfer of heat energy from one place to another with or without the help of a medium can take place in three ways

1. Conduction
2. Convection
3. Radiation.

1. Conduction

It is our common experience that when one end of a long metal rod is heated, the other end of it gradually warms up.

Obviously, heat propagates from the hotter region of the rod to its cooler end, without any actual movement of its constituent particles. This process of transfer of heat is called conduction.

Definition: The process by which heat energy is transmitted through a substance from a region of higher temperature to a region of lower temperature without any movement of its constituent particles from one place to another is called conduction.

By the process of conduction, heat is not only transferred within the same object but heat is also transferred from one object to another provided:

The two objects are in contact with each other, and They are at different temperatures

So long as there is a temperature difference between the two objects (or between two regions at the same object), the transfer of heat continues.

Once the temperature of the two objects (or two regions of the same object) becomes the same, heat transfer ceases.

The substance through which heat can flow easily is called a good conductor of heat. The substances through which heat cannot flow easily are called the bad conductors of heat.

Generally, metals are good conductors of heat. For example, copper, steel, brass, etc. are good conductors. So cooking utensils (sometimes the base of the cooking utensils) are made up of these materials.

So, heat quickly reaches the foodstuffs that are being” cooked. Substances like wool, stone, glass, wood, water, air; diamond, paper, etc. are bad conductors of heat. Generally, most liquids and gases are also, bad conductors.

1. Demonstrative Experiments On Conduction Of Heat Describe a demonstrative experiment to show that water is a bad conductor of heat.

Apparatus needed: A glass test tube, some water, a small piece of ice, some iron wire, a test tube holder, bunsen burner.

Procedure: Let us take a small piece of ice and wrap it with iron wire such that when dropped inside a test tube filled with water, it sinks to the bottom of the test tube.

Now, by holding the test tube in a tilted position with the help of a test tube holder, the upper layer of water in the test tube is strongly heated till it starts boiling.

Observation: Even when the upper layer of water starts boiling, it is found that the piece of ice wrapped with iron wire does not melt.

Inference: This shows that water is a bad conductor of heat. Because when the heat energy was supplied to the upper layer of water, the water in the upper part of the test tube started boiling.

But the heat energy was not transferred to the lower part of the test tube. So the ice did not melt.

WBBSE Class 8 Science practice long answer questions on heat

Question 7. Describe a demonstrative experiment to show that the volume of ice decreases as the ice melts, but the volume of wax increases as it melts. What is the value of latent heat of vapourization of steam in an SI system?
Answer:

Apparatus needed: Two glass test tubes, some water, some wax, a small piece of ice, a small piece of wax, Bunsen burner.

Procedure: Let us take two test tubes – one is % filled with wax and the other is filled with ice.

Both the test tubes are then heated separately ‘to completely melt the wax and ice. Now some solid wax is carefully dropped into the test tube containing liquid wax and some solid ice is dropped into the test tube containing water.

Observation: It is found that the wax sinks In the molten wax and the ice floats on water.

Inference: The solid wax sinks in the molten wax. This confirms that solid wax has a higher density than molten wax (liquid state).

Liquid wax has less density because the volume of wax increases as solid wax melts. But ice floats on water.

This proves that the density of ice is less than that of water. This is because the volume of water decreases as it freezes to ice.

The latent heat of vapourization of steam as per the SI system is 2260000 J/kg.

Question 8. Write short notes on Sea breeze and Land breeze.
Answer:

Sea Breeze And Land Breeze

Water has high specific heat than that land masses. So, during day time, absorption of heat by water will produce less temperature increase compared to the land masses.

As a result, the air above the warmer land mass becomes more warm compared to air over the sea. So, the air over the land moves up and creates a partial vacuum.

Relatively colder air from the sea moves toward the land to fill up the vacuum. This flow of air from the sea towards land is called sea breeze.

After sunset, both land and sea will radiate heat and become cooler. But due to the low specific heat of land mass, the rate of decrease of temperature of the land mass will be much more than that of seawater (having higher specific heat).

So at night, the land mass becomes colder than seawater. Now, the air over the sea will become warmer and moves upwards, creating a partial vacuum.

This is filled up by the movement of colder air from the land. This flow of air from land toward the sea is called the land breeze.

3. In a refrigerator the cooling unit is placed at the top, Why?
The reason is that air in contact with the cooling unit is cooled and becomes heavier. So it moves to a lower region.

To fill the vacuum formed near the said unit, warmer air from other parts of the refrigerator moves up. This air also cools and moves downwards. Thus the whole refrigerator is cooled.

Had the cooling unit been kept at the bottom, a small quantity of air would have been cool. This air being heavier would have remained at the bottom.

The warmer and lighter air would never come down near the cooling unit. Thus there would have been little and partial cooling inside the refrigerator,

WBBSE Chapter 1 exercises long answer solutions on heat

Question 9. In a refrigerator, the cooling unit is placed at the top, not at the bottom. Why? Calculate the amount of heat required to raise the temperature of 150g of water from 30°C to its normal boiling point.
Answer:

In a refrigerator, the cooling unit is placed at the top, not at the bottom.

H = m.s. (t2 – t2)

Here, m = 150g, s = 1 cal/g.°C, (t2 – t1)

= (100 – 30) °C = 70°C

The heat required, H = 150 x 1×70 = 10, 500 cal.

Question 10. What are the properties of the heat transmitted by the radiation process?

Uses Of Bad Conductors Of Heat

1. Handles of cooking utensils are generally made up of or covered with bad conductors such as wood, cane, and bakelite so that the utensil can be held by the handle with our hands even when the rest of the utensil is very hot.

2. In winter, it feels warmer and more comfortable, if we wear two garments made up of thin threads rather than one made up of thick threads, the total thickness of the garments being the same in both cases.

This is because when we wear two garments, the air is trapped between the two. Air being a bad conductor of heat prevents the outflow of heat from our body to the colder surroundings.

So two shirts, one above the other, give more warmth than a single shirt of thickness equal to that of the two together.

2. Woollen clothes are more comfortable in winter. This is because wool itself is a bad conductor of heat. It has innumerable tiny pockets of air enclosed within its fibers.

Air is also a bad conductor of heat. So wool and air pockets together prevent the outflow of heat from our body to the colder surroundings.

4. Ice blocks are covered with sawdust. Saw dust is a bad conductor of heat. It prevents the easy flow of heat from the hotter surroundings to the ice blocks. Thus melting of ice is partly avoided.

5. Ice is a bad conductor of heat. So igloos are built with ice blocks. When the temperature of the surroundings is much below zero degrees centigrade, it is warmer inside the igloos.

6. Building materials should be bad conductors of heat. Hay and mud are bad conductors of heat. So mud houses or houses made up of hay are warmer in winter while colder in the summer.

7. ln winter, birds fluff their feathers. This is because when birds bulge out feathers, layers of air are trapped within the feathers. These layers of air prevent the outflow of heat from their body to the colder surrounding.

8. Table mats are made of poor conducting materials like rubber, spun jute, etc. This is because when hot utensils are placed on it, heat from the hot utensils cannot pass on easily through the mat, and the surface of the wooden table is not damaged.

9. In winter, a new cotton quilt is more comfortable than an old one. In a new quilt, cotton fibers remain in a loose state with plenty of air pockets in between.

Air being a poor conductor of heat prevents the flow of heat through it. But in an old quilt, fibers are in a compressed state, and the quantity of trapped air within the fibers is small. So, it is less effective in preventing heat flow across it.

10. Water is a bad conductor of heat. So the upper surface of the water of a pond is warmer in summer than its lower surface.

In winter, the situation is reversed. The upper layer of water in the pond is colder compared to its bottom layers.

11. A thick glass tumbler is cracked when hot water is poured on it. This is because glass is a poor conductor of heat.

So, when hot water is poured into it, the part of the glass in direct contact with the hot water is heated and it expands.

But glass is a poor conductor, the heat is not evenly distributed throughout the tumbler. So the heated portion (in direct contact with hot water) expands more than the portion which is not in direct contact with hot water. Due to uneven expansion of a different portion of the same tumbler, it cracks.

Question 11. Which one is preferable as building material in or hay and mud? Why? Explain how all processes of heat conduction are prevented in a thermos flask.
Answer:

Building materials should be bad conductors of heat.

Hay and mud, being bad conductors of heat, make the house warm in winter and cool in summer. Tin, on the other hand, is a good conductor of heat and it allows outside heat to come in during summer and also allows inside heat to go out during winter.

Thus a house made of tin is not comfortable. The different forms of heat loss are minimized in thermos flasks in the following ways:

Conduction loss: Since there is a vacuum between the two walls, heat cannot be conducted by means of conduction. Further, the cork cap and glass wool (insulator) prevent the conduction of heat.

1. Convection loss: Vacuum also prevents convection loss of heat.
2. Radiation loss: The shining glass minimizes radiation loss.

Question 12. Mention one difference between sensible heat and latent heat.
Answer:

Difference between sensible heat and latent heat

lOg of ice at 0°C is melted to water at 0°C. The temperature of the water is further increased to 10°C. Calculate the total amount of heat required in the process.

Sensible heat is expended to raise the temperature of a substance. Latent heat is expended to bring about a change in the molecular structure of a substance to facilitate a change of state without altering the temperature of the substance.

The Total Heat Required Is Calculated In Two Steps:

1. Melting of ice at 0°C:

H1 = mL Here, m = lOg of ice L = 80 cal/g.

∴H1 = 10 x 80 = 800 Cal.

(ii) Raising the temperature of water:

H2 = ms (t2-t1) Here, m = 1Og of water (at 0°C) s = 1 cal/g °C

(t2-t1) = 10°C H2 = 10x1x10 = 100 cal.

∴H1 + H2 = 800 + 100 = 900 cal of heat.

Question 13. State the differences between evaporation and boiling.
Answer:

Differences between evaporation and boiling

Examples of long answer questions about heat for Class 8

Question 14. Compare Conduction, Convection, and Radiation.
Answer:

Comparison Of Conduction, Convection, and Radiation

Question 15. Calculate the amount of heat required to raise the temperature of a copper vessel of mass 1OOg from 50°C to 90°C.
Answer:

(Given, the specific heat of copper = 0.09 cal/g. °C). During the heating of water, the burner is placed at the top of the vessel containing water. It is found that heating has not occurred properly. Why?

Numerical problem: Do it yourself.

Water is heated by convection currents in which hot water rises and cold water sinks again and again till the entire mass is heated.

the water to be heated. Convection does not occur if water is heated at the top. If water taken in the vessel is heated at the top, then the hot water (being lighter) stays at the top of the vessel.

As a result, the water at the bottom of the vessel remains cold in such a case since no hot water can sink from the top to down below towards the bottom of the vessel. Hence, heating does not occur uniformly.

Chapter 1 Physical Environment Short Answer Type Questions

Question 1. What is meant by specific heat? What is its unit in CGS and SI unit? The specific heat of copper is 0.09 cal/g.°C. What do you mean by that?
Answer:

Specific Heat:-

Specific heat of a substance is the heat required to raise the temperature of the unit mass of the substance by 1°C.

So, a unit of “s” is calorie per gram per degree centigrade (cal/g.°C) (in CGS unit) and Joule per kilogram per degree centigrade (j/kg.°C) (in SI unit).

The specific heat of copper is 0.09 cal/g.°C. This means that to raise the temperature of 1 gram of copper by 1°C, 0.09 calorie heat energy is required.

Question 2. What do you mean by latent heat?
Answer:

Latent Heat:-

When a substance undergoes a change of state without changing temperature it either absorbs or gives out a certain quantity of heat. Latent heat is the quantity of heat absorbed or liberated by a substance of unit mass when it undergoes a change of state at a particular temperature.

Question 3. The latent heat of the fusion of ice is 80 cal/g. What do you mean by that?
Answer:

Given That 

The latent heat of the fusion of ice is 80 cal/g.

This means the amount of heat required to effect the change of state of 1 g of ice at OX and 1 atmospheric pressure to lg of liquid water at OX and 1 atmospheric pressure is 80 calories.

WBBSE Class 8 Heat short answer questions

Question 4. What do you mean by the melting point and boiling point & of a substance?
Answer:

Melting Point And Boiling Point Of A Substance

The melting point of a pure substance is the temperature at which the substance undergoes a change of state from solid to liquid under a particular pressure.

For example, at 1 atm pressure, the melting point of ice is 0°C. The ice melts at this condition and the temperature remains constant till all the ice melts.

The boiling point of a pure substance is the temperature at which the substance undergoes a change of state from liquid to gas at a particular pressure.

For example, at 1 atm pressure, the boiling point of pure water is 100°C. Water starts boiling at this condition and the temperature remains constant till all the water vaporizes.

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

Question 5. When two pieces of ice are pressed together for some time and then released, they form a single lump. Why?
Answer:

This is because when pressure is applied, then at the contact area between two pieces of ice, the melting point decreases.

So some ice melts. When pressure is released, the melting point again returns to the original value. So the water formed at the contact area again freezes, forming a single lump of ice.

Question 6. Discuss briefly the effect of the presence of an impurity in the substance on its melting point. The presence of impurities or presence of any other substance decreases the melting point of the substance.
Answer:

For example, the melting point of ice is 0eC at normal pressure. But if some salt is added, its melting point becomes much lower than 0°C.

Again, a mixture of metals (called alloys) has a melting point lower than the melting point of any of its constituents.

[For example The melting point of a mixture of 87% (by weight lead in antimony is 246X, while the melting point of pure lead is 327X and that of pure antimony is 631X.]

Question 7. In summer, when we perspire heavily, it feels comfortable if we stand beneath a fan. Why? In summer, when we perspire heavily, it feels comfortable if we stand beneath a fan.
Answer:

This is because the airflow produced by the revolving fan facilitates the process of evaporation. When sweat evaporates from our body it takes up heat from our body. As a result, the body cools down, making us feel comfortable.

Question 8. In summer dogs are very often seen panting. Why? The body of the dog is covered with furs. So, sweat is not produced on their body. They feel comfortable by hanging out their tongues and keep panting.
Answer: 

Due to this the liquid on top of the tongue evaporates, taking up heat from the body, thus bringing down the body temperature.

Short answer questions on heat for Class 8

Question 9. During cooking, the cooking utensil is generally covered with a lid. Why?
Answer: When covered with a lid, the vapor produced in the utensil cannot escape easily, and increases the pressure over the liquid surface.

We know that as pressure increases, the boiling point increases. So, water boils at a higher temperature (greater than 100X). So the food materials get properly boiled and cooked faster.

Question 10. What do you mean by “freezing mixture”? When salt is mixed with ice, its temperature decreases. This mixture is called a freezing mixture.
Answer: 

When ice and common salt are mixed in a 3:1 weight ratio, the temperature becomes approximately – 23°C. This is a freezing mixture.

This is frequently used for the preservation of fish, meat, etc. and to carry medicine at low temperatures from one place to another place.

Question 11. How the cloud is formed?
Answer:

Formation Of Cloud:

The water that evaporates from sea, ocean, river, lakes, etc. mixes with hot air. Air containing water vapor is lighter than “dry” air (i.e., having much less water vapor).

So, it goes upwards towards a higher altitude. But with increasing altitude, the air pressure reduces. The air cools down and the water vapor of the air condenses as water droplets on the tiny dust particles floating in the air, forming clouds.

Question 12. What do you mean by good conductor and bad conductor of heat? Give examples.
Answer:

Good Conductor And Bad Conductor Of Heat

The substitute which heat can flow easily is called the educator of heat

The substance rough in which heat cannot flow easily is called a bad conductor of heat Generally metals are good conductors of heat. For example, copper, and steel.

Substances like wool, stone, glass, wood, water, air, diamond, paper, etc. are bad conductors of heat. Generally, most liquids and gases are also bad conductors.

Question 13. Explain the reason(s).
Answer:

1. Handles of cooking utensils are generally made up of bad conductors.
2. In winter, wearing two shirts is more comfortable than wearing a single shirt made up of thick threads.
3. Woolen clothes are more comfortable in winter.
4. Ice blocks are covered with sawdust.
5. Igloos are made of ice.
6. In winter, birds fluff their feathers.
7. table mats are made of poor conducting materials.

WBBSE Chapter 1 heat short answer solutions

Question 2. In winter, a new cotton quilt is more comfortable than an old one.
Answer:

 A thick glass tumbler cracks when hot water is poured on it.

Handles of cooking utensils are generally made up of or covered with bad conductors such as wood, cane, and bakelite so that the utensil can be held by the handle with our hands even when the rest of the utensil is very hot.

In winter, it feels warmer and more comfortable, if we wear two clothes made up of thin threads rather than one cloth made up of thick threads, the total thickness of the clothes being the same in both cases.

This is because when we wear two clothes, the air is trapped between the two. Air being a bad conductor of heat prevents the outflow of heat from our body to the colder surroundings.

So two shirts, one above the other, give more warmth than a single shirt of thickness equal to that of the two together.

Question 3. Woolen clothes are more comfortable in winter. This is because wool itself is a bad conductor of heat. It has innumerable tiny pockets of air enclosed within its fibers. Air is also a bad conductor of heat.
Answer: 

So wool and air pockets together prevent the outflow of heat from our body to the colder surroundings.

Question 4. Ice blocks are covered with sawdust. Saw dust is a bad conductor of heat.
Answer:

It prevents the easy flow of heat from the hotter surroundings to the ice blocks. Thus melting of ice is delayed.

Question 5. Ice is a bad conductor of heat. So igloos are built with ice blocks.
Answer:

When the temperature of the surroundings is much below zero degrees centigrade it is warmer inside the igloos.

Question 6. In winter, birds fluff their feathers.
Answer:

This is because when birds bulge out feathers, layers of air are trapped within the feathers. These layers of air prevent the outflow of heat from their body to the colder surrounding.

Table mats are made of poor-conducting materials like rubber, spun jute, etc. This is because when hot utensils are placed on it,

the heat from the hot utensils cannot pass on easily through the mat, and the surface of the wooden table is not damaged.

Heat concepts summary with short answers for Class 8

Question 7. In winter, a new cotton quilt is more comfortable than an old one.
Answer:

In a new quilt cotton fibers remain in a loose state with plenty of air pockets in between.

Air being a poor conductor of heat prevents the flow of heat through it. But in an old quilt, fibers are in a compressed state, and the quantity of trapped air within the fibers is small. So, it is less effective in preventing heat flow across it.

A thick glass tumbler is cracked when hot water is poured into it. This is because glass is a poor conductor of heat.

So, when hot water is poured into it, the part of the glass in direct contact with the hot water is heated and it expands.

But glass is a poor conductor, the heat is not evenly distributed evenly throughout the tumbler. So the heated portion (in direct contact with hot water) expands more than the portion which is not in direct contact with hot water.

Due to the uneven expansion of different portions of the same tumbler, it cracks.

Question 14. Explain why fuse wire is made up of lead and tin.
Answer:

The melting point of a mixture of lead and tin is lower than the melting point of either of them.

When excess current flows through this wire, heat is produced and the wire melts easily at lower temperatures, thereby preventing further passage of electricity through it and thus protecting the costly and important electrical instruments and gadgets.

Question 15. During winter, we must not ignite a kerosene lamp or fire inside a confined room-explain why?
Answer:

During winter, if some kerosene lamp or fire is ignited inside a confined room, then it may prove fatal to sleep inside that room.

Since kerosene lamp or fire is burning, oxygen is continuously converted to carbon dioxide and poisonous carbon monoxide.

Since it is winter, doors, and windows are tightly shut, which prevents fresh air from entering the room.

Gradually the concentration of oxygen decreases and the concentration of poisonous carbon monoxide gas within the room increases and it may eventually kill a human being if exposed to this poisonous gas for a longer time.

WBBSE Class 8 Science practice short answer questions on heat

Question 16. What is trade wind?
Answer:

Trade Wind:-

The equatorial regions which receive sun rays directly are very hot compared to the polar regions, where the sun rays fall obliquely.

The hot air of the equatorial zones being lighter rises up and the colder air from the polar regions fills this gap. This flow of air from the polar region to the equatorial region is called trade winds.

Question 17. Eagles can fly at a high altitude without flapping their wings. Why?
Answer:

This is possible because if they are flying over the rising convection currents of air, which is moving upwards, then this lends support to float and they can remain floated without flapping their wings.

Question 18. Explain with a reason(s).
Answer:

1. In summer we prefer to wear white clothes.
2. Cloudy nights are hotter than cloudless nights.
3. The outer surface at bottom of the cooking utensils is coated black.

In summer we prefer to wear white clothes. Since the white surface is a good reflector and a poor absorber of heat,

the white clothes we wear reflect most of the radiant heat of the sun and so we are relieved of intense heat.

Cloudy nights are hotter than cloudless nights. The reason is, after sunset, the heated surface of the earth begins to cool down by radiating heat.

If there is a cloud, the heat radiated by the earth is reflected by the clouds back to the earth. So the earth remains warm.

On a cloudless night, the earth is cooled by radiating the absorbed heat (absorbed during day-time) during the night without any chance of being reflected back to the earth.

The outer surface at bottom of the cooking utensils is coated black. The reason is that a blackened surface absorbs heat very well and so cooking is done more quickly with such a utensil than with a new one with a polished outer surface.

Examples of heat short answer questions for Class 8

Question 19. Give examples of two instances in our day-to-day life where heat is transferred by radiation through the air. Instances of radiation in our daily life
Answer:

When we sit in front of a room heater, we feel the heat radiated out by the heater. When we come out in the sunshine, we feel hot due to solar radiation.

When we stand next to a burning fire, we can feel the heat of the fire in our bodies. This heat is transferred from the fire to our body through radiation.

Question 20. One end of the objects like a steel spoon, a plastic scale, and a divider is put in a beaker of hot water. In which of these objects the other end will get hot?
Answer:

Steel spoons and dividers are made of metals which are good conductors of heat Hence they conduct heat quickly and as a result, the other ends get hot within a short span of time.

Since a plastic scale is a bad conductor of heat, it does not conduct heat rapidly and hence, it heats up very slowly.

Chapter 1 Physical Environment Answer in One or Two sentences

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

1 Calorie:-

This is the amount of heat required to increase the temperature of g of pure water by 1°C.

Question 2. What is meant by the specific heat of a substance?
Answer:

Specific heat of a substance:-

Specific heat of a substance is defined as the amount of heat required to raise the temperature of a unit mass of substance byl°C.

Question 3. What is the unit of specific heat in the SI unit?
Answer:

Specific heat of a substance:

The unit of specific heat is Joule per kilogram per degree centigrade (J/kg °C).

Question 4. Name two substances that do not have fixed melting points.
Answer:

The substances which do not have fixed melting points are wax, butter, etc.

Question 5. Give an example of one substance that increases in volume when it melts.
Answer:

When solid wax melts, its volume increases.

WBBSE Class 8 Heat concise answer questions

Question 6. What happens to the melting point of ice when pressure is increased?
Answer:

When pressure is increased, the melting point of ice is decreased.

Question 7. What do you mean by freezing mixture? When salt is mixed with ice, its temperature decreases.
Answer:

Freezing Mixture:-

This mixture is called a freezing mixture. For example, when ice and common salt is mixed in a 3:1 weight ratio, the temperature becomes approximately – 23°C. This is a freezing mixture.

Question 8. What happens to the melting point of wax if pressure is increased?
Answer:

The melting point of wax increases when pressure is increased.

Question 9. What do you mean by the latent heat of vaporization?
Answer:

latent heat of vaporization:-

Latent heat of vaporization is the quantity of heat absorbed by a substance of unit mass when it undergoes a change of state from liquid to vapor at a particular temperature.

Question 10. What do you mean by the boiling point of a liquid?
Answer:

The boiling point of a liquid:-

At a particular pressure boiling point of a liquid is the temperature at which the boiling starts and the temperature remains constant till all the liquid is vaporized.

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

Question 11. If some salt is dissolved in water, how it will affect the boiling point of water?
Answer:

When some salt is dissolved in water, its boiling point will increase.

Question 12. What should be the characteristic property of a fuse wire and why?
Answer:

The fuse wire should be made up of a low-melting solid. When a large amount of electricity suddenly flows through it,

heat is produced and the wire melts easily at lower temperatures, thereby preventing further passage of electricity through it and thus protecting the costly and important electrical instruments and gadgets.

Question 13. What is the process of transfer of heat within a solid substance?
Answer:

Heat is transferred within a solid substance from one part to another by conduction.

Question 14. How heat is transferred from the sun to the earth?
Answer:

Heat is transferred from the sun to the earth by radiation.

Very short answer questions on heat for Class 8

Question 15. Among the different processes for the transfer of heat, which one is the fastest?
Answer:

Radiation is the fastest of the three processes by which heat is transferred.

Question 16. In the thermos-flask, the region between two glass walls is evacuated. Why?
Answer:

The region between two glass walls is evacuated, this means that there is no medium.

That way, the chances of loss of heat from inside the flask to the outside are minimized by eliminating the possibility of heat transfer by conduction and convection. Both these processes require a medium for the transfer of heat.

Question 17. What are the magnitude of the latent heat of the vaporization of water (at 100°C and 1 atmospheric pressure) and the latent heat of fusion of ice (at 0°C and 1 atmospheric pressure)?
Answer:

The latent heat of the vaporization of water is 537 calories per gram per degree centigrade and the latent heat of the fusion of ice is 80 calories per gram per degree centigrade (at 0°C and 1 atmospheric pressure).

Question 18. Ame is two substances that do not melt when they are heated.
Answer:

Iodine and camphor are the two solid substances (at room temperature) which, when heated, are directly converted to vapor i.e. undergo sublimation without melting.

WBBSE Class 8 Science practice concise answers questions on heat

Question 19. Compute the temperature rise that would take place when 100 cal of heat is supplied to 5g of water.
Answer:

We know, H = m x s x (t2-t1).

Here, H = 100 cal, m = 5g, s = 1 cal/g. °C.

∴t2-t1=H/m xs =100/5×1=20°c

∴Rise in temperature = 20°C.

Question 20. What happens due to the exchange of latent heat?
Answer:

Change of state of matter without temperature change.

Question 21. In which type of heat transfer method particles of the medium are displaced?
Answer:

Convection (by establishing convection current).

Question 22. If Y cal of heat is required to raise the temperature of Xg of mass by 1°C, then what is the specific heat of the material?
Answer:

From H = m x s x t, we have s = H/mxt where t=(t2-t1)

Putting the values of H = Y cal, m =Xg and t = 1°C,

Specific heat, S= Y/x.1=y/x

WBBSE Chapter 1 heat quick answer solutions

Question 23. Among Digha and Darjeeling, where is the boiling point of water lower?
Answer:

The boiling point of water is lower in Darjeeling since it is a hill station.

Question 24. Which physical quantity determines the flow of heat energy?
Answer:

Temperature of a substance.

Question 25. A metal spoon is placed in one of two identical cups of hot coffee. Why does the cup with the spoon have a lower temperature after a few minutes?
Answer:

Heat energy is removed from coffee mostly by conduction through the spoon. This cannot happen in the other cup.

Heat concepts summary with very short answers for Class 8

Question 26. On a bright sunny day, a boy standing on a seashore is flying a kite. In which direction will the kite fly?
Answer:

The hot air of land will take the kite towards the sea.

Question 27. Which of the following is a better reflector of heat and which one is a better absorber of heat? 1. Dull black surface 2. Shiny white surface.

Answer:

1. Dull black surface: better absorber of heat
2. Shiny white surface: better reflector of heat.