WBBSE Notes For Class 8 School Science Chapter 3 Some Common Gases

Chapter 3 Some Common Gases Some Equipment Used In the Laboratory

Experiments are the most important part of the study of science. Any theory must be verified by experiments and theories are developed on the basis of experiments.

Experiments are probably the only tool available in our hands by which we can understand the underlying logic of various naturally observed phenomena.

Experiments are commonly carried out in laboratories, which need to be equipped properly to carry out various hands-on experiments.

On school premises also, you will find a separate room meant for doing different experiments. In any laboratory, you will find some basic instruments and apparatus, with which experiments related to chemistry can be carried out. Below we will briefly describe these basic instruments and apparatus.

 

Thermometer

A common mercury thermometer is used in laboratories to measure the temperature of any object under investigation. The scale of the thermometer may be in Centigrade (in which the freezing point of water at 1 atmospheric pressure is 0°C and the boiling point of water at 1 atmospheric pressure is 100°C)

Or in Fahrenheit (in which the freezing point of water at 1 atmospheric pressure is 32°F and the boiling point of water at 1 atmospheric pressure is 212°F). The range of temperature is usually between 0°C to 100°C or may be from 0°C to 200°C.

For temperatures higher and lower than this, thermometers with a suitable range are used. In sophisticated laboratories, electronic apparatus is used to measure the temperature of an object.

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Electrical Cell

Electrical cells are used as a source of electricity. Generally, dry cells, which are better known as batteries, are used.

More than one battery can be connected to meet the requirement for higher energy. During the combination of batteries, the “+” end of one battery must be in contact with the end of another battery, or vice versa.

Otherwise no current will flow. When the batteries are connected to any electrical circuit, the proper ends of the battery must be connected.

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Switch

Switches are an important part of an electrical circuit. With this the flow of electricity through an electrical circuit can be started or stopped, as and when required.

(We can find switches in our home also, with which we can switch on or off any electrical equipment such as light, fan, television, etc.).

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Wires

Different electrical connections are done using copper wires insulated with polymers like PVC (polyvinyl chloride).

PVCs have good insulating properties, so live wires are covered with them for safety. Long wires are cut to pieces to get wires of suitable lengths required for a particular circuit.

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Bulb

To check whether electricity is flowing through a circuit or not, the easiest way is to connect a bulb to the circuit. If the bulb glows,

when an electric current is switched on, then it confirms that electricity is flowing through the circuit.

Otherwise, it will not glow. Generally, small bulbs such as LEDs, are preferably used nowadays. They have a long life and they do not get fused during repeated use.

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Chemical Balance

This is an instrument by which we can weigh any object very accurately. It is also very sensitive (i.e. it can detect even a very small difference in mass as low as 0.1 mg or lower).

These balances are accurate and give the same result in successive weighing. The upper limit of a chemical balance is generally 100 g or 200 g.

Generally, there are two pans in it – on the left pan the sample to be weighed is placed and on the right pan required weights are placed. Forceps are used to place weights on the pan or to remove from there.

This is because oils, dust or dirt present in our hands may alter the exact masses of the weights. Nowadays in advanced laboratories, electronic balance is used which can detect very small weights with utmost accuracy.

(You must have seen similar balances in some grocery shops, but those are not equipped to measure very small weights, but they can weigh up to a few kilograms of substance.)

 

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Clamp and stand

Iron stands with a heavy base and clamps are used to hold something, particularly glass apparatus such as a burette, separating funnel, condensers etc. during an experiment.

Nowadays, Teflon-coated stands and clamps are available in modern laboratories. The Teflon coatings prevent rusting and ensure longer life.

 

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Bunsen burner and spirit lamps

Bunsen burner is used widely in most laboratories for attaining the moderately high temperatures required during any chemical reaction. Here, LPG (liquefied petroleum gas) is ignited to produce flames.

A regulator is there near the base which is used to regulate the flow of air to increase (or decrease) the temperature. Generally, the maximum temperature is attained by adjusting the regulator so as to allow more air than is required to produce a non-luminous flame.

(But too much air produces a “noisy” flame, not suitable for the experimental purpose). Spirit lamps are also frequently used for carrying out small and simple experiments.

 

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Various Glass Apparatus Used In Laboratory

All the above apparatus are made of glass. (Nowadays “plastic” apparatus made of polymer are being used in modern laboratories).

In order to avoid the introduction of impurities during the analysis or synthesis of any substance, borosilicate glass (or Pyrex glass) is preferred over ordinary soda glass.

They are least affected by acidic or alkaline solutions. Generally, a glass apparatus should not be heated in a naked flame.

 

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Wire gauze with an asbestos centre should be placed between the glass apparatus and the flame. For special purposes, heat-resistant high silica glasses can be used.

Glass containers of different sizes and shapes are used as per requirement. Test tubes are the most common glass apparatus used in laboratories. It is a glass tube with one open end.

Its wall may be thin (thin glass test tube) or thick (hard glass test tube). Hard glass test tubes are sometimes held directly over the flame during some experiments.

Beakers are cylindrical glass apparatuses with a spout (a squeezed side at its mouth). The presence of a spout makes pouring liquid from it convenient.

 

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Also, when the beaker is covered with an ordinary watch glass, it acts as an outlet for any gas evolving from the beaker during a chemical reaction.

Round bottom flasks are round bottom, narrow neck glass containers, most suitable when heating of reaction mixture is required during any chemical reaction.

The round-shaped bottom provides more surface area through which heat can enter and the solution within it is efficiently heated.

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Conical flasks (or Erlenmeyer’s flasks) are cone-shaped glass vessels with small, narrow necks.

Conical flasks of the capacity of 100 mL, 250 mL and 500 mL are frequently used in laboratories.

Woulfe’s bottle is a glass bottle with double mouths. It is usually used for the preparation of gases (such as hydrogen).

Through one mouth, chemical substances (reactants) are introduced into the bottle and the gas produced during the chemical reaction comes out through the other mouth.

 

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This mouth is connected by a bent glass tube the other end of which is introduced to another larger, cylindrical jar, called a gas jar, in which the liberated gas is collected and stored for future use.

Watch glass is like a small, circular, glass plate used for carrying/testing a small amount of solid or liquid substances.

 

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Funnel

It is a cone-shaped glass apparatus fitted with a stem (narrow glass tube) at its bottom.

A funnel is used for transferring liquids or solids from one container to the other (for example a solution of acid is transferred from a glass bottle to a measuring cylinder).

 

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Test Tube Holder And Test Tube Rack

A test tube holder is used to hold a test tube (containing any solution) with thin metallic tongs during heating. This way direct contact between the hand and the test tube is avoided.

Test tube racks are meant for placing the test tubes containing solids or liquids vertically. The bottom of a test tube is round, so it cannot stand on its own. Test tube racks are made up of wood or plastic.

 

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Measuring Cylinder

A measuring cylinder is used to measure the volume of liquid. It is a cylindrical tube, and on its wall markings are there, from which we can estimate the volume of any liquid poured into it.

Measuring cylinders of different capacities (say 50 mL, 100 mL, etc.) are frequently used. Tripod stand and wire gauze

For heating, a solution is placed in a glass apparatus (say a beaker or a conical flask), and a tripod stand made of pig iron is used.

A wire gauge (preferably asbestos centred) is placed on the tripod stand and the glass apparatus is placed on it.

The Bunsen burner is placed below the wire gauze. Wire gauze evenly distributes the heat coming from the flame throughout its surface.

 

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Pipette And Burette

Burettes and pipettes are used for transferring a definite volume of liquid from one container to other. They are frequently used during titration experiments.

Burettes are long, cylindrical tubes of uniform bore throughout the graduated length. A burette is closed at the bottom by means of a glass stopcock.

Burettes are usually of 50 mL capacity and graduate to the tenths of a millilitre. Burettes of another capacity (say 25 mL, 10 mL, etc.) are also used. It is used to deliver variable volumes of liquid.

Pipette consists of a cylindrical bulb joined at both ends to narrow tubes. This type of pipette is commonly known as a transfer pipette.

It has only one mark and it is used to deliver a small, definite volume of liquid. (Another type of pipette known as graduated pipette is also available where the stems are graduated and are used to deliver various small volumes of liquid by using a single pipette.)

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Filter Paper

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

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

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

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

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Oxygen

Nitrogen and oxygen are the main components of the atmosphere by volume.

Oxygen is exchanged between the atmosphere and living species through photosynthesis and respiration.

During photosynthesis, oxygen is produced along with the formation of glucose from carbon dioxide and water in presence of sunlight.

Respiration is the opposite process of photosynthesis.

Here, oxygen is combined with glucose producing water and carbon dioxide and energy is released which sustains all the physicochemical processes occurring within the living bodies.

Scientists believe that the earth was created approximately 4500 million years ago, and life first emerged on earth about 3500 million years ago.

For the first two billion years after the earth was formed, its atmosphere was anoxic (i.e. free from oxygen).

About 2.5 billion years ago oxygen in the atmosphere increased from a trace amount to approximately 1% of the earth’s atmosphere.

This was probably due to a type of bacteria, called cyanobacteria, the first organism capable of producing oxygen through photosynthesis.

In presence of sunlight and with the help of a special protein, they started producing oxygen by splitting water. The level of oxygen in the atmosphere slowly started rising.

Cyanobacteria are found even today in wetlands, ponds and paddy fields. Then came the green algae and other plants.

They too could produce oxygen. The level of oxygen in the atmosphere started rising very slowly.

Another jump in the level of oxygen in the earth’s atmosphere took place approximately 500 million years ago.

The level of oxygen increased to levels closer to the composition of the atmosphere which is found today.

(However, scientists are still not sure what exactly caused this second jump in oxygen level at that time).

Oxygen is essential for almost all living organisms found on earth. It is utilized during respiration.

During respiration, energy is liberated from food kinds of stuff like glucose within the living cells.

The liberated energy is utilized to maintain all types of physicochemical processes occurring within the body of a living organism, necessary to remain alive.

Respiration is a very efficient process for producing energy from intracellular glucose.

But we should point out that not all living beings are dependent on oxygen. There are places on earth where oxygen is not available.

Into the depth of the marshes and in the sludge of urban sewers, where oxygen is not available, some life forms are available.

These are called obligate anaerobes.

The mechanism of production of energy intracellularly is different from ours. (In feet they will die if they come in contact with oxygen).

But oxygen can also produce harmful chemicals like hydrogen peroxide and superoxides, which can cause irreparable damage to the living cells even when present in minute amounts.

These chemicals can damage DNA. Some scientists have linked them with the process of ageing in living organisms.

They are of the opinion that free radicals like superoxides trigger and increase cell death mechanisms within the body.

Fortunately, there are certain enzymes present in our body which can specifically destroy such harmful species.

For example, the enzyme, catalase, can destroy hydrogen peroxide and form oxygen and water.

\(2 \mathrm{H}_2 \mathrm{O}_2 \rightarrow 2 \mathrm{H}_2 \mathrm{O}+\mathrm{O}_2\)

Oxygen has also found major applications in several industrial processes. We will discuss them later.
Let us now discuss the various methods for the preparation of oxygen in the laboratory.

 

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