Composition of Air and Experimental Findings

Introduction:

Air is a homogeneous mixture of gases. The Earth is surrounded by a thick blanket of air called the atmosphere.

The planet Earth has endowed us with the necessities of life, like air, water, food, etc. Among these, the air is the most vital for life, and hence, indispensable. Man can live for several days without food or water, but he will die in a few minutes without air. Heat is the form of energy without which man cannot exist on Earth. Air is also present in water in a dissolved state. The dissolved air is very important for the survival of marine life. Air is used for respiration by all kinds of living beings.

Since all natural processes are connected to air, we cannot imagine life without air. Although it is invisible, the presence of air is felt in every activity in nature.

In ancient times, the air was considered an element. In 1774, Lavoisier showed that air was a mixture and the main gases in it were nitrogen and oxygen. Apart from these two gases, some other gases, like carbon dioxide, water vapor, and inert gases are also present in the air.

So let us learn about the air and its constituents responsible for life on the Earth in this session.

Explanation:

Air is considered a matter:

Air is matter, and hence, has mass and occupies volume. Because:

1. We can feel the air as we move the cycle. Moving air is called wind.
2. Air exerts pressure. This can be seen if we blow a balloon and allow the air to escape.
3. The air escapes with a swishing sound.
4. As air is a gaseous mixture, it can be compressed.

Activity: To show that the air occupies space

Procedure:

1. Take a glass bowl and fill it with water up to half.
2. Take an empty open bottle. Turn it upside down.
3. Sink the open mouth of the bottle into the container filled with water, as shown below.
4. Observe the bottle. Now tilt the bottle slightly. When the bottle is tilted, bubbles are seen in the water.

Observation:

The air was occupying space inside the tumbler, and on tilting, the air was displaced, and bubbles came out because of that.

Conclusion:

This activity shows that air occupies space.

 It fills all the space inside the bottle.

Activity: To show that air has mass

Procedure –

An inflated balloon is placed on one side of a simple scale, and the scale tilts towards one side.

Figure: Air has mass

Observation – The balloon has mass which makes the scale tilt.

Conclusion – This activity proves that air has mass.

Activity – To show air exerts pressure.

Procedure –

1. Take the empty can, fill it with water, and then heat the can.
2. After some time, when it starts to convert into steam, the cap should be placed tightly in such a way that steam cannot escape from the can.

Observations –

1. The steam condenses, and the pressure inside the can also reduce as the steam reduces.
2. The air out of the can exerts pressure and makes the can crumple.

Figure: air exerts pressure

Conclusion – This proves that air exerts pressure.

Activity – To show that air is highly compressible.

Procedure –

A leak-proof syringe, which is empty and filled with air, is pushed toward the end.

Observation –

On pressing the piston of the syringe, the piston is moved inwards.

The air inside the syringe has maximum molecular space and is highly compressible.

Conclusion – Air is highly compressible.

Composition of Air:

The study of air and its importance started back in the 15th century when air was considered an element. Later, in 1674, Mayo, for the first time, predicted the presence of two constituents in the air: the active component and the inactive component. Ever since then, the air has no longer been believed to be an element. In the later years, Lavoisier also found the ratio of air’s active and inactive components to be 1: 4 by volume.

The major components of air are oxygen and nitrogen.

Air also contains carbon dioxide, water vapor, and inert gases present in the atmosphere.

Apart from these gases, air also contains some gases like sulfur dioxide, hydrogen sulfide, and nitrogen dioxide. Some impurities like dust particles, smoke, and germs are also present.

Figure: Composition of air

Lavoisier’s experiment: Shows that nitrogen and oxygen are the main components of air:

Procedure:

1. Lavoisier took some mercury in a retort and heated it.
2. The other end of the retort was connected to a bell jar containing air.
3. The bell jar was kept in an inverted position over mercury contained in a trough, as shown in the figure below.

Figure: Lavoisier’s experiment

Observations:

1. A red layer of mercuric oxide was formed on the hot surface of the mercury in the retort.
2. The level of mercury in the bell jar rose by 1/5th of the total volume of the bell jar.

Conclusions:

1. 1/5^th portion of air in the bell jar was used up by mercury in the retort to form the red substance. This air was active air. This could be re-obtained by strongly re-heating the red substance. This gas supported burning better than air and also supported life. Lavoisier named this active air ‘oxygen’.

2. The 4/5^th air in the bell jar was tested by putting a burning candle into a gas. The flame was extinguished. It did not support combustion.

The remaining 4/5^th portion of air in the bell jar was inactive air as it did not support burning or life.

The gas is named azote, meaning unsuitable for life. Later on, Lavoisier named it ‘nitrogen’.

From the above, it is clear that air by volume contains 1/5^th of oxygen and 4/5^th of nitrogen.

I.e., nitrogen and oxygen are the two main gases present in the air in the ratio of 4:1 by volume.

The rest of the gases are present in a very small amounts.

Activity 1: To show that air contains oxygen (an active part) and nitrogen (an inactive part).

Apparatus required- A glass container, a cork, a glass jar, and candles.

Procedure:

1. Take a glass container and fix a candle at the center of the bottom.
2. Fill the container with some water.
3. Place an empty, inverted dry glass jar over it.
4. Mark five marks above the surface of water on the jar at equal distances as shown in the figure below.
5. Lift the jar, light the candle, and cover it with the inverted glass jar. Observe carefully.

Does the candle continue to burn or get extinguished? Does the level of water inside remain the same or not?

Observation:

1. After some time, it is seen that the candle is extinguished.
2. The water level is raised in the gas jar. The increased level of water is 1/5 of the part of the jar containing air.
3. This part is active air, i.e., oxygen, which helps the candle burn.
4. When it is used up, the candle starts burning. This 3/4^th part of air still present in the jar is inactive air, which does not support burning and is nitrogen.

Figure: Air contains oxygen and nitrogen.

Activity 2: To show that air contains carbon dioxide.

Apparatus required – A test tube, lime water, two outlets of tube

Procedure:

1. Take the test tube fitted with a rubber cork.
2. Fit the long-bent tube through one other bore.
3. Pour some freshly prepared lime water[calcium hydroxide solution, Ca(OH)₂] into the test tube.
4. Blow the air by an air pump through the long tube.

Figure: Air contains Carbon dioxide

Observations:

The air bubbles appear in the lime water, and after some time, the lime water will turn milky.

Conclusion:

Air which is pulled contains carbon dioxide, which turns lime water milky.

Do you know:

Lime water is used as a chemical test to identify carbon dioxide gas because two substances react to form insoluble white solid calcium carbonate (CaCO₃), which turns the lime water milky.

Activity 3: To show that water vapor is present in the air

Apparatus required – A clean glass tumbler, cube of ice

Procedure:

1. Take the glass tumbler and dry it from the outside.
2. Fill it in half with ice cubes and water.
3. Leave the tumbler untouched for some time.
4. Observe the sidewalls of the tumbler.

Observation:

You will observe that fine water droplets get deposited on the outer wall of the glass tumbler. These droplets have certainly not passed through the material of the glass tumbler from inside.

These water droplets must have come from the air. Due to the cold surface of the glass tumbler, the water vapor in the air gets condensed into water droplets.

Figure: Experiment to show air containing water vapor

Conclusion:  Water vapor condenses on the cooler surface of the beaker, which proves air contains water vapor.

Activity 4: To show that air contains dust particles

Procedure:

1. Find a sunny room in your home.
2. Close all the windows and door with curtains pulled down to make the room dark.
3. Now, open the window or door facing the Sun just a little allowing sunlight to enter the room only through that slit.
4. look carefully at the incoming beam of sunlight.

Figure: Observing the presence of dust in the air with sunlight

Observation:

The tiny shining particles move in the beam of sunlight.

Conclusion:

This shows that air also contains dust particles. The existence of dust particles in the air differs from time to time and from place to place.

Examples:

1. During winters, you might have observed a similar beam of sunlight filter through the trees in which dust particles appear to dance merrily around!
2. We inhale air when we breathe through our nostrils. Fine hair and mucus are present inside the nose to prevent dust particles from getting into the respiratory system.

Air is a Mixture:

The air around us is a mixture of gases; there are enough evidence to show that air is a mixture and not a compound.

Characteristics of Air as a Mixture:

1. The composition of air differs from place to place and from time to time.
2. Liquid air has no fixed boiling point.
3. Air has no definite chemical formula.
4. Air can be separated into various components by physical methods.
5. The properties of different gases are retained in the air.

6. Air does not exhibit definite chemical properties of its own.

Summary

• Air is found everywhere. We cannot see the air, but we can feel its presence. Air is the most vital thing in life, and hence, indispensable.
• Air occupies space. Air is considered a matter.
• In ancient times, the air was considered an element. In 1774, Lavoisier showed that air was a mixture, and the main gases in it were nitrogen and oxygen.
• Air also contains carbon dioxide, water vapor, dust particles, and inert gases present in the atmosphere.
• Apart from these gases, the air also contains some gases like sulfur dioxide, hydrogen sulfide, and nitrogen dioxide.
• There is enough evidence to show that air is a mixture and not a compound.