Chapter 15 Air Around Us

Air contains 21% of oxygen, 78% nitrogen, 0.03% carbon dioxide other gases like argon, dust particles, water vapours present in 0.01% quantity. Oxygen gas is very necessary for the respiration of organisms. Oxygen helps in burning; nitrogen gas is present in the large amount but it cannot help in burning process.

Oxygen gas is essential for the respiration in the environment or atmosphere. In respiration, oxidation of food is involved. We inhale oxygen and exhales carbon dioxide. If there is no oxygen, there will be no life on the Earth.

Air supports burning. This can be proved by an experiment.

1. Take two burning candles.
2. Put one of the burning candles inside the inverted glass.
3. After sometime you will see that the candle which is outside, is still burning.
4. The candle which is below the inverted glass is extinguished because of lack of oxygen there.

This shows that air supports burning process.

Vacuum is a place where air is absent.

A lump of cotton wool shrink in water because cotton wool absorbs water and due to absorption of water the air inside the cotton wool is driven out by water. That’s why cotton wool shrinks in water.

Air is dissolved in water and this can be showed by an experiment.

Diagram:

1. Take water in a tumbler or any other utensil.
2. Heat it, till the vapours appear.
3. After getting much heat, you will see that the water starts boiling.
4. Now you can see so many bubbles are there in water.
5. These all are air bubbles.

This proves that air dissolves in water.

Atmosphere

Atmosphere may be defined as the layer of gases which contains oxygen, carbon dioxide, nitrogen and many other gases; dust particles and water vapours that are present around earth.

Carbon dioxide

Green plant use carbon dioxide to prepare food for their own, which they get from the environment. Human beings exhales carbon dioxide and which is then is used by plants.

The activities which are possible due to the presence of air.

1. Respiration: Inhalation of oxygen and exhalation of carbon dioxide, this process called respiration which can be done only in the presence of air.
2. Oxidation: All oxidation processes is proceeded in the presence of air.
3. Combustion: For combustion oxygen is required which is present in air.
4. Transpiration: Loss of water occurs due to presence of air.
5. Evaporation and condensation, flying of birds are some of other activities which are possible only in the presence of air.

Animals breaths out carbon dioxide. This carbon dioxide can be used by plants to prepare their food, this process is called photosynthesis which makes the balance on earth. Animals and plants by exchange of gases help each other to survive.