1.  What is air ?
There is air around us , we can't see air, but we can feel it . Air doesn't have colour or odour. Living organisms need air to breath.
2. How can you prove that burning materials requires oxygen ?
Light two candles. Keep one candle closed by a glass tumbler. The candle which is kept closed in put out after a little while . But the one which is not closed burns continously oxygen. Present in the air supports burning of materials.

3. What will happen if we keep lime water in the outside atmosphere air ?
A white layer forms on the surface of lime water. The carbondioxide that is present in the air turns lime water milky.

4. Write any two uses of Carbondioxide ?
*  Carbondioxide  is needed for the plants to prepare food.
*  Carbondioxide makes soft drinks fizzle.

5. Write any two uses of oxygen ?

*  The burning of materialsrequires oxygen.
*  Oxygen is needed for the respiration of man, animals and plants.

6.  Why we say that soft drinks are harmful to breeth ?
When we open the air tight cool drink bottles, we can see some gas bubbles coming out, it is nothing but the carbondioxide  present in cool drinks which are harmful to our health.

7. Which gas is present more in air and what is its use?
Among the gases present in the air four fifth of air consist of Nitrogen. It is necessary for the growth of plants.

8. How can we say that air is a mixture ?
Gases like Nitrogen, Oxygen, Carbondioxide, water vapour and dust particles are mixed in air.

9. Explain the percentage of gases in air .
Nitrogen              -   78%
Oxygen                -   21%
Carbondioxide     -   0.03%
Other gases         -   0.97%

10. Which gas is in more percentage during expiration ? why ?
Nitrogen is the gas which is present more in air. About four fifth of air consist of Nitrogen and also Nitrogen is not needed for our body.

11. What is atmosphere ?
Due to the gravitational force of the earth, there is a blanket of air about 1000 km thick around the earth, this is called atmosphere.

12. What is the use of atmosphere ?
The atmosphere helps the living organism to live, by providing the suitable temperature on the earth.

13. What is gravity ?
The capacity of the earth to attract all the matters to its centre is called gravity .

14. What happened in Bhopal on 1984 ?
Bhopal, the capital city of Madhya Pradesh there was a leakage of Methyl isoyanide gas   from a factory. This resulted in the death of thousands of people.

15. What is inspiration ?  Which gas is taken up by the man during inspiration ?
The process of taking up of gases is called as inspiration . Oxygen is taken up by the man during inspiration .

16. What is respiration ?
Respiration means the exchange of gases between the body cells and the environment.

17. What is photosynthesis ?
Photosynthesis is the process of preparation of food by plant with the help of carbondioxide, sunlight and water.

18.Which gas is in less percentage during expiration ? why ?
Carbondioxide is less percentage during expiration because carbondioxide is only about 0.05% in air.

19. How the ozone layer protect the earth ?
Ozone layer act as an umbrella and save the earth from ceaseless atomic rays and cosmic dust.

20. What is the use of water vapours ?
Water vapours rise so slowly and merge as thick soft clouds and forms rain.