1. What is an atmosphere?

A thick layer which surrounds the earth is called an atmosphere.

2. What are the constituents of air?

Air is a mixture of several gases. The gases mainly present in air are nitrogen, oxygen , small amount of CO₂, helium, organ, water vapour and dust particles and certain other impurities.

3. What is Dry ice?

Dry ice is just like ordinary ice, but it does not change into liquid on warming it changes directly into a gaseous form.

4. Define respiration?

Food  is the source of energy. The energy stored in food is released by a process called respiration.
           Or
The process of oxidation of food materials in the living cells with the release of carbon dioxide  (CO2), water and energy.

5. What is liquid air?

Liquid air is a mixture of oxygen, nitrogen and inert gases, all in liquid state. It is used by mountaineers, deep sea divers and astronauts.

6. Define photosynthesis?

Green plants use carbon dioxide from the air, water from the soil and light from the sun to make their own food in the form of sugar or starch. This process is called photosynthesis.

7. What is meant by combustion?

The process of burning fuel in the presence of oxygen releasing a large amount of heat and light is called combustion.

8. What are the various air pollutants produced on burning fuel?

The various air pollutants produced  on burning fuel are carbon monoxide (CO), sulphur dioxide (SO₂), smoke, carbon dioxide (CO₂), unburnt carbon particles lead compound.

9. What is a fuel?

Any material that on burning produces heat energy is known as fuel.

10. Name some fuels which are commonly used in our daily life?

The fuels which are commonly used in our daily life are wood, cow dung cakes, coal, LPG, CNG.

11. What is acid rain?

The gases like carbon dioxide and nitrogen dioxide when combine with rain water form acids like sulphuric acid and nitric acid, which can damage historical monuments like the Taj Mahal and the Red Fort.

13. What is meant by rusting?

A reddish brown, soft, porous, flaky coating on the surface of iron articles is called rust and the process of rust formation is called rusting.

14. Define corrosion?

Rusting is a slow continuous oxidation process and it gradually eats iron, due to which an iron article loses its strength. This process is called corrosion.

15. Write two common uses of air?

• Oxygen is also used in hospitals for artificial respiration.
• Oxygen is used by deep sea divers, mountaineers, aviators, miners and astronauts for respiration.
• Air guides the movements of parachutes, gliders and air crafts.
• Wind guides the movements of parachutes, gliders and air crafts.