1. What are the properties of air?

• It is colorless that is it has no color and tasteless.
• It is transparent that is we can see through it.
• Air occupies space. It fills all the space in a contains which is otherwise empty and seems to be empty.
• Air exerts pressure.

2. What will happen if the amount of oxygen in increased in the air?

Air will become more combustible and many substances may catch fire undesirably.

3. List five activities that are possible due to the presence of air.

Air is very important for living beings. Its various activities are listed between:

1. It helps in burning.
2. It is essential for the respiration in living being.
3. It is helpful in movements of sailing yachts, gliders, parachutes and aircrafts.
4. Air current makes the windmill to rotate. The wind mill is used to draw water from tube wells and to run flour mills. Wind mill also generate electricity.
5. Birds, bats and insects fly in air.
6. Air also helps in the dispersal of seeds and pollen of flowers of several plants.
7. Compressed air is used in tyres in vehicles.
8. Nitrogen of the air is used on a large scale to manufacture fertilizers.
9. Helps in winnowing.
10. Helps in hearing sounds.
11. CO₂ present in air is used by plant in making their food.
12. Activities like cloud formation, thundering, etc, occur in air.

4. Name the gases present in air. Which one of them is most abundant?

N₂, O₂, Ar (Argon), CO₂, water vapor, Neon (Ne), Helium (He), Krypton (Kr), Xenon (Xe) are present in air, Nitrogen is most abundant.

5. How does oxygen occur in nature?

Oxygen is found in free state in air as diatomic molecule (O₂) to the extent of 21% volume or 23% by weight. In the combined state, it is present in water (89% by weight), Earth's crust (about 50%) and in plants and animal tissues (50-70%).

6. Which gas in the atmosphere is essential for respiration?

Oxygen gas helps in the breakdown of the food to provide energy. The process is called respiration. Thus, oxygen helps in respiration.

7. What is atmosphere?

Our Earth is surrounded by air in the form of a thin layer. This thin layer is called atmosphere. The atmosphere extends several kilometers. Air becomes thinner and thinner as we go high up from the surface of Earth. The atmosphere is quite active due to the movement of air, with respect to the Earth. The process like, cloud formation, thundering, rain etc., occur in atmosphere.

8. Nature maintains a balance of carbon dioxide and oxygen in the atmosphere. Comment.

Nature in its own way has been maintaining a balance of carbon dioxide and oxygen in the atmosphere. Atmosphere and oceans are continuously exchanging these gases with the rocks, plants and living organisms. However, due to rapid industrialization in the past, the concentration of CO₂ near the Earth's surface is increasing though in very small proportions till now.

9. Describe the composition of air.

Air contains mostly nitrogen (78.03%) and oxygen (20.99%). The remaining 1% (by volume) is shared by argon (0.34%), carbon dioxide (0.33%), other noble gases, oxides of nitrogen and sulfur etc., in the decreasing order.

10. How can we see dust particles present in the air?

To see the dust particles present in air, we have to close all the doors and windows of a room to make it dark. Then, if we open the door/windows facing the sun, just a little, in such a way that it allows sunlight to enter the room only through a slit. We can see some tiny shining dust particles moving rapidly in the slanting beam of sunlight.