Lump of cotton wool has air among gaps of cotton fibres. When dipped in water, air from these gaps is replaced by water. The cotton lump becomes heavy and also shrinks due to removal of air gaps.

The following activities are possible due to the presence of air
(i) Respiration by animals.
(ii) Photosynthesis by plants.
(iii) Power generation by windmills.
(iv) Burning of fuels and substances.
(v) Scattering of seeds and pollens of plants (pollination).
(vi) Movements of sailing yachts, gliders, parachutes and aeroplanes.

There is an interdependence of plants and animals on each other for exchange of gases. Plants take carbon dioxide from the atmosphere and prepare food. This is called photosynthesis. During this process, plants give out oxygen to the atmosphere. Animals inhale this oxygen and gives out carbon dioxide to the atmosphere. Therefore, plants and animals help each other in the exchange of gases in the atmosphere.

Air is dissolved in water in the form of oxygen. When water bottle is left in the open sunlight, tiny bubbles were formed all around inside the bottle because air dissolved in water starts escaping in the form of tiny bubbles due to heat from the sun.

Nitrogen in air is used for manufacturing food by the plants produced by nitrogen fixation of atmospheric nitrogen into nitrates which mix with the soil. The plants absorb these as their food, so it is necessary and important for living organisms.

(a) Carbon dioxide
(b) 0.03%
(c) (i) Respiration of living organisms.
      (ii) Burning of fuels by human beings.
(d) All the food which animals eat is made from carbon dioxide gas of the air.

Fire or burning is supported by air. In order to stop it,
(a) the towel or woollen blanket used, cuts off the supply of air (oxygen) which is supporting burning. As this supply is cut out, fire eventually stops.
(b) a fire extinguisher contains carbon dioxide gas which does not support burning. So, when fire extinguisher is used over the area, CO2 cuts off the oxygen supply from air causing fire to extinguish.

i) Air does not have a fixed shape or size of its own because it is a gaseous mixture whose particles are very loose, thus not able to form a fixed shape.
ii) If air is blown in a balloon of round shape, it takes its shape. Similarly, air can be enclosed in packets of chips different in shape and size.
iii) Like water, air also has no smell. It carries the odour of whatever is present in the surroundings. We can smell a nice perfume or a bad smell because air carries the smell.

(a) (i) Nitrogen, (ii) CO₂, water vapour, other gases and dust particles, (iii) Oxygen
(b) Component (iii) is oxygen. It is used by humans and animals for respiration. It also helps in combustion burning.
(c) Yes, the composition of air is not always exactly the same. It changes slightly from place to place.

Air is present everywhere and it contains some gases, water vapours and dust particles that stick to the window surface and are visible due to the transparency of windows.
These particles attract outdoor elements Iike pollens, etc which collect on glass window and together form a layer. That is why, transparent glass windows appear hazy if not wiped off regularly.

The air is considered a mixture because of the following reasons:
(a) Air consists of many gases like oxygen, nitrogen, carbon dioxide, etc.
(b) The composition of air varies from place to place.

No, winnowing is not possible in a closed room. The reason is that winnowing needs wind, which occurs only when air moves. Since, there is no wind. in a closed room, no winnowing can take place.

Both plants and animals are interdependent on each other for their survival. Plants cannot survive for long without animals, as they need CO₂ for photosynthesis, which is exhaled by animals. Similarly, in absence of plants, there will be no oxygen for animals to breathe in Hence we can say that both need each other and the balance of oxygen and carbon dioxide in the atmosphere is maintained by their presence.

(a) A windmill is a machine that harnesses wind energy to grind grain, pump water, or generate electricity.
(b) The windmill is used to draw water from tube wells and to run flour mills. They are also used to generate electricity.

The activities that are possible due to the presence of air are as follows :
(i) Respiration and water cycle.
(ii) Flying of birds and aeroplanes.
(iii) Dispersal of seeds.