Air is a mixture of gases, water vapour and dust particles. Largely, it consists of
(i) Nitrogen (78%)
(ii) Oxygen (21%)
(iii) Carbon dioxide (0.03%)
(iv) Other gases like helium, hydrogen, etc (0.17%)
(v) Water vapour and dust particles.

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.

Some uses of air are
(i) For respiration, all living organisms need air.
(ii) For burning of any substance, air is need.

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.

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.

Respiration by animals and plants and burning of fuel.

Nitrogen does not support burning.

1% of air includes carbon dioxide (C02), water vapours , helium, hydrogen, dust particles and smoke.

Carbon dioxide turns lime water milky. It is a constituent of air.

The dust particles present in air stick to transparent glass of windows. These particles also attract other elements like pollen to collect on glass. That's why, transparent glass of windows if not wiped appears hazy.

Air is important for life on Earth due to following reasons:
(i) It plays an important role in water cycle.
(ii) It helps in the movement of sailing yachts, gliders, parachutes and aeroplanes.
(iii) It helps birds, bats and insects to fly.
(iv) It helps in dispersal of seeds and pollen of flowers of several plants

(a) Akshay and his friends went deep sea diving where the level of oxygen is reduced. Due to lack of sufficient oxygen supply, problems in breathing occur.
(b) By carrying oxygen cylinders, this situation can be overcome.
(c) Two more situations include (i) Mountain climbing. (ii) Patients with breathing problems.
(d) Plants and animals living in the water use dissolved oxygen, i.e. oxygen gas dissolved in water for respiration through special organs or pores.
(e) Akshay is responsible and adventurous. He is cautious and observant of his surroundings.

On a sunny day, go to a room which is a little bit dark. In this room, choose a thin opening through which light can enter the room. If, there is no opening in the room, then make one. From this opening we will see that a sharp beam of sunlight enters the room. In this beam, we will observe that there are some tiny solid-like particles moving freely. These particles are the dust particles, and this shows the presence of dust particles in air.