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.

Take some water in a glass vessel or beaker. Heat it slowly on a tripod stand well before the water begins to boil. Look carefully at the inner surface of the vessel, you will see tiny bubbles inside. These bubbles come from the air dissolved in water.

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.

Water does not enter into an inverted glass bottle because even when the bottle appears empty, its not. Air is present inside the bottle or any other area which is empty. All the space in the bottle is occupied by air. This air prevents the water from entering inside the bottle.
As we tilt the bottle, air present inside moves out. As air comes out, the space becomes empty allowing the entry of water. The space vacated by leaving of air is then occupied by water.

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.

Nitrogen does not support burning.

To prevent inhalation of smoke given out by automobiles, policemen use masks.

We might not have seen air but its presence can be felt.

Take a burning candle which is fixed in a tumbler containing water. The candle is covered by an inverted glass. After some time, the covered candle gets extinguished and water rises upto 1/5th of the glass. It shows that air supports burning.

The amount of oxygen decreases at high altitude. So, for normal breathing, mountaineers carry oxygen cylinders with them, while climbing high mountains.

Humans and animals use oxygen present in the environment to carry out respiration and release CO₂ as a by- product of this process. Plants perform photosynthesis by using this CO₂ present in the atmosphere accumulated due to activities of humans and animals. They release oxygen as their reaction by product which also gets out in the atmosphere. Plants depend on humans and the activities performed by them like burning of fuels, etc for CO₂ and humans depend on plants for a continuous oxygen supply. Thus, these two processes maintain a unique balance between O₂ and CO₂ concentration in our environment.

Air is a mixture of gases. The major composition of air are oxygen, nitrogen, carbon dioxide, etc. The percentage composition of These dust particles are produced due to constituents of air are given below:

Nitrogen - 78%, oxygen - 20.9%, argon - 0.9%, carbon dioxide - 0.035 - 0.0,45%, water vapour - 0.03%, dust, etc.