Although you cannot ‘see’ air, you need it for living. It has no colour, taste or smell. But it has weight and force and is made up of a mixture of a variety of individual gases. It surrounds or envelops the terrestrial globe to form the earth’s atmosphere.
A gas, or air which is a mixture of gases, does not have a definite shape or size but it fills up any available space. Actually, everything around us is made up of molecules - our bodies, rocks, plants, water, steam, gases all contain these tiny particles. The molecules in hard solid things are packed close together. The molecules in a liquid are less closely packed and can move about easily. But the molecules of a gas are thinly distributed and are free to move about freely.
The most important of all the gases present in the air is oxygen. We cannot live without this. We breathe air to give oxygen to our bodies to keep alive. But only about one fifth of the air is made up of oxygen. About four fifth is occupied by nitrogen. The‘ rest of the air is formed by a mixture of many gases. And due to pollution, the air that surrounds us also has dust particles and harmful matter in it. There is also some water vapour present in the air. But these things dust, water vapour, dirt are not really part of air it self. Although light and invisible, air has weight. This is why it is attracted and held to the earth by gravity and does not fly away into outer space.