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Chapter 3 Air

Air is an invisible mixture of gases made up of a mass of millions of tiny gas molecules that
moves randomly and at high speed. Without air the earth would be lifeless.

Composition of thE atmosphErE

The atmosphere consists of a mixture of gases which are present in varying proportions.
Its major constituents are nitrogen 78% and oxygen 21%. The balance 1% includes carbon
dioxide, helium, ozone, argon and hydrogen. Impurities like dust, smoke, salts, carbon
dioxide and other chemicals are also present in the atmosphere. In its lower layer, the
atmosphere contains water vapour too. Nitrogen is the largest component and is essential
for survival and growth of plants.

Oxygen is the second most plentiful gas. All living beings
breathe in oxygen while the green plants excrete out
oxygen during photosynthesis. Green plants use carbon
dioxide present in the atmosphere to make their food.

Thus, the plants help in maintaining a balance
between oxygen and carbon dioxide percentage in the

atmosphere through the process of photosynthesis. If
the plant cover is reduced, it will effect the balance Composition of Atmosphere
between oxygen and carbon dioxide levels in the atmosphere.

The word ‘atmosphere’ comes from the Greek term ‘atmos’ means 'vapour' and ‘sphaira’ means
'sphere'. It means the air between the earth’s surface and other space.

struCturE of thE atmosphErE

Like the lithosphere, the atmosphere is also found in different layers. Thus, our atmosphere
has a layered structure, as the concentration of different
gases varies with the height from the earth’s surface.
The denser and heavy gases like— nitrogen, oxygen,

carbon dioxide etc. are found near the earth’s surface,
while lighter gases are found at higher altitudes.
Based on temperature, the atmosphere is divided vertically

into five layers. These five layers are troposphere,
stratosphere, mesosphere, thermosphere and exosphere.
Brief description of each layer is given below.

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