Air is neither a pure substance nor a single gas. It is a mixture of gases, important among them being nitrogen, oxygen, carbon dioxide, and water vapour. The other minor constituents include noble gases or inert gases like helium, neon, argon and krypton.
Atmosphere
Air is present all around us. All living things need air to
breathe. We breathe in the oxygen that is present in the air.
The blanket of air that surrounds the Earth is called the atmosphere. It extends hundreds of kilometres above the earth. It acts as a
shield for the earth.
The atmosphere is thick near the surface of the earth. It
gets thinner as we go higher up. After a few hundred kilometres, the atmosphere
ends and space begins. Space has no air.
Layers of the
atmosphere
The atmosphere has five layers. These are the troposphere,
stratosphere, mesosphere, thermosphere and exosphere.
Troposphere
The troposphere is the lowermost layer of the atmosphere. It
extends up to 15 kilometres above the surface of the earth. All weather changes
take place in this layer.
The stratosphere starts from where the troposphere ends and
extends to about 35 kilometres. Compared to the troposphere, this layer of the
atmosphere is less dense. The ozone layer, which absorbs the sun’s ultraviolet rays, is located in this layer. Jet planes usually fly in this layer.
Mesosphere
Beyond the stratosphere lies the mesosphere. In this layer,
temperature begins to fall as we go higher. That is known as the ‘cold
layer’.
Thermosphere
The thermosphere is called the ‘hot layer’ because it is
characterized by high temperatures. Space shuttles orbit in this layer.
Another layer, called the ionosphere is a part of the
thermosphere. The radio signals transmitted from the earth are reflected back
to the earth by this layer. So long-distance radio communication is possible
because of the ionosphere.
Exosphere
This is the outermost layer of the atmosphere. The air here
is very thin. Beyond the exosphere is vast, empty space.
Composition of Air
Nitrogen constitutes a major percentage of air (78.03%), oxygen constitutes above 20.9%. The remaining 1.07% is shared by the noble gases, carbon dioxide, water vapour, dust particles etc.,
|
Constituents of air |
Percentage |
|
Nitrogen |
78.03% |
|
Oxygen |
20.09% |
|
Noble gases (Helium, Neon, Argon,
Xenon, Krypton) |
0.94% |
|
Carbon dioxide |
0.03% |
|
Water vapour |
ppm – ppb level |
|
Impurities like dust pollutants,
sulphur dioxide, nitrogen dioxide, and carbon fluoride. |
ppm – ppb level. They may vary from
place to place. |
Nitrogen
The percentage of nitrogen in air, by volume, varies from
77.9% to 78.03%.
Significance of
nitrogen in the atmosphere
It dilutes the activity of oxygen. Thus, the combustion of
fuel during burning takes place at a moderate rate.
The compounds of nitrogen are of vital importance to plants
as they help the plants to manufacture proteins. However, free nitrogen cannot
be absorbed by plants.
Symbiotic bacteria are present in nodules of pod-bearing
plants such as peas, grams, sweet peas, etc., The bacteria absorb nitrogen
directly from the air and convert it into compounds of nitrogen. These compounds
are absorbed by the plants for the manufacture of proteins.
Nitrogen being an active gas is used in the preservation of
foods. The containers containing food are flushed with nitrogen to drive out
oxygen and then sealed. Because, of the absence of oxygen, the bacteria do not
grow and hence food remains fresh.
Oxygen
The percentage of oxygen in air by volume varies from 20.8%
to 20.9%.
Significance of oxygen
in the atmosphere
It supports all animal and plant life on the earth, as it is
essential for respiration.
It is essential for the combustion of fuels. Man burns an
enormous amount of fuel for his energy needs. Oxygen helps in the combustion
of fuels to form carbon dioxide and water vapour as principal products.
In nature, it dissolves in water. This in turn helps to keep
the water fresh and is a source of respiration for water animals.
Carbon Dioxide
The percentage of carbon dioxide in air, by volume, varies
from 0.03% to 0.04%. The areas where a lot of fuel burns have a higher percentage
of carbon dioxide.
Significance of carbon
dioxide in the atmosphere
It acts as food for plants. During photosynthesis, plants
absorb carbon dioxide and water vapour. These are converted into carbohydrates
and oxygen in the presence of chlorophyll and sunlight.
Carbon dioxide, dissolved in water, can dissolve rocks
containing calcium carbonate (chalk) or magnesium carbonate. The salts, so formed,
are calcium bicarbonate and magnesium bicarbonate. These dissolved salts give a taste to natural water. Furthermore, they satisfy the needs of plants for
growth, as calcium and magnesium ions are needed by the plants.
It does not allow the earth to radiate heat energy rapidly at
night. It is because the carbon dioxide molecules present in the air reflect back
heat radiations.
Water vapour
The percentage composition of water vapor in air, by volume,
varies from 0.4% to 4%.
Significance of water
vapour in the atmosphere
The presence of water vapour in the air determines climatic
conditions. Excessive water vapour causes rain. The amount of rain produced
depends upon the geographical location and the surroundings. Depending upon the
temperature, the water vapour produces snow, fog, mist, hail and other phenomena.
The presence of water vapour controls the rate of evaporation
from plants and animals.
The presence of water vapour is very essential for the growth
of plants.
The presence of water vapour is very essential for the health and
comfort of animals. Excess or deficient water vapours in the air can lead to several problems related to good health.
A group of gases that do not react chemically with any
substance are called noble gases. They are helium, neon, argon, krypton and
xenon. Their percentage composition in the air by volume is 0.94%.
Significance of noble
gases in air
Apparently, they do not have any use for the animals or
plants. They can be separated from the air and put to several industrial uses.
Helium gas is very light and is used for filling weather
observation balloons.
Argon gas is used for filling electric bulbs at low pressure.
Neon gas is used for making advertising glow tubes, which you
see at night in big shops, hotels, etc.
Impurities
The percentage of impurities in the air is variable, depending
upon the location of the place. In general, the air contains a large amount of
impurities in industrial areas.
Dust particles
Small amounts of dust particles do no harm. As a matter of
fact, they help in the precipitation of water vapour and, hence, in the
formation of clouds and rain. However, an excessive amount of these can cause
serious respiratory problems.
Smoke particles
Smoke particles are present in large amounts where coal is
used as fuel. They also act as nuclei for the precipitation of water vapour.
However, an excessive amount of these can lead to serious respiratory problems.
Sulphur dioxide
nitrogen dioxide, carbon monoxide and hydrogen sulphide
These gases are present in variable amounts in big cities,
industrial districts, etc., They can cause the following problems.
They can cause serious respiratory problems.
They can destroy the ozone layer, which protects us from
ultraviolet radiation.
They can cause acid rain, which rains crops and forests.