All living things require air. We have not seen air but we must have felt its presence in so many ways. For example, when the leaves of the trees rustle or the clothes hanging on a clothes line sway
Air is present everywhere around us. It fills all the empty space . It has no colour and one can see through it. It is transparent. 
The presence of air can be felt by a simple experiment. 

Activity : 

Take an empty glass bottle. Turn it upside down. Now dip the open mouth of the bottle into a bucket filled with water as shown in the figure. We observe that water does not enter the bottle. 
Now tilt the bottle slightly. We observe that now water enters the bottle and we can see bubbles coming out of the bottle with a bubbly sound. This shows that air was present in the bottle. The bottle was not empty at all. In fact it was filled completely with air even when it is upside down. That is why water does not enter the bottle when It is pushed in an inverted position as there was no space for air to escape. 
When the bottle was tilted the air was able to come out in the form of bubbles and water filled up the empty space that the air has occupied. 


Air is a mixture of several gases, water vapour and fine dust particles. A human being respires about 22,000 times everyday and takes in about 16 kg of air. Air contains mainly nitrogen, oxygen and smaller amounts of argon, carbon dioxide, water vapours and traces of helium, neon, krypton and xenon. 


The transparent , thick and invisible envelope of air surrounding us is called atmosphere. It consist of a uniform mixture of permanent gases called dry air and varying amounts of other materials including organic and inorganic impurities such as smoke, pollen grains and dust particles. 
The atmosphere is divided in various layers to facilitate the study of it. 
The different layers of atmosphere (starting from the surface of earth) are troposphere, stratosphere, mesosphere, thermosphere and exosphere. 
Troposphere : This is the lowest layer of the atmosphere. It extends to a height of about 11 km above the earth's surface. All weather phenomena, such as clouds, fog, rainfall, snowfall, storms and lightning occur here.
Stratosphere; This layer lies above the troposphere and it extends to a height of about 50km above the earth's surface. There are virtually no clouds and very little dust and water vapour convection. The stratosphere contains Ozone which absorbs the harmful ultraviolet radiation of the sun . 
Mesosphere: The layer lies above the stratosphere and it extends to a height of about 80km above the earth's surfa
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