Theory:

Presence of air:
Air is required for all the living things in the world. We can indeed feel the presence of air even though we cannot see it. We can notice its presence in our day-to-day life while flying a kite, on swaying of clothes, fluttering pages of a book, winnowing, and during storms.
 
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Flying kite
 
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Swaying clothes
 
During storms, you would have noticed that the trees get uprooted, and the rooftop even get blown off.
 
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Storm in coastal areas
Wind:
A moving air is known as wind.
A firki and a weathercock is a good example which shows moving air. A firki rotates when it is held in an open area. It moves or rotates towards the direction of the wind.
 
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Firki
 
A weathercock helps to show the direction in which the air is moving. It is very useful in predicting the weather.
 
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Weathercock
Empty bottle experiment:
Even empty bottles and containers are filled with air, although they look empty. Let us demonstrate the presence of the air.
 
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Experimental set-up
  1. A large bowl is filled with water.
  2. Now, the open mouth of an empty bottle and dip it into the water-filled bowl.
  3. In the inverted position of the bottle, water does not enter into it due to the presence of air.
  4. Air cannot escape out of the bottle since there is no space inside. Air prevents the water from entering into the bottle.
  5. When the bottle is tilted sideways, the air which is present inside the bottle goes out in the form of bubbles.
  6. Now, the water enters into the bottle by occupying the space inside.
It is observed that even an empty bottle is filled with air, though it looks empty to our naked eyes. This experiment shows that the air occupies space.
Properties of air:
Air has various properties. Some of them are:
  • It occupies space
  • It has no colour
  • It is transparent
  • It dissolves in water
  • It is tasteless and odourless
  • It can even be compressed
Atmosphere:
Atmosphere is the envelope of air that covers the earth and extends up to many kilometers above the earth's surface. This layer is essential for life on our planet as it provides us with oxygen to breathe.
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Earth's atmosphere
As we move higher above the earth's surface, the air gets thinner and thinner.
Mountaineers or people who climb the mountains carry oxygen masks or cylinders filled with oxygen with them.
 
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Mountaineer with oxygen mask
 
Deep-sea divers also carry oxygen cylinders for breathing under the sea.
 
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Deep-sea divers with oxygen mask