Theory:

In olden days, people believed that air is just one single substance. But, experiments have proved that air is a mixture of many gases.
The primary constituent of air is nitrogen, and the second major component is oxygen. It also contains carbon dioxide, water vapour, and dust particles with some other gases like argon, neon, krypton, xenon etc. This composition slightly changes from place to place.
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Burning candle experiment
1. Two lighted candles of the same length are taken and are placed on the table.
 
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2. Now, one of the candle is covered with a transparent glass tumbler.
 
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3. After some time, the candle covered with glass tumbler gets extinguished. But, the other one continues to burn.
 
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This happens due to the limited amount of gas which present inside the glass tumbler. When most of the gas component gets exhausted inside the glass, the candle stopped burning. At the same time, the other candle was burning due to the continues supply of the air.
Oxygen
In the above experiment, the gas component, which supported the burning of the candle, is known as oxygen. One-fifth of the atmosphere consists of oxygen. It occupies around \(21\%\) of the atmosphere.
Oxygen is consumed in the process of respiration by all the living things on the earth.
 
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Oxygen supports combustion.
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Nitrogen
We have seen that after the candle got exhausted, some air was still present inside the glass. This major component of air that does not support burning is called nitrogen. Around four-fifth of the atmosphere consists of nitrogen. It occupies around \(78\%\) of our atmosphere.
Plants use nitrogen to make proteins which is necessary for their growth. It also acts as a good fertilizer. All the living things need nitrogen for their growth.
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Carbon dioxide
For instance, whenever a fire happens inside a closed room, people trapped in may feel suffocation. It is due to the carbon dioxide which is filled inside the room.
 
Carbon dioxide occupies less than \(1\%\) of our atmosphere. Plant and animal matter produce carbon dioxide on consuming oxygen for respiration.
 
Plants prepare their food with the help of carbon dioxide.
 
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Burning of dried leaves and crop remains, pollutes our environment. This is due to the emission of carbon dioxide gas while burning.
 
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Reference:
https://image.shutterstock.com/z/stock-photo-air-composition-pie-chart-155679464.jpg
https://image.shutterstock.com/image-photo/burning-wood-night-campfire-touristic-600w-776560174.jpg
https://image.shutterstock.com/image-vector/breathing-inhalation-exhalation-operation-respiratory-600w-1609567627.jpg
https://image.shutterstock.com/image-photo/small-trees-different-sizes-grow-600w-1722400228.jpg
https://image.shutterstock.com/image-photo/woman-man-workers-suffocating-construction-600w-1177559314.jpg
https://www.shutterstock.com/image-vector/process-tree-produce-oxygen-illustration-1114795847
https://www.shutterstock.com/image-vector/photosynthesis-cellular-respiration-day-night-two-1599621970
https://image.shutterstock.com/image-photo/burning-field-background-farmers-burn-600w-1618052449.jpg
https://image.shutterstock.com/image-photo/scientific-experiment-showing-that-presence-600w-144374707.jpg