Dear Student
1. Take a candle and fix it on a table.
2. The candle is lighted.
3. The candle will continue to burn due to continuously available fresh air providing the required oxygen for combustion.
4. Cover the burning candle by putting an inverted gas jar over it.
5. After sometime, the candle stops burning and gets extinguished.
6. When burning candle is covered with gas jar, then the candle takes away the oxygen necessary for burning from the air enclosed in the gas jar.
7. After some time, when all the oxygen of air inside the gas jar is used, then the burning candle gets extinguished.
This shows that air (oxygen) is necessary for combustion of substances.
Regards