Question

how will you prove that air supports burning

Answer 1

Take a candle. Place it in a tub. Light the candle and also fill the tub with some water. Cover the candle with an inverted glass. You will find that the candle blows out after burning for some time, and the water level inside the inverted glass rises up to some extent.
The component, oxygen, of air inside the glass is used up in burning. Therefore, water occupies that space. This shows that air supports burning.

Answer 2

The process of burning is only be done in presence of O2.
And since O2 is only found in the air so air supports burning.
Even for example if u light a candle it will burn in presence of air but if u suddenly put a glass upon it or cover it totally without disturbing the candle so that no air passes inside or comes outside u will find that the fire is blown off.. It's only because of lack of oxygen or air.