if hydrogen is combustable gas why does it not support burning ?

Most of the gases burn but they do support burning. I think your're confused a little about this. Oxygen it supports burning. You must have studied that oxygen is essential for the burning of combustible substances. But gases like hydrogen can't burn by itself and it needs oxygen to burn. And when hydrogen, a gas that burns combines with oxygen, a gas that supports burning, it forms water which is neither combustible nor supports combustion. this is just an extra part and it shows what chemical change is and i just told ..

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