Question

A substance x when heating in presence of air it turns black.on heating this black

Answer 1

HERE IS YOUR ANSWER

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The substance X is copper.
When copper is heated in air , it is oxidised to form copper oxide which is black in nature.

2Cu + O2 -----> 2CuO

This black substance when treated with hydrogen returns to brownish colour indicating its reduction back to copper.

CuO + H2 ------> Cu + H2O

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