Question

when hydrogen gas is passed over a black coloured substance 'X', the coating of on its surface turns brown.
this is due to:

i) oxidation of X
ii) reduction of X
iii) neither oxidation nor reduction of X
iv) removal of impurities from the surface of X

Q. name the substance X?

Answer 1

The substance X is copper (II) oxide (CuO). Copper metal is reddish brown in colour and when burnt it air, it turns black due to the formation of copper oxide. When hydrogen gas is passed over this CuO, reverse reaction takes place and copper is obtained back.

CuO + H₂ → Cu + H₂O

Thus, the brown colour of the metal is restored due to reduction of CuO.