(a) When copper powder is heated in the presence of air (oxygen), copper reacts with oxygen to form copper oxide. The copper oxide formed is black in colour. The black colour is formed due to the oxidation of copper takes place.
The chemical equation of the reaction that takes place is given below: \(2 \mathrm{Cu}+\mathrm{O}_{2} \rightarrow 2 \mathrm{CuO}\)
Copper oxide
(b) When hydrogen gas is passed over hot copper oxide, the black coating on the surface turns brown as the reverse reaction takes place and copper is obtained.
The reaction that takes place is:
\(\mathrm{CuO}+\mathrm{H}_{2} \rightarrow \mathrm{Cu}+\mathrm{H}_{2} \mathrm{O}\)
