When the copper powder is heated in a china dish, the copper powder surface becomes coated with black colour substance due to the formation of copper oxide by surface oxidation. Copper reacts with oxygen in the air upon heating and forms copper oxide. The black colour is due to the formation of this copper oxide.
2‌Cu( s)+O2(g)→2‌CuO
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.
CuO+H2→Cu+H2O