The chemical reactions are:
2Cu + O₂ → 2CuO
CuO + H₂ → Cu +H₂O
When copper powder is heated in a china dish, in the presence of oxygen, a black residue is formed on the dish due to the formation of copper(II) oxide.
This phenomenon is known as surface oxidation.
The followiing reaction is shown below.
2Cu + O₂ → 2CuO
Again when CuO is treated with hydrogen gas, CuO gets reduced to Cu, a brown residue.
CuO + H₂ → Cu +H₂O
