Why do copper objects develop a green coating in air?
When a copper vessel is exposed to moist air for a long time it develops a green layer on its surface. Copper corrodes by oxidation in which it reacts with oxygen in the air to form copper oxide.
Copper oxide then combines with carbon dioxide to make copper carbonate, which gives it a green colour. This process is called corrosion of copper.
The green material is a mixture of copper hydroxide (Cu(OH)$$_2$$) and copper carbonate (CuCO$$_3$$). The following is the reaction:
2Cu(s)+H$$_2$$O(g)+CO$$_2$$ +O$$_2$$
$$→$$ Cu(OH)$$_2$$ +CuCO$$_3$$ (s)
Copper(II) carbonate is a blue-green compound.