Copper becomes green when exposed to moist air for a long period. This is due to:
Copper metal when exposed to air turns green in colour due to corrosion. When copper vessel is exposed to air in rainy season, the metal reacts with gases and moisture and atmospheric gases to form a mixture of copper carbonate and copper hydroxide. This gives a green colour to the surface of copper metal.
The reaction is as follows:
2Cu + H2O + CO2 + O2 → Cu(OH)2 + CuCO3