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Copper is exposed to air.

1. When copper is exposed to air for a considerable time, then copper reacts slowly with the carbon dioxide and water of air to form a green coating of copper carbonate on the surface of the object.
2. The green copper carbonate is a mixture of copper carbonate and copper hydroxide, CuCO3.Cu(OH)2.
3. Since, copper is low in reactivity series, therefore the corrosion of copper metal is very slow.