When a piece of copper is heated in the air it combines slowly with the oxygen and two oxides are formed: cuprous oxide (Cu20) and cupric oxide (CuO). The cuprous oxide is red and the cupric oxide is black. In a moist atmosphere, the carbon dioxide of the air combines with these oxides forming a green layer which contains the hydroxide, Cu(OH)2, and the carbonate, CuC03 - verdigris.

Copper is not affected by hydrochloric acid, but a weak, cold or hot concentrated acid will quickly dissolve it.