There are two stable copper oxides, copper(II) oxide (CuO) and copper(I) oxide (Cu2O).
Copper(I) oxide or cuprous oxide (Cu2O) is an oxide of copper. It is insoluble in water and organic solvents. Copper(I) oxide dissolves in concentrated ammonia solution to form the colorless complex [Cu(NH3)2]+, which easily oxidizes in air to the blue [Cu(NH3)4(H2O)2]2+. It dissolves in hydrochloric acid to form HCuCl2 (a complex of CuCl), while dilute sulfuric acid and nitric acid produce copper(II) sulfate and copper(II) nitrate, respectively.
Copper(I) oxide is found as the mineral cuprite in some red-colored rocks. When it is exposed to oxygen, copper will naturally oxidize to copper(I) oxide, but this takes extensive time. Artificial formation is usually accomplished at high temperature or at high oxygen pressure. With further heating, copper(I) oxide will form copper(II) oxide.
Formation of copper(I) oxide is the basis of the Fehling's test and Benedict's test for reducing sugars which reduce an alkaline solution of a copper(II) salt and give a precipitate of Cu2O.
Cuprous oxide forms on silver-plated copper parts exposed to moisture when the silver layer is porous or damaged; this kind of corrosion is known as red plague.
Copper(II) oxide or cupric oxide (CuO) is the higher oxide of copper. As a mineral, it is known as tenorite.