compounds of copper and oxygen: copper (I) oxide Cu2O; copper (II) oxide, CuO; copper (III) oxide, Cu2O3; and copper peroxide, CuO2. Copper (III) oxide Cu2O3 is unstable.
Copper (II) oxide, or cupric oxide, CuO, occurs in nature as the black mineral tenorite (melaconite). It is unstable and begins to decompose at 800°C; it is readily soluble in cyanide solutions and in acids (the latter is used in copper hydrometallurgy).
Cupric hydroxide, Cu(OH)2, is precipitated out in the form of blue particles upon the interaction of copper (II) salts with alkalis in solutions. Freshly precipitated Cu(OH)2 is soluble in alkalis; however, its acidic nature is hardly evident. Its corresponding salts (for example, Na2CuO2) are called cuprites. In hydrous ammonia, cupric hydroxide forms a blue solution, [Cu(NH3)4](OH)2; this solution is known to dissolve cellulose. Upon dilution or acidification, the cellulose once more precipitates out. This process is used in the manufacture of rayon.
Copper (II) oxide serves as a green dye and a blue dye in the glass-making and enamel industries; it is also used in the manufacture of ruby glass.
Copper (I) oxide, or cuprous oxide, Cu2O, forms the brownish red mineral cuprite. It melts at 1230°C without decomposing and is readily reduced to metal form by the action of hydrogen and carbon monoxide. Cu2O dissolves completely in dilute sulfuric acid only in the presence of oxygen; in concentrated H2SO4 it dissolves with the formation of SO2. Cu2O is obtained by roasting copper in an insufficient quantity of air. It is used in tinting glass and enamel; it is also used as a pesticide.