Copper, one of the seven metals of alchemy, takes its name from the Latin cyprium (Cyprus). It has two main oxidation states:
- Cuprite (Cu2O) is copper(I) oxide.
- Tenorite CuO is copper (II) oxide.
Copper with a charge of +1 is called cuprous; with a +2 charge it is called cupric. Copper (II) carbonate (often called copper carbonate or cupric carbonate) occurs naturally as malachite (CuCO3.Cu(OH)2) and azurite (Cu3(CO3)2(OH)2).
Copper does not react in water, although it is subject to oxidation in air, which gives the metal a familiar green or brown patina.
The colours of copper(II) complexes are explained by the behaviour of visible light passing through an aqueous solution. The visible spectrum for a solution of Cu(II), [Cu(H2O)6]2+, has an absorption band which spans the red-orange-yellow portion of the spectrum. Green, blue and violet are transmitted by a solution. So in relationship to the artist's colour wheel - colours on one side of the wheel are absorbed and their complementary colours are transmitted within a solution of copper sulphate.