Ferrous Sulfate: Iron Displaces Copper

Iron is more reactive than copper, so iron put into copper sulfate solution takes the copper's place: CuSO₄ + Fe → FeSO₄ + Cu. The copper comes out as a metal and the solution turns into iron(II) sulfate, ferrous sulfate. (Industry gets most of it as a by-product of pickling steel in sulfuric acid, or by oxidising pyrite.) From a cool solution it crystallises as the blue-green heptahydrate, the mineral melanterite; above 56.6 °C the tetrahydrate forms instead. Ferrous sulfate (copperas) was used for iron gall ink, as a mordant in wool dyeing and to make Mars pigments.