FeCl2 + Cl = FeCl3.
Ferrous hydroxide exposed to moist air is rapidly changed into ferric hydroxide:
2Fe(OH)2 + H2O + O = 2Fe(OH)3.
Ferrous sulphate (copperas, green vitriol) (FeSO4·7H2O). Ferrous sulphate is the most familiar ferrous compound. It is prepared commercially as a by-product in the steel-plate mills. Steel plates are cleaned by the action of dilute sulphuric acid upon them, and in the process some of the iron dissolves. The liquors are concentrated and the green vitriol separates from them.
Ferrous sulphide (FeS). Ferrous sulphide is sometimes found in nature as a golden-yellow crystalline mineral. It is formed as a black precipitate when a soluble sulphide and an iron salt are brought together in solution:
FeSO4 + Na2S = FeS + Na2SO4.
It can also be made as a heavy dark-brown solid by fusing together the requisite quantities of sulphur and iron. It is obtained as a by-product in the metallurgy of lead:
PbS + Fe = FeS + Pb.
It is used in the laboratory in the preparation of hydrosulphuric acid: