~Ferrous sulphate~ _(copperas, green vitriol)_ (FeSO_{4}.7H_{2}O). 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: FeSO_{4} + Na_{2}S = FeS + Na_{2}SO_{4}. 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: FeS + 2HCl = FeCl_{2} + H_{2}S. ~Iron disulphide~ _(pyrites)_ (FeS_{2}). This substance bears the same relation to ferrous sulphide that hydrogen dioxide does to water. It occurs abundantly in nature in the form of brass-yellow cubical crystals and in compact masses. Sometimes the name "fool's gold" is applied to it from its superficial resemblance to the precious metal. It is used in very large quantities as a source of sulphur dioxide in the manufacture of sulphuric acid, since it burns readily in the air, forming ferric oxide and sulphur dioxide: 2FeS_{2} + 11O = Fe_{2}O_{3} + 4SO_{2}. ~Ferrous carbonate~ (FeCO_{3}). This compound occurs in nature as siderite, and is a valuable ore. It will dissolve to some extent in water containing carbon dioxide, just as will calcium carbonate, and waters containing it are called chalybeate waters. These chalybeate waters are supposed to possess certain medicinal virtues and form an important class of mineral waters. ~Ferric salts.~ Ferric salts are usually obtained by treating an acidified solution of a ferrous salt with an oxidizing agent: 2FeCl_{2} + 2HCl + O = 2FeCl_{3} + H_{2}O, 2FeSO_{4} + H_{2}SO_{4} + O = Fe_{2}(SO_{4})_{3} + H_{2}O. They are usually yellow or violet in color, are quite soluble, and as a rule do not crystallize well. Heated with water in the absence of free acid, they hydrolyze even more readily than the salts of aluminium. The most familiar ferric salts are the chloride and the sulphate. ~Ferric chloride~ (FeCl_{3}). This salt can be obtained most conveniently by dissolving iron in hydrochloric acid and then passing chlorine into the solution: Fe + 2HCl = FeCl_{2} + 2H, FeCl_{2} + Cl = FeCl_{3}. When the pure salt is heated with water it is partly hydrolyzed: FeCl_{3} + 3 H_{2}O <--> Fe(OH)_{3} + 3HCl. This is a reversible reaction, however, and hydrolysis can therefore be prevented by first adding a considerable amount of the soluble product of the reaction, namely, hydrochloric acid.