FeS + 2HCl = FeCl2 + H2S.
Iron disulphide (pyrites) (FeS2). 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:
2FeS2 + 11O = Fe2O3 + 4SO2.
Ferrous carbonate (FeCO3). 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:
2FeCl2 + 2HCl + O = 2FeCl3 + H2O,
2FeSO4 + H2SO4 + O = Fe2(SO4)3 + H2O.
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 (FeCl3). This salt can be obtained most conveniently by dissolving iron in hydrochloric acid and then passing chlorine into the solution:
Fe + 2HCl = FeCl2 + 2H,