The members of the aluminium group form hydroxides, which are much weaker bases than are the hydroxides of the bivalent group just considered. Their salts with strong acids are considerably hydrolyzed and react strongly acid, and their salts with very weak acids, like carbonic acid and hydrogen sulphide, are decomposed so readily by water, that only ferric sulphide is capable of existence in its presence. When the sulphide, Al_{2}S_{3}, prepared by heating aluminium with sulphur, is added to water, it is totally decomposed into the hydroxide and hydrogen sulphide (p. 186); and ‹if aluminium chloride is treated with ammonium sulphide› in aqueous solution, ‹aluminium hydroxide, and not its sulphide, is precipitated›. The latter result may be interpreted in two ways, both of which, in the ultimate analysis, mean that hydrogen sulphide is too weak an acid to form a stable sulphide with aluminium hydroxide in the presence of water, the difficult solubility of aluminium hydroxide and the limited solubility of hydrogen sulphide being favoring factors (see p. 186). In a solution of aluminium chloride, the salt of a very weak base with a strong [p190] acid, more or less of the salt is hydrolyzed, and we have a condition of equilibrium as expressed in the equation AlCl_{3} + 3 H_{2}O ⇄ Al(OH)_{3} + 3 HCl. The addition of ammonium sulphide to such a solution would neutralize the free hydrochloric acid, and the action would proceed to completion towards the right, hydrogen sulphide being liberated, by the action of the acid on the ammonium sulphide. As hydrogen sulphide is too weak an acid to combine, appreciably, with aluminium hydroxide, and as the latter is difficultly soluble, the hydroxide is precipitated. According to the degree of dilution, more or less of the hydrogen sulphide also escapes. Besides this interpretation of the precipitation of aluminium hydroxide under these conditions, we may also consider the following: any aluminium sulphide, formed the first moment, would remain largely ionized and would be immediately converted, by the ions of water, into aluminium hydroxide and hydrogen sulphide. The net result of the action is the precipitation of aluminium hydroxide and the evolution of hydrogen sulphide: 2 AlCl_{3} + 3 (NH_{4})_{2}S + 6 H_{2}O → 2 Al(OH)_{3} ↓ + 6 NH_{4}Cl + 3 H_{2}S ↑ or 2 Al^{3+} + 3 S^{2−} + 6 HOH → 2 Al(OH)_{3} ↓ + 3 H_{2}S ↑. A similar result is obtained when the solution of a chromium salt is treated with a solution of ammonium sulphide. Only ferric hydroxide is capable of forming a sulphide, ferric sulphide, Fe_{2}S_{3}, which is precipitated when solutions of ferric salts are treated with ammonium sulphide.[382] Ammonium sulphide will, consequently, precipitate aluminium and chromium ‹hydroxides› and ferric, ferrous, nickel, cobalt, manganese and zinc ‹sulphides›, from a solution of the chlorides of the metals.