«Aluminium Hydroxide an Amphoteric Hydroxide.»—Whereas the hydroxides of the alkali and alkaline earth metals are bases, pure and simple, aluminium hydroxide shows the properties both of a base and of an acid; it is an ‹amphoteric› hydroxide, the term "amphoteric" indicating the combination of acid with basic properties in any compound. Aluminium hydroxide dissolves in acids. From its solution in hydrochloric acid, an ‹aluminium› salt, aluminium chloride AlCl_{3}, 6 H_{2}O, is obtained. It also combines with strong bases, dissolving for instance in a solution of sodium hydroxide and forming an ‹aluminate›, NaAlO_{2}. The two salts mentioned are typical representatives of the two series of salts, which aluminium hydroxide is capable of forming. This dual character of the hydroxide raises a number of interesting questions, which one meets with quite frequently in the study of analytical reactions. One may ask, first, how aluminium hydroxide can ionize both as an acid and as a base; second, whether any reason can be given, why it should show the dual nature; and third, if it is both base and acid, why it does not neutralize itself. According to the best knowledge we have on the subject, the molecule of aluminium hydroxide has the following ‹structure› or arrangement of its atoms: Al(—O—H)_{3}. It is readily seen that the cleavage of the molecules may produce, [p172] either aluminium and hydroxide ions, characteristic ions of a base, or aluminate[354] and hydrogen ions, characteristic ions of an acid: Al^{3+} + 3 ^{−}OH ⇄ Al(—0—H)_{3} ⇄ AlO_{2}^{−} + H^{+} + H_{2}O. The ionization of the hydroxide both as an acid and as a base is, thus, quite possible on the basis of the molecular structure assigned to it. In fact, all of the so-called oxygen acids are considered to be hydroxides—we have sulphuric acid, O_{2}S(OH)_{2}, phosphoric acid, OP(OH)_{3}, etc.,—exactly as the bases, Mg(OH)_{2}, etc., are hydroxides.