III.

Compounds

Tin forms stannic acid, H₂SnO ₄, when heated in air or oxygen at high temperatures. It dissolves in hydrochloric acid to form stannous chloride, SnCl₂, and in aqua regia to form stannic chloride, SnCl₄, and it reacts with sodium hydroxide solution to form sodium stannite and hydrogen gas. In cold and very dilute nitric acid, tin dissolves to form stannous nitrate and ammonium nitrate; in concentrated nitric acid, it produces metastannic acid, H₂SnO₃. Stannous sulphide, SnS, is yielded as a dark brown precipitate by the action of hydrogen sulphide on a solution of stannous chloride. Stannic sulphide, SnS₂, is produced by passing hydrogen sulphide through a solution of stannic salt. The two hydroxides of tin, Sn(OH)₂ and Sn(OH)₄, are produced by adding a soluble hydroxide to solutions of stannous and stannic salts. Stannous oxide, SnO, a black insoluble powder, is obtained by heating stannous oxalate in the absence of air. In the presence of air, stannous oxide burns to form the dioxide, or stannic oxide, SnO₂, a white insoluble solid. The dioxide may also be prepared by heating stannic acid or by heating tin metal in air at high temperatures.