Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/514

Rh 502 CHE MIST 11 Y [OXIDES OP THE molecule of the latter is not represented by the formula Se0 2, but that it is more complex; and although the vapour density of selenium dioxide is such as to negative this assumption for the gaseous substance, there is no evidence to show that the expression Se0 2 is the correct molecular formula for the solid oxide. Sulphur trioxide or sulphuric anhydride is a white, highly volatile substance which dissolves in water, forming sulphuric acid, H 2 SO 4, the combination being attended with the development of so much heat that when the anhydride comes in contact with the water a violent hissing is produced, just as when a red-hot iron is plunged into water. Selenium trioxide has not been isolated, but selenic acid, H 2 SeO 4, the acid corresponding to sulphuric acid, is known, and very closely resembles the latter com pound in many of its properties. Tellurium trioxide, TeO 3, is an orange-j-ellow solid, insoluble in water, and therefore altogether different in properties from sulphur trioxide ; moreover, the resemblance between the com pound H 2 TeO 4, from which tellurium trioxide is obtained by heating to a temperature below redness, and sulphuric and selenic acids scarcely extends beyond the similarity of their formulae, since telluric acid is nearly insoluble in water, and its solution has a metallic rather than an acid taste, and reddens litmus but slightly. Sulphur dioxide is usually prepared for laboratory pur poses by heating a metal, such as copper or mercury, with concentrated sulphuric acid ; the most probable explana tion of its formation in this manner appears to be that the metal acting upon the acid produces the corresponding sulphate and hydrogen, and that the latter whilst in the nascent state acts upon another portion of the acid, reduc ing it to sulphurous acid, which splits up into sulphur dioxide and water ; thus- Hg + H 2 S0 4 = 2H + HgS0 4 Mercury. Sulphuric acid. Hydrogen. Mercuric sulphate. 2H + H 2 S0 4 = H 2 S0 3 + H 2 Hydrogen. Sulphuric acid. Sulphurous acid. Water. H 2 S0 3 = S0 2 Sulphurous acid. Sulphur dioxide. Water. H 2 0. We may also suppose, however, that the metal merely withdraws an atom of oxygen from the sulphuric acid, and that the oxide produced dissolves in the acid, forming a sulphate 24 + H 2 S0 3 Sulphuric acid. Mercuric oxide. Sulphurous acid. HO Hg + HS0 = Mercury. HgO + H 2 S0 4 = HgSO 4 + H 2 O. Mercuric oxide. Sulphuric acid. Mercuric sulphate. Water. The kind of action represented by the first of these equations undoubtedly takes place when carbon in the form of charcoal is heated with sulphuric acid, whereby carbon dioxide, sulphur dioxide, and water are produced C + 2H 2 SO 4 = CO 2 + 2SO 2 + 2H 2 0. Carbon. Sulphuric acid. Carbon dioxide. Sulphur dioxide. Waler. When copper is employed not only are sulphur dioxide, water, and cupric sulphate produced, but cuprous and cupric sulphides, Cu. 2 S and CuS, it is stated, are also formed ; their production, however, is due to secondary action. Selenium dioxide is obtained either by burning selenium in a stream of oxygen, or by evaporating a solution of selenious acid to dryness. Sulphurous acid is always prepared by passing sulphur dioxide gas into cold water ; the combination is attended with a slight elevation of temperature. Selenious acid is produced in a similar manner by dissolving the oxide in water, or by dissolving selenium in concentrated nitric acid, and evaporating to expel the excess of nitric acid. Tellurous acid may be obtained by dissolving tellurium in nitric acid of specific gravity 1*25, and pouring the solution, after the lapse of not more than a few minutes, into water. If the precipitation be delayed for a longer time, the oxide TeO 2 is thrown down instead of the hydrate. It is best prepared by decomposing tellurium tetrachloride with water TeCl 4 + 3H 2 = H 2 Te0 3 + 4HC1. Tellurium tetrachloride. Water. Tellurous hydrate. Hydrogen chloride. It is stated that anhydrous sulphurous acid, H 2 S0 3 , has been obtained in crystals by cooling a saturated aqueous solution to C. ; a crystalline hydrate of the composition H 2 S0 3 + 8H 2 O was obtained by Pierre by cooling to 6 C. a saturated solution through which a current of the gas was being transmitted. The solution of sulphur dioxide in water is strongly acid, and effervesces with carbonates ; by passing a current of the gas through water in which metallic hydroxides or carbonates are dissolved or suspended metallic sulphites are produced. Two classes of metallic sulphites may be thus formed : the acid sulphites or bisulphites in which one-half the hydrogen in sulphurous acid is displaced by a metal ; and normal sulphites, in which the whole of the hydrogen is displaced. It is also possible to displace the two atoms of hydrogen in sulphurous acid by two different metals, and thus to obtain so-called double salts. The following table shows the composition of some of the sulphites, disregarding the water of crystallization which several of them contain : Potassium hydrogen sulphite KHS0 3 Potassium sulphite K 2 SO 3 Sodium hydrogen sulphite NaHS0 3 Sodium sulphite Na 2 S0 3 Calcium sulphite CaS0 3 Calcium hydrogen sulphite CaH. 2 (S0 3 ) 2 The acid sulphites of barium, strontium, calcium, and magnesium, and the acid and normal sulphites of the alkali metals, are soluble in water, although only the sulphites of alkali metals are freely soluble; but most other sulphites are insoluble, and may be prepared by precipita tion with an alkaline sulphite. The acid sulphites are prepared by saturating a solution of the metallic hydroxide or carbonate with sulphur dioxide gas, and then adding to it as much of the hydroxide or carbonate as it originally contained ; thus Na 2 C0 3 + OH 2 + 2S0 2 = 2NaHS0 3 + CO 2 Sodium carbonate. Water. Sulphur dioxide. 2NaHSO 3 Sodium hydrogen sulphite. Na 2 CO 3 Sodium carbonate. = 2Na 2 S0 3 + Sodium sulphite. H 2 Water. h C0 2. Carbon dioxide. The alkali and alkaline-earth metals are the only ones, however, which readily produce acid sulphites ; in fact, the tendency to form acid salts is almost restricted to these metals in the case of all acids, and it is difficult to obtain acid salts containing heavy metals. A solution of sulphurous acid slowly absorbs oxygen from the air and is converted into sulphuric acid ; in like manner, the sulphites, particularly if in solution, become converted into sulphates on exposure to the air. All sulphites are decomposed at a red heat, either into sulphate and sulphide, or into sulphur dioxide and metallic oxide. They are also decomposed by all acids excepting carbonic and boric acids, sulphurous acid being liberated. Sulphur ous acid possesses considerable bleaching powers, and is ex tensively employed in bleaching straw, wool, and many other articles, which would be injured by chlorine. The articles to be bleached are moistened and suspended in closed chambers in which sulphur is burnt ; the sulphur dioxide produced is then absorbed by the damp goods, and their colour is discharged. The manner in which it acts is not wall understood, but it appears to be by form ing colourless compounds with the colouring matters ; it does not, like chlorine, decompose colouring matters, for