Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/534

522 CHEMISTRY [SILICON. hydrogen. When heated by itself, it is decomposed into amorphous silicon and free hydrogen. It is insoluble in water, and is not altered by dilute hydrochloric or sul phuric acid ; potassium hydroxide, however, decomposes it readDy SiH 4 + 2KOH + H 2 = K 2 Si0 3 + 4H 2 . Silicon hydride. Potassium silicate. It precipitates silicide of copper from a solution of copper sulphate, and metallic silver mixed with a black substance, which is, perhaps, silver silicide, from a solution of silver nitrate. By heating silicon in chlorine gas, or in an atmosphere of bromine, or iodine, it is converted into the compounds SiCl 4 , SiBr 4 , and SiI 4 ; the chloride and bromide are best prepared, however, by passing chlorine or bromine vapour over an intimate mixture of silica and charcoal heated to redness. Silicon tetrachloride and tetrabromide are colour less liquids, boiling at 50 3 C. and 153 C. respectively ; the tetriodide crystallizes from its solution in carbon di- sulphide in colourless octahedra (which melt at 120 5 C.), and boils at about 290 C., distilling without alteration in a stream of carbon dioxide. They are quickly decomposed by water ; thus SiCl 4 + 2H 2 O = Si0 2 + 4HC1 . "When silicon tetriodide is heated to near its boiling point with finely divided metallic silver it is converted into disili- con hexiodide, Si 2 I 6 ; this crystallizes from carbon disulphide in colourless hexagonal prisms which are decomposed on fusion into the tetriodide and a body which is perhaps the subiodide, Si 2 I 4 . By the action of bromine, this compound is converted into the corresponding bromide, Si 2 Br G , and by gently heating it with mercuric chloride the chloride Si 2 Cl 6 is obtained. The bromide forms crystals which distil without decomposition at about 240 C. ; disilicon hexachloride is a colourless mobile liquid, which boils at 146-148 C., the corresponding carbon chloride, C 2 C1 6 , being a crystalline solid which boils at 330 C, ; it is very slowly decomposed when heated in closed vessels to 350 C. into the tetrachloride and silicon, and is entirely con verted into these substances at 800 C. A lower liquid chloride, which probably has the composition Si 2 Cl 4 , is obtained in small quantity together with the hexachloride when the vapour of the tetrachloride is passed over fused silicon heated to a very high temperature. By passing the vapour of silicon tetrachloride through an exhausted porcelain tube containing fragments of felspar heated nearly to the fusing point of the latter, it is partially converted into the oxychloride (SiCl 3 ),0; this is a colourless liquid boiling at 13G-139 C., which is decomposed by water in the same manner as silicon tetrachloride. When the vapour of this compound mixed with oxygen is passed several times through a heated tube, silicon tetrachloride is reproduced, and a complex mixture of higher liquid oxychlorides is obtained containing the compoundsSi,OoCl in (b. p. 152-154 C.), Si 4 4 01 8 (b. p. about 200 C.), Si 8 O 10 Clj 2 (b. p. about 300 C.), together with still higher oxychlorides. By passing hydrochloric acid over crystallized silicon heated to barely visible redness, a mixture is produced of silicon tetrachloride with the compound SiH01 3 , which from its relation in composition to trichloromethane or chloroform, CHC1 3 , is termed silicon-chloroform. The corresponding iodo-derivative, SiHI 3 , is formed in a similar manner by the action of hydriodic acid gas mixed with hydrogen. Silicon chloroform is a colourless extremely mobile liquid, and boils at about 36 C. ; it is converted by chlorine at ordinary temperatures into the tetrachloride, and when heated with bromine it furnishes silicon tri- chlorobromide, SiCl.^Br. which boils at 80 C. Silicon iodoform, SiHI 3 , is a colourless, strongly refracting liquid; it boils at about 220 C. When a mixture of silicon tetrachloride vapour and hy drogen sulphide gas is passed through a tube heated to redness, the compound SiCl 3 .SH is obtained ; it is a colour less liquid boiling at 96 C. Hydrofluoric acid acts readily on silicon dioxide and silicious substances generally, forming silicon tetrafluoride : Si0 2 + 4HF = SiF 4 + 2H 2 0. Silicon tetrafluoride is a colourless gas, which liquefies under very strong pressure, and solidifies at - 140 C. ; it is not inflammable. It is readily absorbed by water, which decomposes it into silica and hydrofluosilicic acid : 3SiF 4 + 2H 2 = 2H 2 SiF + SiO 2 . This acid is not known except in solution, but its salts, which are readily produced by neutralizing its solution with metallic oxides, hydroxides, or carbonates, are stable compounds, and are mostly soluble in water. By prolonged ignition they are decomposed into silicon fluoride which escapes, and a metallic fluoride which remains behind. Potassium fluosilicate, K 2 SiF 6 , is one of the least soluble of the potassium salts, and consequently this acid is often used to precipitate potassium from solutions of its salts. By passing the tetrafluoride over silicon heated nearly to the softening point of porcelain, it is converted into a subfluoride, which probably has the composition Si 2 F 6 ; it is a white, very light powder. Only one oxide of silicon is known, viz., the dioxide or silica, SiO 2 . According to Troost and Hautefeuille, its formation from amorphous silicon and oxygen is attended with the development of a very large amount of heat, viz., (Si , O 2 ) = 219,240 units of heat. According to the same observers, in the formation of the tetrachloride from amorphous silicon and chlorine, and in its decomposition by 140 times its weight of water, the following amounts of heat are developed : (Si,Cl 4 ) =157,010 (SiCl 4 ,Aq) = 81,040 and from the amount of heat developed on dissolving amorphous and crystallized (? adamantine) silicon in a mixture of nitric and hydrofluoric acids they calculate that the transformation of the former into the latter is attended with the development of 8120 units of heat. When obtained by igniting the gelatinous silica from the decomposition of the haloid silicon compounds by water, or of the soluble silicates by acids, silica is a snow- whits amorphous powder ; but it occurs native as quartz in large transparent crystals of the hexagonal system, of the specific gravity 2 5 to 2 - 8, and of extreme hardness. Native silica of all kinds is insoluble in water and in all acids except hydrofluoric acid ; this is also true of artificial ignited silica. The gelatinous hydrate which separates on the addition of acids to solutions of alkaline silicates is also almost insoluble in water and in acids, but that which results from the decomposition of silicon fluoride is stated to be soluble in a considerable quantity of water. Gelatinous silica dissolves without difficulty in solutions of potassium or sodium hydroxide at the ordinary temperature, and all forms of silica are more or less readily dissolved by solutions of these alkalies, especially when heated with them under pressure ; in this manner, according to the proportions of silica and alkali employed, various alkaline silicates are formed which are the more soluble the greater the proportion of alkali metal they contain. Those which contain a larger proportion than is indicated by the formula N"a 2 SiO 3 , but still a sufficient quantity to make them completely soluble in water, are termed " water-glasses ; " water-glass is now largely manu

factured and employed for hardening and preserving stone,