Only one oxide of silicon, namely the dioxide or silica, is defi nitely known (see SILICA) although SiO has been described.
Halides.--Silicon fluoride, SiF 4, is formed when silicon is brought into contact with fluorine; or by decomposing a mixture of acid potassium fluoride and silica, or of calcium fluoride and silica with concentrated sulphuric acid. It is a colourless, strongly fuming gas with suffocating smell and solidifies at —97° C. It is decomposed with great violence when heated in contact with either sodium or potassium. It combines directly with ammonia to form the compound and is absorbed by dry boric acid and by many metallic oxides. Water decomposes it into silico fluoric acid and silicic acid : hy drogen fluoride being an intermediate product. It combines directly with acetone and with various amines.
It is very unstable, decomposing slowly, even at ordinary tem peratures, into hydrogen, silicon fluoride and silicon: It burns with a pale-blue flame forming silicon fluoride, silico fluoric acid and silicic acid. It is decomposed readily by water, alcohol and ether, yielding respectively silicofluoric acid, H2SiF6, ethyl orthosilicate, Si(OC2H5)4, and ethyl orthosilicoformate, Sill (0C2H5) 3.
Silicofluoric or hydrofluosilicic acid, H2SiF6, is obtained as shown above, and also by the action of sulphuric acid on barium silicofluoride, or by absorbing silicon fluoride in aqueous hydro fluoric acid. The concentrated solution deposits a hydrated form, The anhydrous acid is not known, since on further heating alone or in aqueous solution it gradually decomposes into silicon fluoride and hydrofluoric acid.
Silicon chloride, was prepared by J. J. Berzelius by the action of chlorine on silicon, and is also obtained when an inti mate mixture of silica and carbon (or sulphur) is heated in a stream of chlorine and the products of reaction fractionated. It