Properties. As obtained by chemical processes silicon dioxide is an amorphous white powder. In the crystallized state it is very hard and has a density of 2.6. It is insoluble in water and in most chemical reagents, and requires the hottest oxyhydrogen flame for fusion. Acids, excepting hydrofluoric acid, have little action on it, and it requires the most energetic reducing agents to deprive it of oxygen. It is the anhydride of an acid, and consequently it dissolves in fused alkalis to form silicates. Being nonvolatile, it will drive out most other anhydrides when heated to a high temperature with their salts, especially when the silicates so formed are fusible. The following equations illustrate this property:
Na2CO3 + SiO2 = Na2SiO3 + CO2,
Na2SO4 + SiO2 = Na2SiO3 + SO3.
Silicic acids. Silicon forms two simple acids, orthosilicic acid (H4SiO4) and metasilicic acid (H2SiO3). Orthosilicic acid is formed as a jelly-like mass when orthosilicates are treated with strong acids such as hydrochloric. On attempting to dry this acid it loses water, passing into metasilicic or common silicic acid:
H4SiO4 = H2SiO3 + H2O.
Metasilicic acid when heated breaks up into silica and water, thus:
H2SiO3 = H2O + SiO2.
Salts of silicic acids,—silicates. A number of salts of the orthosilicic and metasilicic acids occur in nature. Thus mica (KAlSiO4) is a salt of orthosilicic acid.
Polysilicic acids. Silicon has the power to form a great many complex acids which may be regarded as derived from the union of several molecules of the orthosilicic acid, with the loss of water. Thus we have
3H4SiO4 = H4Si3O8 + 4H2O.