Silicic acid

In chemistry, a silicic acid (sɪˈlɪsɪk) is any chemical compound containing the element silicon attached to oxide () and hydroxyl () groups, with the general formula or, equivalently, . Orthosilicic acid is a representative example. Silicic acids are rarely observed in isolation, but are thought to exist in aqueous solutions, including seawater, and play a role in biomineralization. They are typically colorless weak acids that are sparingly soluble in water. Like the silicate anions, which are their better known conjugate bases, silicic acids are proposed to be oligomeric or polymeric. No simple silicic acid has ever been identified, since these species are primarily of theoretical interest. Depending on the number of silicon atoms present, there are mono- and polysilicic (di-, tri-, tetrasilicic, etc.) acids. Well defined silicic acids have not been obtained in a form that has been characterized by X-ray crystallography. Silicic acids can be seen as hydrated forms of silica, namely . Indeed, in concentrated solutions, silicic acids generally polymerize and condense, and ultimately degrade to silicon dioxide and water. The intermediate stages may be very thick liquids or gel-like solids. Dehydrating the latter yields a hard translucent form of silica with atomic-scale pores, called silica gel, which is widely used as water absorbent and drying agent. Silica dissolves very sparingly in water and is present in seawater at concentrations below 100 parts per million. In such dilute solutions, silica is assumed to exist as orthosilicic acid. Theoretical computations indicate that the dissolution of silica in water proceeds through the formation of a complex and then orthosilicic acid. The silicon–oxygen double bond of metasilicic acid, implied by the formula , is hypothetical or highly unstable. Such double bonds can be hydrated to a pair of hydroxyl () groups: =Si=O + H2O =Si(-OH)2 For example, \overset{metasilicic\ acid}{H2SiO3} + H2O \overset{orthosilicic\ acid}{H4SiO4} or \overset{disilicic\ acid}{H2Si2O5} + 2 H2O \overset{pyrosilicic\ acid}{(HO)3Si-O-Si(OH)3} Alternatively, metasilicic acid is liable to form cyclic polymers , which can be opened by hydration to chain polymers .