Thread Content
Salts of silicic acid are called silicates; they are actually a general term for compounds formed by the combination of silicon, oxygen, and metals. It is widely distributed in the Earth’s crust and is a major component of most rocks (such as granite) and soils. Most are crystals, which can be divided into two categories: those soluble in water and those insoluble in water. Water-soluble silicates have a general composition of aM2O·bSiO2·xH2O (where M is an alkali metal element). Among soluble silicates, sodium silicate Na2SiO3 is the most common, and its aqueous solution is known as water glass. It is a salt of a weak acid and a strong base; its aqueous solution is strongly alkaline. In chemical manufacturing, it can sometimes be used as a substitute for caustic soda, which is why it is also known as saponifying alkali. Water glass can be used as an adhesive, fireproof coating, and preservative for eggs. Except that alkali metal silicates are soluble salts, many other silicates are insoluble and have very complex structures as well. They are widely distributed in nature and come in many species. Feldspar, mica, asbestos, talc, and others are the most common natural silicates. Their chemical formulas are complex; they are generally considered to be compounds formed by the combination of acidic oxides and basic oxides. The chemical formulas of silicates are often expressed using the formulas of common oxides. For example: Feldspar – K2O·Al2O3·6SiO2 or K2Al2Si6O16; Mica – K2O·3Al2O3·6SiO2·2H2O or K2H4Al6(SiO4)6; Kaolin – Al2O3·2SiO2·2H2O or Al2H4Si2O9; Asbestos – CaO·2MgO·4SiO2 or Mg2Ca(SiO3)4; Talc – 3MgO·4SiO2·H2O or Mg3H4(SiO3)4; Zeolites – Na2O·Al2O3·2SiO2·nH2O or Na2Al2(SiO4)2·nH2O. Among the insoluble silicates, some are synthetically produced, such as silica-alumina adsorbents. Due to the strong chemical bonds between silicon-oxygen atoms, silicates exhibit good chemical stability, mechanical strength, fire resistance, and a high melting point. They are not only the main raw materials for the silicate industry but also important materials for the construction industry.