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With the chemical formula H2SiO3, it is actually a hydrate of silica, SiO2·H2O. Silicic acid is a weak dibasic acid that is almost insoluble in water, with K1 ≈ 10-9 and K2 ≈ 10-12. Silicic acid cannot be produced by the direct reaction of SiO2 and water; instead, it is obtained by reacting silicates with acids. The resulting silicic acid, depending on the concentration of the solution, either forms a sol or turns into a gel-like precipitate. Silicic acid can also condense to form polyacids. The silicic acids corresponding to various silicates include orthosilicic acid H4SiO4 (SiO2·2H2O), metasilicic acid H2SiO3 (SiO2·H2O), and trisilicic acid H4Si3O8 (3SiO2·2H2O), among others. Their composition can be expressed by the general formula xSiO2·yH2O, where x and y are positive integers. Silicic acids with x>1 are known as polysilicic acids; those that have been identified include orthosilicic acid, metasilicic acid, disilicic acid H6Si2O7, and trisilicic acid. Since metasilicic acid has the simplest formula, in reactions silicic acid is generally written as H2SiO3. If the water contained in silicic acid gel is removed by drying, it turns into a white, transparent, porous solid substance known as silica gel. It has a strong adsorption capacity and can absorb various gases and vapors. Therefore, it is often used as a carrier for adsorbents, desiccants, and certain catalysts.