A group of seven hydrated forms of SiO₂, including the following silicic acids: tetra, H₂Si₄O₉, meta-di, H₂Si₂O₅, meta-tri, H₄Si₃O₈, meta, H₂SiO₃, ortho-tri, H₈Si₃O₁₀, ortho-di, H₆Si₂O₇ and ortho, H₄SiO₄. The latter formula is often written as Si(OH)₄. Silicic acids and silicate anions polymerize through formation of multiple Si-O-Si bonds. The polysilic structure can be linear or cyclic and is not uniform in size.