Hexafluorosilicic acid is a chemical compound that is an acid. Its chemical formula is H2SiF6. It is made by the reaction of hydrofluoric acid with silicon dioxide when the hydrofluoric acid is produced. It can be reacted with bases such as sodium hydroxide to produce a fluorosilicate salt such as sodium fluorosilicate. It is used to make artificial cryolite. It releases hydrogen fluoride when it is heated, so it is toxic like hydrofluoric acid. It is used in fluoridation of water. 