Hexafluorosilicic acid is an inorganic compound with the chemical formula  (also written as ). It is a colorless liquid rarely encountered undiluted. Hexafluorosilicic acid has a distinctive sour taste and pungent smell. It is mainly produced as a precursor to aluminum trifluoride and synthetic cryolite. It is commonly used as a source of fluoride for water fluoridation. Concentrated hexafluorosilicic acid is corrosive and can attack the skin, but in everyday use, it is so diluted as to be harmless.