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Hydrofluoric acid

- a compound of hydrogen and fluorine dissolved in water to produce an extremely corrosive acid traditionally used to etch glass. Fluorine is the most reactive of the halogens and extremely dangerous to handle. Paradoxically, many fluorine compounds are extremely stable, and used to inhibit corrosion, adhesion and oxidation, eg sodium monofluorophosphate in toothpaste, bromochlorodimethylfluoride in fire extinguishers and chlorinated fluorocarbons as nearly inert working fluids in heat pumps. Unfortunately these compounds become unstable when exposed to intense UV radiation and ozone, hence the problems in the upper atmosphere. All of which makes it seem particularly extraordinary that hydrofluoric acid is used in stone cleaning.