The hydride of fluorine and the first member of the family of halogen acids. Anhydrous hydrogen fluoride is a mobile, colorless liquid that fumes strongly in air. It has the empirical formula HF, melts at −83°C (−117°F) and boils at 19.8°C (67.6°F). The vapor is highly aggregated, and gaseous hydrogen fluoride deviates from perfect gas behavior to a greater extent than any other gaseous substance known. Aggregate formation in both the vapor and liquid phase arises from unusually strong hydrogen-bond interactions. Hydrogen fluoride is prepared on the large industrial scale by treating fluorspar (calcium fluoride, CaF₂) with concentrated sulfuric acid. The crude product is purified by fractional distillation to yield a product containing more than 99.5% HF; the remaining impurities are principally water and small amounts of sulfur dioxide, silicon tetrafluoride, and boron trifluoride. Very dry hydrogen fluoride can be obtained by electrolysis or by treatment with reagents such as fluorine or cobaltic fluoride that react with water.