This webpage is maintained by the Centers for Disease Control and Prevention (CDC). The webpage provides information and resources on hydrofluoric acid. "[...] Hydrogen fluoride is a chemical compound that contains fluorine. It can exist as a colorless gas or as a fuming liquid, or it can be dissolved in water. [...] When hydrogen fluoride is dissolved in water, it may be called hydrofluoric acid. [...] Hydrogen fluoride can be released when other fluoride-containing compounds such as ammonium fluoride are combined with water."
Centers for Disease Control and Prevention: http://www.cdc.gov/