Hydrogen cyanide (also called prussic acid) is a chemical compound with the formula HCN and structural formula H−C≡N. It is a highly toxic and flammable liquid that boils slightly above room temperature, at 25.6 °C (78.1 °F). HCN is produced on an industrial scale and is a highly valued precursor to many chemical compounds ranging from polymers to pharmaceuticals. Large-scale applications are for the production of potassium cyanide and adiponitrile, used in mining and plastics, respectively. It is more toxic than solid cyanide compounds due to its volatile nature. A solution of hydrogen cyanide in water, represented as HCN(aq), is called hydrocyanic acid. The salts of the cyanide anion are known as cyanides.
Whether hydrogen cyanide is an organic compound or not is a topic of debate among chemists. It is traditionally considered inorganic, but can also be considered a nitrile, giving rise to its alternative names of methanenitrile and formonitrile.