A colorless, flammable, extremely poisonous liquid. Salts derived from it have many industrial uses, such as hardening iron and steel, extracting metals from ores, electroplating metallic surfaces, and making acrylonitrile, from which acrylic fibers and plastics are produced. It is also used to make dyes and poisons. A solution of hydrogen cyanide in water forms a colorless acid called hydrocyanic acid. Chemical formula: HCN.