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a highly volatile, colourless, and extremely poisonous liquid (boiling point 26 C [79 F], freezing point -14 C [7 F]). A solution of hydrogen cyanide in water is called hydrocyanic acid, or prussic acid. It was discovered in 1782 by a Swedish chemist, Carl Wilhelm Scheele, who prepared it from the pigment Prussian blue. Hydrogen cyanide and its compounds are used for many chemical processes, including fumigation, the case hardening of iron and steel, electroplating, and the concentration of ores. It also is employed in the preparation of acrylonitrile, which is used in the production of acrylic fibres, synthetic rubber, and plastics.