Hydrogen Cyanide, also called hydrocyanic acid and prussic acid, extremely poisonous, colorless liquid with a bitter-almond odor. The compound's chemical formula is HCN, the term cyanide applying to the -CN structure. HCN melts at -14° C (6.8° F) and boils at 25.7° C (78.2° F). A few milligrams of the substance and of related cyanides can be rapidly fatal to humans, acting by blocking the ability of cells to use oxygen. The compound mixes with water, alcohol, and ether in all proportions. When impurities are present, HCN molecules can combine to form a black solid; the reaction may be explosive unless inhibited. HCN is flammable and can form explosive mixtures with air. It was once produced from the pigment Prussian blue, hence its secondary name. Now it is prepared commercially by the reaction of methane with ammonia in the presence of a platinum catalyst.

One major use of HCN has been in the production of acrylonitrile, a starting compound for various products; however, this process has been replaced by another process using propylene. HCN has many other industrial uses, including the making of plastics and several important cyanide compounds. The latter include sodium cyanide (NaCN) and potassium cyanide (KCN), both of them important in metallurgy. They are used in recovering gold and silver from ores, for example, and NaCN is used in the hardening of steel. Several cyanide compounds are employed in the electroplating of metals such as silver, gold, copper, and platinum.

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