Hydrogen cyanide (HCN) is a chemical compound with the formula HCN and structural formula H−C≡N. It is a colorless, extremely poisonous, 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. It is more toxic than solid cyanide compounds due to its volatile nature.

Some key properties of HCN include:

• Structure: Hydrogen cyanide is a linear molecule, with a triple bond between carbon and nitrogen. The tautomer of HCN is HNC, hydrogen isocyanide.

• Acidity: Hydrogen cyanide is weakly acidic with a pKa of 9.2. It partially ionizes in water solution to give the cyanide anion, CN−. A solution of hydrogen cyanide in water, represented as HCN, is called hydrocyanic acid. The salts of the cyanide anion are known as cyanides.

• Uses: HCN is the precursor to sodium cyanide and potassium cyanide, which are used mainly in gold and silver mining and for the electroplating of those metals. Via the intermediacy of cyanohydrins, a variety of useful organic compounds are prepared from HCN including the monomer methyl methacrylate, from acetone, the amino acid methionine, via the Strecker synthesis, and the chelating agents EDTA and NTA.

HCN is also a rapid-acting lethal agent that inhibits aerobic respiration at the cellular level, preventing cells from utilizing oxygen. It is highly toxic and exposure can occur through inhalation or skin absorption. The odor of HCN is often described as bitter almonds or marzipan, but some people cannot detect it. Air purifying gas masks with filters treated to absorb cyanide can be used for protection.