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Hydrogen cyanide



Name Hydrogen cyanide
Chemical formula HCN
Appearance Colourless liquid


Formula weight 27.0 amu
Melting point 260 K (-13 °C)
Boiling point 299 K (26 °C)
Density 0.7 ×103 kg/m3
Solubility very soluble


ΔfH0gas 135.14 kJ/mol
ΔfH0liquid 109 kJ/mol
ΔfH0solid 100 kJ/mol
S0gas, 1 bar 201.82 J/mol·K
S0liquid, 1 bar 113.01 J/mol·K
S0solid ? J/mol·K


Ingestion Extremely toxic. Early symptoms include nausea, vomiting and abdominal pain.
Inhalation Extremely dangerous. Early symptoms include slow breathing rate, irritation.
Skin Poisoning is thought to be possible through the skin.
Eyes Dilated pupils are a symptom of poisoning.
More info Hazardous Chemical Database
SI units were used where possible. Unless otherwise stated, standard conditions were used.

Disclaimer and references

Hydrogen cyanide, also known as formonitrile, is the chemical compound with chemical formula HCN (with the carbon atom (C) being triple bonded to the nitrogen atom (N)).

Hydrogen cyanide is highly poisonous. It is colorless and has a faint, bitter, almond-like odor. Some people are unable to smell cyanide at all, due to a genetic trait. The melting point is at -14°C and the boiling point is at 26°C.

Hydrogen cyanide is weakly acidic and partly converts to the cyanide ion CN in aqueous solution. Such a solution is called prussic acid or hydrocyanic acid. The salts of hydrogen cyanide are known as cyanides.

Hydrogen cyanide is produced in large quantities all over the world by the chemical industry where it is used in tempering steel, dyeing, explosives, engraving, the production of acrylic resin plastic, and other organic chemical products. It can be produced by reacting a cyanide salt with a strong acid, or directly from ammonia and carbon monoxide.

Fruits that have a pit, such as cherries or apricots, often contain small quantities of hydrogen cyanide in the pit. Bitter almonds from which almond oil and flavoring is made also contain hydrogen cyanide. Hydrogen cyanide is contained in the exhaust of vehicles, in tobacco smoke and in the smoke of burning plastics. A deep blue pigment called Prussian Blue, used in the making of blueprints, also contains hydrogen cyanide.

An HCN concentration of 300 parts per million of air will kill a human in a few minutes. The toxicity is caused by the cyanide ion. The mechanism of this toxicity, and the uses of the poison, are described on the cyanide page.

Hydrogen cyanide gas in air is explosive at concentrations over 56,000 ppm.