Hydrocyanic Acid - HCN

What is Hydrocyanic Acid?

Hydrocyanic acid is a liquid of hydrogen cyanide in water. The chemical formula of Hydrocyanic acid is HCN. Hydrocyanic acid is a colorless liquid whose vapor is lighter than air and dissipates rapidly. It is usually sold commercially as an aqueous solution containing 2 to 10% hydrogen cyanide.The aqueous solutions of hydrogen cyanide decomposes slowly to form ammonium formate. It is very poisonous colourless gas with a characteristic fragrance of bitter almonds. It is a dangerous transparent liquid its storage and transport are prohibited.

Other names – Prussic acid, acetonitrile, formonitrile


Hydrocyanic acid


687 kg/m³

Molecular Weight/ Molar Mass

27.0253 g/mol

Boiling Point

25.6 °C

Melting Point

-13.4 °C

Chemical Formula


Hydrocyanic Acid Structure – HCN

Hydrocyanic Acid Structure

Physical Properties of Hydrocyanic Acid – HCN


Bitter almond smell


Pale blue

Covalently-Bonded Unit


Heat capacity

35.9 J K−1 mol−1

Hydrogen Bond Donor



Miscible in water and ethanol

Chemical Properties of Hydrocyanic Acid – HCN

  • Hydrocyanic acid reacts with base like sodium hydroxide forms sodium cyanide and water. The chemical equation is given below.
  • HCN + NaOH → NaCN + H2O

  • Hydrocyanic acid reacts with potassium hydroxide forms potassium cyanide and water. The chemical equation is given below.
  • HCN + KOH → H2O + KCN

Uses of Hydrocyanic Acid – HCN

  • Used as a fugiment to kill pests such as rodents in warehouses, grain storage bins, greenhouses and holds of ships where its high toxicity and ability to penetrate obscure spaces are advantageous.
  • Used in the manufacture of cyanide salts, acrylonitrile and dyes. It is also used as a horticultural fumigant.
  • Used as a highly valuable precursor in the production of several chemicals.

Practise This Question

0.1 mol of methyl amine (Kb = 5 × 104) is mixed with 0.08 mol of HCl and the solution is diluted to 1 litre. The hydrogen ion concentration of the resulting solution is: