Nitrous Acid - HNO2

What is Nitrous Acid?

Nitrous acid is unstable and weakly acidic compound. It exists in the solution of nitrate salts only. Nitrous acid is a mixture of nitric acid and nitrous gas but with a smaller proportion of oxygen than what exists in nitric acid. Nitrous acid was discovered by Scheele. It is obtained by distilling perfectly dry nitrate of lead in a retort, fitted with a receiver. A liquid comes over which is nitrous acid.


Nitrous Acid


1 g/cm³

Molecular Weight/ Molar Mass

47.013 g/mol

Boiling Point

158 °C

Compound Formula


Nitrous Acid Structure – HNO2

Nitrous Acid

Physical Properties of Nitrous Acid – HNO2


Unpleasantly bitter or pungent


Pale blue solution

Specific Density

1.35 g/cm3

Systematic name‎

‎Dioxonitric(III) acid

Chemical Properties of Nitrous Acid – HNO2

  • It is strongly acidic, extremely volatile and rises in thick fumes; boils at the low temperature of 82oC and the specific gravity is 1.45
  • In vapour state nitrous acid remains unchanged by the action of heat, but when mixed with water an effervescence takes place with the evolution of nitrous gas.
  • Nitrous acid in its fuming state is totally irrespirable but supports the combustion of phosphorus or charcoal, when these are introduced into it in a state of combustion.

Uses of Nitrous Acid – HNO2

  • Used in the preparation of diazonium salts from amines and in the preparation of azo dyes in sandmeyer reaction.
  • Used to remove the toxic nature in potentially explosive compound sodium azide.
  • Nitrous acid is also used as an oxidizer in liquid fuel rockets.

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