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Hydrazoic acid

IUPAC name hydrogen azide
CAS number 7782-79-8
Molecular formula HN3
Molar mass 43.03 g/mol
Melting point

−80 °C (193.15 K)

Boiling point

37 °C (310.15 K)

Acidity (pKa) 4.6 to 4.7
Molecular shape Linear
Main hazards Highly toxic, explosive.
R-phrases R1, R2
Related Compounds
Other anions Hydrogen chloride
Other cations Sodium azide
Related acids Hydrochloric acid
hydrocyanic acid
Related compounds Hydrazine
Except where noted otherwise, data are given for
materials in their standard state
(at 25 °C, 100 kPa)

Infobox disclaimer and references

Hydrazoic acid, HN3, is a colorless, volatile, and extremely explosive liquid at room temperature and pressure.



Hydrazoic acid is used primarily for preservation of stock solutions, and as a reagent.


It was first isolated in 1890 by Theodor Curtius (Berichte, 1890, 23, p. 3023).


It is soluble in water, and the solution dissolves many metals (e.g. zinc, iron) with liberation of hydrogen and formation of salts (azides, formerly also called azoimides or hydrazoates).

All the salts are explosive and readily interact with the alkyl iodides. In its properties it shows some analogy to the halogen acids, since it forms poorly soluble lead, silver and mercurous salts (in water). The metallic salts all crystallize in the anhydrous form and decompose on heating, leaving a residue of the pure metal. It is a weak acid (pKa 4.6-4.7).


The acid is usually formed by acidification of an azide salt like sodium azide. Normally solutions of sodium azide in water contain trace quantities of hydrazoic acid in equilibrium with the azide salt, but introduction of a stronger acid can convert the primary species in solution to hydrazoic acid. The pure acid may be subsequently obtained by fractional distillation as an extremely explosive colorless liquid with an unpleasant smell.


Hydrazoic acid is volatile and highly toxic. Its unbearable smell and the violent headache caused by breathing the vapor conspire to make accidental poisoning impossible. The compound acts a non-cumulative poison.


  • Dictionary of inorganic and organometallic compounds, Chapman & Hall

This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Hydrazoic_acid". A list of authors is available in Wikipedia.
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