A halous acid, also known as a halogenous acid,[1] is an oxyacid consisting of a halogen atom in the +3 oxidation state single-bonded to a hydroxyl group and double-bonded to an oxygen atom.
One method is to add oxygen to the corresponding hypohalous acid.
Another method of oxidizing hypobromous acid can be used: The oxidized bromine-containing acid need not contain oxygen originally, as in this comproportionation reaction: Bromous acid was originally prepared using another method of adding oxygen to hypobromous acid, where the free oxygen was freed from water when the free hydrogen was taken by NO3 (to form nitric acid) that was freed from silver nitrate when the silver was taken by elemental bromine to form silver bromide.
Chlorous acid may be generated from salts of the conjugate base, chlorite, which is more stable than chlorous acid.
Chlorous acid is the only isolatable halous acid,[5] and while it has stable salts, they tend to decompose rapidly, some even explosively, upon heating.