Chlorous acid is an inorganic compound with the formula HClO<sub>2</sub>. It is a weak acid. Chlorine has oxidation state +3 in this acid. The pure substance is unstable, disproportionating to hypochlorous acid (Cl oxidation state +1) and chloric acid (Cl oxidation state +5):
Although the acid is difficult to obtain in pure substance, the conjugate base, chlorite, derived from this acid is stable. One example of a salt of this anion is the well-known sodium chlorite. This and related salts are sometimes used in the production of chlorine dioxide.
HClO<sub>2</sub> can be prepared through reaction of barium or lead chlorite and dilute sulfuric acid:
Chlorous acid is a powerful oxidizing agent, although its tendency to undergo disproportionation counteracts its oxidizing potential.
Chlorine is the only halogen to form an isolable acid of formula HXO<sub>2</sub>. Fluorine is resistant to oxidation, having a âÂÂ1 oxidation state even in hypofluorous acid, and is thus unable to form fluorous acid, or any higher oxoacids; despite the name, fluorite minerals are chemically fluoride compounds. Neither bromous acid nor iodous acid has ever been isolated. A few salts of bromous acid, bromites, are known, but no iodites.