A weak acid, HNO₂, known only in solution and in the gas phase. It is prepared by the action of acids upon nitrites, preferably using a combination that removes the salt as an insoluble precipitate (e.g. Ba(NO₂)₂ and H₂SO₄). The solutions are unstable and decompose on heating to give nitric acid and nitrogen monoxide. Nitrous acid can function both as an oxidizing agent (forms NO) with I⁻ and Fe²⁺, or as a reducing agent (forms NO₃⁻) with, for example, Cu²⁺; the latter is most common. It is widely used (prepared in situ) for the preparation of diazonium compounds in organic chemistry. The full systematic name is dioxonitric(III) acid.