A weak dibasic acid, H2SO3, known in the form of its salts: the sulphites and hydrogensulphites. It is considered to be formed (along with sulphuric acid) when sulphur dioxide is dissolved in water. It is probable, however, that the molecule H2SO3 is not present and that the solution contains hydrated SO2. It is a reducing agent. The systematic name is trioxosulphuric(IV) acid. See also sulphuric acid.