In aqueous solution, sulphur dioxide is combined in small proportions with water, creating sulphurous acid, which is a weak acid.
The chemical reaction that describes the formation of sulphurous acid is presented below:
`SO_2 + H_2O -> H_2SO_3`
Sulphur dioxide is, therefore, the anhydride of sulphurous acid. Most of the sulfur dioxide in water solution is physically dissolved as sulphur dioxide, `SO_2` . When sulphurous acid is heated or evaporated, it releases sulphur dioxide, therefore, sulphurous acid can not be isolated.
Sulphurous acid (`SO_2` in `H_2O` solution) is a strong reducing agent. It reduces the permanganate to `Mn^(2+)` by dissolving manganese dioxide.
`MnO_2 + H_2SO_3 -> MnSO_4 + H_2O`
Sulphurous acid reduces potassium chromate to chromium sulphate.
`K_2Cr_2O_7 + 4H_2SO_3 -> K_2SO_3 + Cr_2(SO_4)_3 + H_2O`
Halogens oxidize sulphurous acid to sulphuric acid. The reaction is virtually irreversible in the case of chlorine, but is reversible in case of bromine, and iodine.
`Cl_2 + H_2SO_3 + H_2O -> 2Cl^- + SO_4^(2-) + 4H^(+)`