Q. Describe Sulphurous Acid ?
Sulphurous acid H2SO3 is known only in solution. It exists mainly as a hydrate of sulphur dioxide SO2. 7H2O two series of salts the bisulphites containing HSO-3 and sulphites containing SO2-3 ions are well known.
Sulphurous acid possesses both oxidising as well as reducing properties. It owes its reducing nature to the ease with which it can get oxidised to sulphuric acid. It reduces I2, to HI, KMnO4 to MnSO4, and K2, Cr207 to Cr2(SO4)3.
H2SO3 + H2O + I2----------------> H2SO4 + 2HI
2KMnO4+ 5SO2+ 2H2O-------> K2SO4+ 2MnSO4+ 2H2SO4
K2Cr2O7+ 3SO2+ H2SO4---------> K2SO4+ Cr2 (SO4) 3 + H2O
In the presence of strong reducing agents like H2S, it can behave as an oxidising agent.
2H2S+ H2SO3-------------------> 3H2O + 3S
Sodium and calcium bisulphite solutions are used in making paper from, wood because they dissolve fibrous material lignin and facilitate pulping of cellulose.