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 SO^2-₃ 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.