Q.Write short notes on bleaching action of Sulphur dioxide.

Unlike chlorine, which bleaches by oxidation, sulphur dioxide bleaches coloured organic matter by a reduction reaction via sulphurous acid — and because the resulting colourless compound is only a reduced form (not permanently destroyed), the colour can reappear on standing in air, making $SO_2$'s bleaching action temporary.

Mechanism: sulphur dioxide dissolves in water (often moisture already present in the material) to form sulphurous acid: $SO_2 + H_2O \rightarrow H_2SO_3$ This sulphurous acid then reacts with the coloured organic substance, reducing it to a colourless compound: $\text{Coloured substance} + H_2SO_3 \rightarrow \text{Colourless compound} + [\text{H}_2SO_4 \text{ or similar oxidised sulphur species}]$ (the nascent hydrogen/reducing power of $H_2SO_3$ is responsible for decolourising the substrate).

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