Q.What happens when sulphur dioxide is passed through an aqueous solution of Fe(III) salt?

Sulphur dioxide, $SO_2$, is an angular molecule with $sp^2$-hybridised sulphur; both S-O bonds are equal in length and are, by resonance delocalisation across the two oxygens, intermediate between a formal single and double bond, so the two S-O bonds are structurally equivalent even though no single Lewis structure shows this directly. $SO_2$ acts as a mild reducing agent: passed into an aqueous solution of an iron(III) salt, it reduces $Fe^{3+}$ to $Fe^{2+}$ while itself being oxidised to sulphate, $2Fe^{3+} + SO_2 + 2H_2O \rightarrow 2Fe^{2+} + SO_4^{2-} + 4H^+$. Its presence is detected by its characteristic pungent, choking smell and by its ability to decolourise acidified potassium permanganate solution or turn moist starch-iodine paper colourless. $SO …