A moist leaf on exposure to $S O_{2}$ became rather colourless. But, it regained colour on exposure to atmospheric air. Explain.

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Solution

Bleaching by sulphur dioxide is temporary as atmospheric oxygen re-oxidizes the bleached matter after some time. Sulphur dioxide is a temporary bleaching agent, unlike Chlorine, which is a permanent bleaching agent.
${S O}_{2} + 2 H_{2} O ⟶ H_{2} {S O}_{4} + 2 H C o l o u r e d f l o w e r + H ⟶ C o l o r l e s s f l o w e r l e s s$
Hence, ${S O}_{2}$ bleaches by reduction.

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