Answer:
In the presence of moisture, sulphur
dioxide is oxidized to \[{{H}_{2}}S{{O}_{4}}\] liberating nascent hydrogen. The
latter reduces coloured substances due to reduction.
\[S{{O}_{2}}+2{{H}_{2}}O\xrightarrow{{}}\,{{H}_{2}}S{{O}_{4}}+2[H]\]
Coloured substance \[+\,2[H]\xrightarrow{\,}\,\]colourless
(bleached)
However, the bleaching by
sulphur dioxide is only temporatory. When the bleached article is exposed to
air, it again gets coloured due to oxidation.
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