The bleaching action of chlorine is permanent, whereas the bleaching action and sulphur dioxide is temporary. In this context :
(i) Give a reason, why chlorine is not used to bleach silk.
(ii) State similarity in the use of sulphur dioxide and chlorine as bleaching agents.
(iii) Explain the bleaching action of sulphur dioxide with the help of chemical equations. (iv) Why is bleaching by sulphur dioxide only temporary?

Explanation

Chlorine bleaches coloured material by oxidation:
`Cl_(2)+H_(2)O to 2HCl+[O]`
Coloured `+[O] to "colourless material"`
Hence, the bleaching is permanent.
Whereas, sulphur dioxide, `SO_(2)` bleaches coloured material by reduction and hence bleaching is temporary. When the bleached colourless material is exposed to air, it gets oxidised and the colour is restored.
`SO_(2)+2H_(2)Oto 2H_(2)SO_(4)+2[H]`
`underset("material")("Coloured")+[H]underset("material")("Colourless")overset("Aerial")underset("Oxidation")(rarr)underset("material")("Coloured")`.