Question

What is similar between sulphur dioxide and chlorine which acts as bleaching agent

Answer 1

Both sulphur dioxide and chlorine are good bleaching agents. Bleaching action of chlorine is based on oxidation while that of sulphur is based on reduction.
Chlorine acts with water to produce nascent oxygen. This oxygen combines with colourd substance and makes it colourless.
Sulphour dioxide removes oxygen from the coloured substance and makes it colourless. But atmospheric oxygen slowly takes place of the removed oxygen and because of which the material regains the colour.
Sulphur dioxide is a temporary bleaching agent, unlike Chlorine, which is a permanent bleaching agent! This is because the mechanism for bleaching is different:

SO2-Reduction => TEMPORARYCl2-Oxidation => PERMANENT
Sulphur dioxide removes oxygen from the coloured substance and makes it colourless. But atmospheric oxygen slowly takes place of the removed oxygen and because of which the material regains the colour.