Question

bleaching action of so2 is due to

Answer 1

Sulphur dioxide is a temporary bleaching agent, unlike Chlorine, which is a permanent bleaching agent! This is because the mechanism for bleaching is different.
The bleaching action sulphur dioxide is due to its reducing action.
The colour of pigment on exposure to SO2 gets reduced to a colourless compound. On standing in an air the colourless reduced product is reoxidised and gain the original colour. Bleaching with sulphur dioxide is therefore of temporary nature. It is generally used for bleaching delegate article like ivory, silk, wool etc.

Sulphur dioxide bleaches by reduction. Presence of moisture is essential for bleaching.
SO2 (g) + 2H2O → H2SO4 (aq.) + 2[H]

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