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WBCS Preliminary Examination 2018 — General Studies · Question 108 of 200

Q108
General ScienceChemistry
Bleaching action of chlorine is by
  1. a.decomposition
  2. b.hydrolysis
  3. c.reduction
  4. d.oxidation

Answer: (D) oxidation

Chlorine bleaches only in the presence of moisture: $Cl_2 + H_2O \rightarrow HCl + HOCl$, and the hypochlorous acid gives nascent oxygen, which oxidises the coloured matter to a colourless product. Bleaching by oxidation is permanent. Sulphur dioxide, by contrast, bleaches by reduction and its effect is temporary (the colour returns on exposure to air).

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