2010 General Studies, all questions

UPSC Prelims 2010 · General Studies

Question 11: Science and Technology

Chlorination is a process used for water purification. The disinfecting action of chlorine is mainly due to

aThe formation of hydrochloric acid when chlorine is added to water
bThe formation of hypochlorous acid when chlorine is added to water
cThe formation of nascent oxygen when chlorine is added to water
dThe formation of hydrogen when chlorine is added to water

AnswerThe answer is (b).

When chlorine dissolves in water it reacts to form hydrochloric acid and hypochlorous acid (HOCl). Hypochlorous acid is a weak acid but a powerful disinfectant: being electrically neutral it passes through the cell walls of microbes and oxidises their enzymes, killing them. Hydrochloric acid is strong but plays no real part in disinfection. Remember: the killing agent in chlorination is hypochlorous acid (with some hypochlorite ion at higher pH).

Why each option is right or wrong
  • (a) HCl does form, but it is not what disinfects; the germicidal agent is the other product.
  • (b) Right: hypochlorous acid enters microbial cells and oxidises them, killing them.
  • (c) 'Nascent oxygen' was the older explanation; the accepted agent is hypochlorous acid.
  • (d) No hydrogen gas is released in this reaction.

Explanation: written by UPSC Grade, not by UPSC.

Practise this paper timedPrevious questionNext question

Source: UPSC, Civil Services (Preliminary) Examination 2010, General Studies, Series A. The paper on upsc.gov.in.