UPSC Prelims 2010 Q11: Science & Technology – Chemistry
Chlorination is a process used for water purification. The disinfecting action of chlorine is
mainly due to
- (a) The formation of hydrochloric acid when chlorine is added to water
- (b) The formation of hypochlorous acid when chlorine is added to water
- (c) The formation of nascent oxygen when chlorine is added to water
- (d) The formation of hydrogen when chlorine is added to water
Answer: (b) The formation of hypochlorous acid when chlorine is added to water
Explanation
Why chlorine disinfects water
Chlorination is one of the most common water-purification methods, working through a specific chemical reaction.
| Option | Verdict | Reason |
|---|---|---|
| (a) Formation of hydrochloric acid | Incorrect | While some HCl forms, it is not the main disinfecting agent |
| (b) Formation of hypochlorous acid | Correct | When chlorine dissolves in water, it forms hypochlorous acid (HOCl), which is the primary active disinfecting agent, effectively killing microorganisms |
| (c) Formation of nascent oxygen | Incorrect | This is not the accepted mechanism for chlorine's disinfecting action in water treatment |
| (d) Formation of hydrogen | Incorrect | Hydrogen gas is not the relevant disinfecting byproduct here |
Chlorination chemistry:
| Point | Detail |
|---|---|
| Reaction | Cl₂ + H₂O → HOCl (hypochlorous acid) + HCl |
| Disinfecting agent | HOCl, a strong oxidizer that destroys microbial cell structures |
| Why effective | HOCl can penetrate microbial cell walls, disrupting enzymes and metabolism |
Answer: (b) The formation of hypochlorous acid when chlorine is added to water.
Why each option is right or wrong
- (a) Incorrect. Hydrochloric acid forms as a byproduct but is not the main disinfecting agent.
- (b) CORRECT. Hypochlorous acid (HOCl), formed when chlorine reacts with water, is the primary disinfecting agent.
- (c) Incorrect. Nascent oxygen formation is not the standard accepted mechanism for chlorine disinfection.
- (d) Incorrect. Hydrogen gas is not relevant to chlorine's disinfecting action.
Key point
Chlorine disinfects water mainly through the formation of HYPOCHLOROUS ACID (HOCl), a potent microorganism-killing agent — not through HCl, nascent oxygen, or hydrogen.
Remember
Chlorination disinfects water via hypochlorous acid (HOCl) formation — the key active disinfecting compound.
Common mistakes
- Confusing the main disinfecting agent (HOCl) with a minor byproduct like HCl.
- Attributing chlorine's action to an outdated or incorrect nascent oxygen theory.
- Not knowing the specific chemical equation behind chlorine's reaction with water.
Exam tip
Chlorine + water → hypochlorous acid (HOCl), the real disinfecting agent. Memorise this specific chemical pathway.
Sources
- NCERT Class XI Chemistry — Hydrogen (water and its treatment)
- American Water Works Association — Water Chlorination Principles and Practices (AWWA Manual M20)