Chlorination – IGCSE Chemistry Definition
IGCSE Chemistry definition of chlorination: adding chlorine to water to kill microorganisms. Covers purpose, advantages, disadvantages, and exam tips for 0620.
Published by IGCSEChemistry.com.my
Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
Chlorination is a short but precise topic that appears within the potable water treatment sequence. Examiners want you to state its purpose clearly and may ask you to discuss both advantages and disadvantages.
The 0620 definition
Chlorination is the addition of chlorine (in controlled amounts) to water in order to kill bacteria and other pathogenic microorganisms.
How it works
Chlorine is a powerful oxidising agent. When dissolved in water, it forms hypochlorous acid (HOCl), which destroys the cell membranes and enzymes of bacteria and viruses.
For IGCSE, you do not need this mechanism — simply state that chlorine kills microorganisms.
Role in water treatment
Chlorination is the final chemical treatment step, after sedimentation and filtration:
- Sedimentation removes large particles
- Filtration through sand beds removes smaller particles
- Chlorination kills remaining bacteria and viruses
The chlorine is added at a concentration high enough to disinfect but low enough to be safe for drinking (typically 0.2–0.5 mg/L residual chlorine).
Advantages and disadvantages
| Advantages | Disadvantages |
|---|---|
| Kills disease-causing organisms | Can produce harmful by-products with organic matter |
| Cheap and effective | Slight taste and smell |
| Provides residual protection in pipes | Some people prefer alternative methods |
| Prevents waterborne diseases (cholera, typhoid) |
Swimming pool chlorination
Chlorine is also added to swimming pools for the same reason — to kill bacteria introduced by swimmers. The “chlorine smell” at pools is actually from chloramines, formed when chlorine reacts with sweat and urine.
Worked exam question
Water is treated before it is supplied to homes. (a) State why chlorine is added to the water. [1] (b) Name the process of adding chlorine to water. [1] (c) State one other step in water treatment and its purpose. [2]
Mark scheme
(a) To kill bacteria / microorganisms [1]
(b) Chlorination [1]
(c) Filtration (through sand) — to remove insoluble particles/solids [1+1] OR sedimentation — to allow large/heavy particles to settle [1+1]
Common exam mistakes
- Saying chlorine “cleans” the water — the correct term is that it kills bacteria/microorganisms. Filtration removes solids; chlorine does not.
- Confusing chlorination with filtering — these are different steps with different purposes.
- Forgetting that chlorine remains in the water supply as a residual disinfectant — this is why tap water may have a slight chlorine smell.
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