CHEMISTRY · Stage 8 · Chapter 8: Pure Substances and Solutions

Chemistry 8.6 Chlorine and water — Water Treatment and Public Health

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Lesson summary

8.6 Chlorine and water — Water Treatment and Public Health — Alhussan International Academy — Chemistry (Stages 7–9) notes: Describe how water is made safe to drink, why chlorine is used, and evaluate the benefits and risks of chlorinating water.Contaminants — untreated water can…

Adjustments
Properties
Production
Swimming pool smell
Advantages
Cancer risk

Part 1 of 6Why treat water

Contaminants — untreated water can contain solid particles, harmful microorganisms and chemical pollutants.
Disease — the World Health Organization estimates that diseases spread by untreated water kill nearly half a million people every year.
- These diseases include cholera, typhoid and dysentery.
Treatment — river and reservoir water must be treated before it reaches taps, and clean water saves more lives than almost any other public health measure.

Part 2 of 6Stages of water treatment

Screening — grids remove large objects such as sticks and rubbish.
Sedimentation — chemicals such as aluminium sulfate make small particles clump together and settle to the bottom of tanks.
Filtration — beds of sand and gravel trap smaller particles, and carbon filters remove tastes and odours.
Chlorination — chlorine is added to kill microorganisms.

Adjustments — pH may also be adjusted, and fluoride is added in some areas for dental health.

Part 3 of 6Chlorine

Action — adding chlorine, or compounds such as calcium chloride or sodium hypochlorite, destroys the bacteria, viruses and parasites that cause waterborne disease.
- Chlorine reacts with water to form chloric(I) acid (hypochlorous acid), the active disinfectant.
Properties — chlorine is a green toxic gas, but in very small amounts in water it is safe.
- It stays in the water and protects it as it travels through pipes.
Production — chlorine is made by electrolysis of sodium chloride solution.
Swimming pool smell — it comes from chlorine reacting with sweat and other substances.
Advantages — it is cheap, effective against many microbes and gives lasting protection.
Disadvantages — it gives a taste and smell, and chlorine gas is dangerous to handle and transport.

Part 4 of 6The possible risk: trihalomethanes

1THMs1 point

— river and lake water contain dissolved substances from the soil, and chlorine reacts with them to make a mixture of products called trihalomethanes.

Their formulae include CHCl₃, CHBrCl₂, CHClBr₂ and CHBr₃.

2Cancer risk1 point

— many scientists have investigated whether THMs in tap water increase the risk of cancer over many years, and different groups reached different conclusions.

Almost all say that if there is an increased risk, it is tiny.

3Decision

— water companies judge that the benefits outweigh the risks, so almost all chlorinate tap water.

Part 5 of 6Investigation in West Bengal

1Question

— what are the health effects of drinking chlorinated water?

2Groups

— Rabi Sharma and Sudha Goel studied three groups: people drinking chlorinated water for 30 years or more, people with little access, and people with no access.

3Sample1 point

— 1810 people in one city took part, and the big sample helps make the evidence reliable.

Only people aged 30 or over were included, because they wanted people who had drunk the water for 30 years.

4Interviews2 points

— they asked about time in the home, age, education, income, health and whether people filter or boil their water.

Education and income were recorded because they affect health.
Health questions covered cancer, waterborne diseases, skin infections and kidney disease.

5Measurements1 point

— THM concentrations were measured at five places in the city water supply.

Almost everyone filtered or boiled the water before drinking it.

6Conclusions1 point

— chlorinated water does not significantly increase the risk of cancer.

It reduces the risk of cholera, typhoid, dysentery, skin infections and kidney disease.

7Reporting

— the report was checked by other scientists and published, and the large number of people and the care taken make the conclusions trustworthy.

Part 6 of 6Alternatives and public health

Method	Notes
Boiling	Kills microbes, but uses fuel and gives no lasting protection
Ultraviolet light	Kills microbes, no taste, no lasting protection
Ozone	Strong disinfectant, expensive
Chlorine	Cheap, lasting protection

Without a treatment plantpeople can use simple filters, solar disinfection (clear bottles left in sunlight) or chlorine tablets.
Testingscientists test water regularly for bacteria, pH, chlorine level and pollutants.
Distillationit makes pure water but is too costly for large amounts of household water.
Scientific knowledgeit helps individuals, organisations and governments make health decisions.

Last updated: 13 October 2026