Explain chlorine's role in water purification
Represent chlorine hydrolysis and the HOCl/ClO⁻ equilibrium involved in water disinfection.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. State conditions for halogen reactions, distinguish halogen oxidising strength from halide-ion reducing strength, and balance redox equations. Chlorine, bromine, iodine and concentrated acids are hazardous; examples teach chemistry, not practical instructions. Written practice is self-checked
Before you begin
- Use molecular halogen formulas X₂, oxidation numbers, ionic equations and electron-transfer ideas.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which active chlorine species forms when Cl₂ reacts with water?
What you will learn
- Write the chlorine–water equilibrium
- Show formation of HOCl and ClO⁻
- Explain their role in killing bacteria without giving treatment instructions
Chlorine water forms hypochlorous acid
Chlorine reacts reversibly with water: Cl₂ + H₂O ⇌ HCl + HOCl. Hypochlorous acid can ionise: HOCl ⇌ H⁺ + ClO⁻.
HOCl and ClO⁻ are active chlorine species that kill bacteria by oxidising cellular components. Their relative amounts depend on the solution conditions; the syllabus outcome asks for the species and their disinfecting role.
Keep the chemistry separate from dosing advice
The equations explain how active species arise in water. Effective and safe drinking-water treatment depends on controlled concentration, contact time, pH, water quality and local standards.
Do not infer that adding household chemicals is a safe treatment method. This lesson teaches the equilibrium and biological role only, not a treatment recipe or dose.
Pause and explain
Which two active chlorine species are identified in the water-purification outcome?
Worked example
Show how dissolved chlorine can produce both HOCl and ClO⁻ in water.
Show the worked solution
- Chlorine first reacts reversibly with water to form HCl and HOCl.
- HOCl can then dissociate to H⁺ and ClO⁻.
- Both HOCl and ClO⁻ are active species that kill bacteria.
Answer Cl₂ + H₂O ⇌ HCl + HOCl; HOCl ⇌ H⁺ + ClO⁻. HOCl and ClO⁻ kill bacteria by oxidation.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Write both equilibrium equations connecting dissolved chlorine, HOCl and ClO⁻.
Give me a hint
First add water to chlorine, then show ionisation of HOCl.
Compare my reasoning
- Cl₂ reacts reversibly with H₂O to form HCl and HOCl.
- HOCl dissociates into H⁺ and ClO⁻.
- Use equilibrium arrows for both reversible processes.
Cl₂ + H₂O ⇌ HCl + HOCl; HOCl ⇌ H⁺ + ClO⁻.
Look for these in your work
- First equation balanced
- Second equation charge-balanced
- Equilibrium arrows used
Explain the role of HOCl and ClO⁻ in water purification at the syllabus level.
Give me a hint
State the species and what they do to bacteria.
Compare my reasoning
- Both are active chlorine species.
- They kill bacteria by oxidising cellular components.
- Their relative amount depends on water conditions.
HOCl and ClO⁻ are active species that kill bacteria by oxidation; their proportions depend on solution conditions.
Look for these in your work
- Both species named
- Action explained
- Conditions acknowledged
A learner asks for an exact household chlorine dose from the equilibrium equation. Explain why the syllabus information cannot provide a safe dose.
Give me a hint
A chemical equation identifies species, not operational treatment parameters.
Compare my reasoning
- The equation shows active species formation but gives no water volume or contamination data.
- Safe treatment depends on concentration, contact time, pH and local requirements.
- A dose cannot be inferred from the equilibrium equation alone.
The equation describes chemistry, not a safe treatment dose; water quality, concentration, pH, contact time and local standards are also needed.
Look for these in your work
- Equation scope stated
- Missing variables identified
- No unsupported dose supplied
Common mistakes
- Omitting the equilibrium arrows
- Calling HOCl an inert by-product
- Writing ClO₂ instead of ClO⁻
- Turning the chemistry lesson into an unsupervised chemical-treatment recipe
What can you explain now?
Which active chlorine species forms when Cl₂ reacts with water?
Compare with the explanation
HOCl
Hypochlorous acid forms in the chlorine–water equilibrium.
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Make it stick
How do Cl₂, HOCl and ClO⁻ connect in water, and what role do the active species play?
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