Use oxidation states to explain chlorine reactions with water and alkali, and connect the products to disinfection and bleaching.
Chlorine in water: one element, two directions
Chlorine reacts reversibly with water to form hydrochloric acid and chloric(I) acid, HClO (also called hypochlorous acid or HOCl). Chlorine starts at oxidation state 0. It becomes −1 in chloride and +1 in HClO, so the same starting element is both reduced and oxidised: disproportionation.
The molecular equation and the ionic form shown below describe the same acid-forming reaction. HClO can dissociate to H⁺ and ClO⁻; the relative proportions depend on pH. ClO⁻ is chlorate(I), also called hypochlorite.
Damp blue litmus is turned red by acidity and then bleached. Universal indicator can similarly show acidity before losing its colour. Bleaching is a chemical change to the dye; do not call it dilution.
Sunlight changes chlorine water
In sunlight, HClO can decompose, releasing oxygen. As HClO is removed, more chlorine reacts with water. The overall change produces chloride, acid and oxygen; the chlorine colour fades.
This overall reaction is not chlorine disproportionation: chlorine is reduced from 0 to −1 while oxygen is oxidised from −2 in water to 0 in O₂. Always assign states for the actual equation, not just the topic heading.
Cold dilute alkali makes chlorate(I)
Chlorine reacts with cold, dilute aqueous sodium hydroxide to make sodium chloride, sodium chlorate(I) and water. Chlorine changes from 0 to −1 and +1, so this is disproportionation. The green chlorine colour disappears as chlorine is consumed.
The resulting chlorate(I)-containing solution is used for bleaching and disinfection. Include the conditions: hot, concentrated alkali can give a different oxyanion, so the equation below is specifically for cold dilute alkali.
NaClO is sodium chlorate(I), because chlorine is +1: (+1) + x + (−2) = 0. NaClO₃ is sodium chlorate(V), with chlorine at +5. The Roman numeral identifies the chlorine oxidation state, not the charge of the whole salt.
Explain both the benefit and the managed risk
Chlorine treatment disinfects water by killing harmful microorganisms. HClO and related active chlorine species act as oxidants. This reduces transmission of waterborne disease and can maintain disinfectant activity as water travels through a distribution system.
Chlorine is hazardous at inappropriate concentrations and can form unwanted by-products with substances in water. The AQA evaluation is that the health benefit of controlled water disinfection outweighs its toxic effects. An answer should name the benefit, acknowledge the risk and refer to controlled low concentrations rather than claiming chlorine is harmless.
Bleach should not be mixed with acids: acidification can release chlorine gas. This is a useful application of the chemistry, not an experiment to try.
Keep the reactions distinct
When calculating amounts, use the balanced equation for the stated conditions. For the cold-alkali reaction, one mole Cl₂ forms one mole NaClO and consumes two moles NaOH. For the overall sunlight reaction, two moles Cl₂ correspond to one mole O₂.
| Conditions | Main chlorine-containing products | Oxidation-state pattern |
|---|---|---|
| Water, ordinary equilibrium | Cl⁻ and HClO / ClO⁻ | 0 → −1 and +1 |
| Water in sunlight, overall change | Cl⁻, with O₂ also produced | Cl: 0 → −1; O: −2 → 0 |
| Cold dilute NaOH | Cl⁻ and ClO⁻ | 0 → −1 and +1 |
Quick checks
Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official AQA mark allocations.
Q1. Show why Cl₂ + H₂O ⇌ HCl + HClO is disproportionation.Show answer
Cl starts at 0 and becomes −1 in HCl and +1 in HClO. The same starting element is simultaneously reduced and oxidised.
Q2. Write the ionic equation for chlorine with cold dilute NaOH and check charge.Show answer
Cl₂ + 2OH⁻ → Cl⁻ + ClO⁻ + H₂O. Total charge is −2 on each side.
Q3. Why is 2Cl₂ + 2H₂O → 4H⁺ + 4Cl⁻ + O₂ not chlorine disproportionation?Show answer
All chlorine is reduced from 0 to −1. Oxygen is oxidised from −2 to 0. Chlorine does not undergo both directions in this overall equation.
Q4. 0.0250 mol Cl₂ reacts completely with excess cold dilute NaOH. Find the NaOH consumed and NaClO produced.Show answer
The ratio Cl₂:NaOH:NaClO is 1:2:1. NaOH consumed = 0.0500 mol; NaClO produced = 0.0250 mol.
Q5. Give a balanced assessment of adding chlorine to drinking water.Show answer
It kills harmful microorganisms and reduces waterborne disease. Chlorine is toxic at inappropriate concentrations, so dosing is controlled. The health benefit of controlled disinfection outweighs the associated toxic effects.
Sources
Sources and examiner guidance (reviewed 2 October 2026)
- Chemrevise — AQA 2.3 Halogens (January 2022) — Primary coverage checklist, pp1–3; original explanations and questions.
- AQA 7405 specification — 3.2.3.1–3.2.3.2 and Required Practical 4.
- AQA 7404/1 2020 mark scheme — Q07.1, p18: disinfection and benefit/risk.
- AQA 7404/1 2021 mark scheme (November archive) — Q05.2–05.3, p15: water and cold alkali equations.
- AQA 7404/1 June 2023 examiner report — Q06, p4: distinguish reaction contexts and balance equations.
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