What mistakes do students make with Uses of chlorine and chlorate(I)?

Explaining the redox by saying chlorine is oxidised and reduced when the question asks for an answer in terms of electrons, which mark schemes ignore. Giving the oxidation state of chlorine in ClO⁻ as −1. Writing the product as chlorate(V), ClO₃⁻, which forms only with hot concentrated alkali.

What are the exam tips for Uses of chlorine and chlorate(I) in Chemistry AQA A-Level?

If the question says 'with reference to electrons', the words gains and loses must appear, or the mark is lost. Work out the oxidation state in ClO⁻ from oxygen at −2 and the overall charge of −1.

AQA · A-Level · Chemistry

Course version: 7405 · Version from Sept 2016

The reaction of chlorine with water to form chloride ions and chlorate(I)…

Full specification wording

Uses of chlorine and chlorate(I): The reaction of chlorine with water to form chloride ions and chlorate(I) ions.

Chlorine dissolves in water and reacts with it: Cl₂ + H₂O ⇌ HClO + HCl, or ionically Cl₂ + H₂O ⇌ 2H⁺ + Cl⁻ + ClO⁻.

Checked against the AQA 7405 specification (opens in a new tab) on 20 September 2026.

Point 1 of 6
2 key terms

Study notes

From the checked teaching notes for this specification point.

The idea

What earns the marks

The equation Cl₂ + H₂O ⇌ HClO + HCl, or the ionic form Cl₂ + H₂O ⇌ 2H⁺ + Cl⁻ + ClO⁻.
Stating that chlorine gains electrons to form Cl⁻.
Stating that chlorine loses electrons to form ClO⁻.
Identifying the change as disproportionation, with chlorine going from 0 to −1 and from 0 to +1.
Naming ClO⁻ as the chlorate(I) ion.

Common mistakes and how to put them right

Explaining the redox by saying chlorine is oxidised and reduced when the question asks for an answer in terms of electrons, which mark schemes ignore.
Giving the oxidation state of chlorine in ClO⁻ as −1.
Writing the product as chlorate(V), ClO₃⁻, which forms only with hot concentrated alkali.
Leaving hydrogen unbalanced in the equation.
Using a single arrow when the reaction is an equilibrium.

Exam tips

If the question says 'with reference to electrons', the words gains and loses must appear, or the mark is lost.
Work out the oxidation state in ClO⁻ from oxygen at −2 and the overall charge of −1.

Try the lesson

The start of the Uses of chlorine and chlorate(I): The reaction of chlorine with water to form chloride ions and chlorate(I) ions. lesson, made from checked teaching notes for this specification point.

Have a go first

Which equation represents the reaction of chlorine with water that forms chloride ions and chlorate(I) ions?

Tip: press 1 to 4 to choose

Listen

Chlorine dissolves in water and reacts with it: Cl₂ + H₂O ⇌ HClO + HCl, or ionically Cl₂ + H₂O ⇌ 2H⁺ + Cl⁻ + ClO⁻. One chlorine atom ends up as chloride at oxidation state −1 and the other as the chlorate(I) ion, ClO⁻, at +1, both starting from 0 in Cl₂. A single element being oxidised and reduced at the same time is disproportionation. If a question asks you to explain the redox in terms of electrons, say it that way: chlorine gains electrons to form Cl⁻ and loses electrons to form ClO⁻. Chloric(I) acid and the chlorate(I) ion are the species that kill bacteria, which is why this equilibrium, rather than chlorine itself, does the work in water treatment. A strong answer earns marks for these points. the equation Cl₂ + H₂O ⇌ HClO + HCl, or the ionic form Cl₂ + H₂O ⇌ 2H⁺ + Cl⁻ + ClO⁻. stating that chlorine gains electrons to form Cl⁻. stating that chlorine loses electrons to form ClO⁻. identifying the change as disproportionation, with chlorine going from 0 to −1 and from 0 to +1. Exam tip: If the question says 'with reference to electrons', the words gains and loses must appear, or the mark is lost.

See it

A top-down flowchart. The top node gives the equation Cl₂ + H₂O ⇌ HClO + HCl, which leads to the ionic form Cl₂ + H₂O ⇌ 2H⁺ + Cl⁻ + ClO⁻. From this ionic equation two branches emerge: one node says chlorine gains electrons, leading to a node for Cl⁻ at oxidation state −1; the other says chlorine loses electrons, leading to a node for ClO⁻ at oxidation state +1. Both of these final nodes lead into a single node stating disproportionation, with chlorine going from 0 to −1 and from 0 to +1. — It shows chlorine reacting with water to give chloride and chlorate(I) ions, with chlorine gaining electrons to form Cl⁻ and losing electrons to form ClO⁻, which is disproportionation.

Key words

Say what each word means before you turn the card.

disproportionation

Try answering aloud or in your head before turning the card.

chlorate(I) ion

Try answering aloud or in your head before turning the card.

See every definition

disproportionation: A single element being oxidised and reduced at the same time, as when chlorine goes from 0 to −1 and from 0 to +1.
chlorate(I) ion: The species ClO⁻, in which chlorine is at oxidation state +1, formed along with chloride ions when chlorine reacts with water.

Label the diagram

Some boxes have lost their names. Pick the right name for each number.

The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. The top node gives the equation Cl₂ + H₂O ⇌ HClO + HCl, which leads to the ionic form Cl₂ + H₂O ⇌ 2H⁺ + Cl⁻ + ClO⁻. From this ionic equation two branches emerge: one node says chlorine gains electrons, leading to a node for Cl⁻ at oxidation state −1; the other says chlorine loses electrons, leading to a node for ClO⁻ at oxidation state +1. Both of these final nodes lead into a single node stating disproportionation, with chlorine going from 0 to −1 and from 0 to +1.

1Box 1
2Box 2
3Box 3

0 of 3 labelled

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