Redox Reactions

This guide covers subtopic 6.4, Redox, for Cambridge IGCSE Chemistry 0620 and Cambridge O Level Chemistry 5070, 2026–2028 series.

Read this before you revise. Cambridge defines oxidation and reduction three different ways — by oxygen transfer, by electron transfer, and by oxidation number — and expects you to move between them freely. In 0620, the oxygen-transfer definition is Core and the electron-transfer and oxidation-number definitions are Extended. In 5070, all three definitions are required outcomes from the start — 5070’s own wording states oxidation as “gain of oxygen, loss of electrons, an increase in oxidation number” as a single combined outcome, not three separate ones. The chemistry is identical either way; only how much of it you’re responsible for differs.

This resource does not use A Level (9701) ideas such as electrode potentials, half-equations built from ionic equations, or disproportionation. Stay within what follows.

Defining oxidation and reduction

CORE (0620) · REQUIRED (5070) — define redox reactions as involving simultaneous oxidation and reduction; define oxidation as gain of oxygen and reduction as loss of oxygen; identify redox reactions as reactions involving gain and loss of oxygen; identify oxidation and reduction in redox reactions.

The classic example is the reduction of a metal oxide by a more reactive metal or by carbon:

CuO + H2 → Cu + H2O

Copper(II) oxide loses oxygen (reduced to copper); hydrogen gains oxygen (oxidised to water). Both happen in the same reaction — that’s why it’s called a redox reaction, a contraction of reduction–oxidation.

EXTENDED / SUPPLEMENT (0620) · REQUIRED (5070) — define oxidation as loss of electrons and reduction as gain of electrons; identify redox reactions as reactions involving gain and loss of electrons.

Displacement reactions are usually explained this way:

Zn + CuSO4(aq) → ZnSO4(aq) + Cu

Zinc atoms lose two electrons each to form Zn²⁺ ions — oxidation. Cu²⁺ ions gain two electrons each to form copper atoms — reduction. Nothing here involves oxygen at all, which is exactly why the electron-transfer definition matters: it covers redox reactions the oxygen definition can’t describe.

Oxidation numbers

CORE (0620) · REQUIRED (5070) — use a Roman numeral to indicate the oxidation number of an element in a compound, for example iron(II) oxide and iron(III) oxide.

EXTENDED / SUPPLEMENT (0620) · REQUIRED (5070) — define oxidation as an increase in oxidation number and reduction as a decrease in oxidation number; identify redox reactions by changes in oxidation number, using the rules that the oxidation number of an uncombined element is zero, the oxidation number of a monatomic ion equals its charge, and the oxidation numbers in a compound (or in an ion) sum to zero (or to the ion’s charge).

Working out an oxidation number

What is the oxidation number of manganese in KMnO₄?

Potassium is always +1, and oxygen is normally −2. The compound is neutral, so the oxidation numbers must sum to zero:

(+1) + Mn + 4(−2) = 0
Mn = 0 − 1 + 8 = +7

Manganese in potassium manganate(VII) is +7 — which is exactly what the “(VII)” in its name tells you.

Spotting redox by oxidation number

Zn + CuSO4(aq) → ZnSO4(aq) + Cu

Zinc goes from 0 (uncombined element) to +2 in ZnSO₄ — the oxidation number increases, so zinc is oxidised. Copper goes from +2 in CuSO₄ to 0 — the oxidation number decreases, so copper is reduced. This agrees with the electron-transfer picture above, because oxidation number and electron loss/gain are two ways of tracking the same thing.

Oxidising agents and reducing agents

EXTENDED / SUPPLEMENT (0620) · REQUIRED (5070) — define an oxidising agent as a substance that oxidises another substance and is itself reduced; define a reducing agent as a substance that reduces another substance and is itself oxidised; identify oxidising agents and reducing agents in redox reactions.

In the zinc/copper sulfate reaction above, Cu²⁺ is the oxidising agent (it oxidises zinc, and is itself reduced to copper), and zinc is the reducing agent.

EXTENDED / SUPPLEMENT (0620) · REQUIRED (5070) — identify redox reactions by the colour changes involved when using acidified aqueous potassium manganate(VII) or aqueous potassium iodide.

Acidified potassium manganate(VII) is a strong oxidising agent: its deep purple colour decolourises as the manganate(VII) ion is reduced, which is how it’s used as a test reagent (see Identification of Ions and Gases for the sulfite and sulfur dioxide tests that rely on this). Aqueous potassium iodide is a reducing agent: in the presence of an oxidising agent, colourless iodide ions are oxidised to iodine, turning the solution brown.

Common mistakes

• Treating “oxidation” as only meaning “gaining oxygen.” That’s true, but incomplete — the electron and oxidation-number definitions describe the same process in reactions with no oxygen at all.
• Forgetting that oxidation and reduction always happen together. A reaction cannot be “just oxidation” — if one species is oxidised, another is reduced.
• Mixing up which species is the oxidising agent. The oxidising agent is the substance that gets reduced — it’s easy to say it backwards under exam pressure.
• Getting oxidation number sign errors. Work systematically: known elements first (Group I = +1, oxygen = −2, hydrogen = +1 in compounds), then solve for the unknown so the total matches the overall charge.
• 0620 Core candidates attempting oxidation-number questions they are not required to answer — or O Level candidates skipping electron-transfer or oxidation-number reasoning because a source called it “Extended.”

Quick revision checklist

All candidates (0620 Core and all 5070): redox as simultaneous oxidation and reduction · oxidation as gain of oxygen, reduction as loss of oxygen · identifying redox by oxygen transfer · Roman numerals for oxidation number

0620 Extended and all 5070 candidates, additionally: oxidation as loss of electrons, reduction as gain of electrons · oxidation number rules and how to calculate one · identifying redox by oxidation number change · oxidising agent and reducing agent, defined and identified · colour-change redox tests using manganate(VII) and iodide

Related resources

• Formulae, Equations and the Mole — ionic charges used throughout oxidation-number rules
• Identification of Ions and Gases — the manganate(VII) decolourisation test
• Rates of Reaction and Reversible Reactions — the Contact process, a redox reaction run reversibly
• Redox Processes: Oxidation Numbers and Electron Transfer — the AS Level continuation of this topic (9701)
• Cambridge IGCSE Chemistry hub · Cambridge O Level Chemistry hub

Written against Cambridge IGCSE Chemistry 0620 and Cambridge O Level Chemistry 5070, 2026–2028 series. Always check the current syllabus for your examination year.