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 6.4.3 reads “Define oxidation in terms of: (a) gain of oxygen (b) loss of electrons (c) an increase in oxidation number”, one outcome listing all three. 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, by carbon, or by hydrogen:

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

• 0620 Extended and 5070: treating “oxidation” as only meaning “gaining oxygen.” For 0620 Core, gain and loss of oxygen is the complete requirement, so a Core answer using the oxygen definition alone is correct. Extended and 5070 candidates must also define oxidation as loss of electrons and as an increase in oxidation number, which 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
• Electrolysis and Fuel Cells — oxidation at the anode, reduction at the cathode
• Alloys and Extraction of Metals — extracting iron and aluminium, both reduction reactions
• 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.