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What is redox in terms of oxygen and electrons, and how are oxidising and reducing agents identified?

Define oxidation and reduction in terms of oxygen and electron transfer, identify oxidising and reducing agents, and use colour changes of common reagents to test for them

A focused answer to the O-Level Chemistry outcome on redox. Oxidation and reduction defined by oxygen and by electron transfer, identifying oxidising and reducing agents, and the colour-change tests that detect them.

Generated by Claude Opus 4.88 min answerUpdated 2026-06-06

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking
The answer
Examples in context
Try this

What this dot point is asking

SEAB wants you to define oxidation and reduction both in terms of oxygen and in terms of electron transfer, identify the oxidising and reducing agents in a reaction, and use the colour changes of common reagents (acidified potassium manganate(VII) and potassium iodide, for example) to test for oxidising or reducing agents. Redox underpins the reactivity series, electrolysis and corrosion, so it ties much of the course together.

The answer

Two definitions of oxidation and reduction

Redox (reduction and oxidation) can be defined two ways, and you need both:

In terms of oxygen: oxidation is the gain of oxygen; reduction is the loss of oxygen. For example, when copper(II) oxide is heated with hydrogen, the copper oxide loses oxygen (reduced) and the hydrogen gains oxygen (oxidised).
In terms of electrons: oxidation is the loss of electrons; reduction is the gain of electrons. A common memory aid is OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).

Both definitions describe the same reactions; the electron definition is the more general one.

Redox reactions happen together

Oxidation and reduction always occur together in a redox reaction: if one substance loses electrons (is oxidised), another must gain them (is reduced). You cannot have one without the other, because the electrons lost by one species are the electrons gained by another.

Oxidising and reducing agents

The agents are named by what they do to the other substance:

An oxidising agent oxidises another substance (takes electrons from it, or gives it oxygen) and is itself reduced. Oxygen and chlorine are oxidising agents.
A reducing agent reduces another substance (gives electrons to it, or takes oxygen from it) and is itself oxidised. Carbon, hydrogen and reactive metals are reducing agents.

A useful check: the oxidising agent is the one that gets reduced; the reducing agent is the one that gets oxidised.

Colour-change tests

Two coloured reagents are used to test for redox behaviour:

Acidified potassium manganate(VII) is purple and turns colourless when it acts as an oxidising agent (it is reduced). So a colour change from purple to colourless shows the presence of a reducing agent.
Acidified potassium dichromate(VI) is orange and turns green when reduced, also a test for a reducing agent.
Potassium iodide solution is colourless and turns brown (iodine is formed) when an oxidising agent is present, so this tests for an oxidising agent.

These colour changes are the standard way to detect oxidising and reducing agents in the practical paper.

Worked example

When carbon is heated with copper(II) oxide, copper and carbon dioxide form: $2\text{CuO} + \text{C} \rightarrow 2\text{Cu} + \text{CO}_2$ . Identify what is oxidised and reduced, and name the oxidising and reducing agents.

Step 1: Track the oxygen

Copper(II) oxide loses oxygen to become copper, so the copper(II) oxide is reduced. Carbon gains oxygen to become carbon dioxide, so the carbon is oxidised.

Step 2: Check with electrons

The copper ions gain electrons to become copper metal (reduction); the carbon loses electrons as it bonds to oxygen (oxidation). Both definitions agree.

Step 3: Name the oxidising agent

The copper(II) oxide oxidised the carbon (it gave up its oxygen), and was itself reduced, so the copper(II) oxide is the oxidising agent.

Step 4: Name the reducing agent

The carbon reduced the copper(II) oxide (it took the oxygen), and was itself oxidised, so the carbon is the reducing agent. This is the same redox idea used to extract metals by reduction with carbon.

Examples in context

Example 1. Bleaching with chlorine. Chlorine acts as an oxidising agent when it bleaches a dye, taking electrons from the coloured molecules and destroying their colour. The same oxidising power that makes chlorine a disinfectant is at work, showing redox behaviour in a familiar process.

Example 2. Rusting as oxidation. When iron rusts, it loses electrons (is oxidised) as it reacts with oxygen and water, while the oxygen is reduced. Recognising rusting as a redox reaction connects the corrosion topic to the electron-transfer definition and explains why sacrificial protection (supplying electrons from a more reactive metal) prevents it.

Try this

Q1. Define oxidation in terms of electrons. [1 mark]

Cue. Oxidation is the loss of electrons.

Q2. In the reaction where hydrogen reduces copper(II) oxide to copper, state which substance is the reducing agent. [1 mark]

Cue. Hydrogen (it removes the oxygen, reducing the copper oxide, and is itself oxidised).

Q3. State the colour change of acidified potassium manganate(VII) when added to a reducing agent, and what it indicates. [2 marks]

Cue. Purple to colourless; it indicates the presence of a reducing agent (the manganate is reduced).

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original5 marksMagnesium burns in oxygen to form magnesium oxide:

2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}

. (a) State which species is oxidised and which is reduced, in terms of oxygen. (b) Explain the same reaction in terms of electron transfer. (c) Identify the oxidising agent.

Show worked answer →

(a) Magnesium gains oxygen, so magnesium is oxidised. Oxygen is gained by the magnesium, so the oxygen is reduced (it is removed from the gas as it combines).

(b) Each magnesium atom loses two electrons to form $\text{Mg}^{2+}$ (loss of electrons is oxidation). Each oxygen atom gains two electrons to form $\text{O}^{2-}$ (gain of electrons is reduction).

Markers reward magnesium oxidised (gains oxygen, loses electrons) and oxygen reduced (gains electrons), and oxygen identified as the oxidising agent.

Original4 marksAcidified potassium manganate(VII) is purple, and acidified potassium dichromate(VI) is orange. (a) State the colour change seen when each is added to a reducing agent. (b) State what these colour changes are used to test for.