SingaporeChemistrySyllabus dot point

What do we really mean when we say something has been oxidised or reduced, and how can we spot it in a reaction?

Define oxidation and reduction in terms of oxygen and of electrons, identify redox reactions, and name simple oxidising and reducing agents

A focused answer to the N(A) Chemistry outcome on redox. Oxidation and reduction defined by oxygen and by electron transfer, how to spot a redox reaction, and what oxidising and reducing agents do.

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

The syllabus wants you to define oxidation and reduction in two ways, by oxygen and by electrons, to identify a redox reaction, and to name simple oxidising and reducing agents. The central idea is that oxidation and reduction always happen together in the same reaction: if one substance loses electrons, another must gain them. A reaction where both happen is called a redox reaction.

The answer

Definition by oxygen

The simplest way to start is with oxygen:

Oxidation is the gain of oxygen.
Reduction is the loss of oxygen.

For example, when magnesium burns it gains oxygen to form magnesium oxide, so magnesium is oxidised. When copper oxide is heated with hydrogen, the copper oxide loses its oxygen, so it is reduced.

Definition by electrons

The more powerful definition uses electrons. A helpful memory aid is OIL RIG:

Oxidation Is Loss of electrons,
Reduction Is Gain of electrons.

So when zinc reacts and becomes $\text{Zn}^{2+}$ , it has lost two electrons, so it is oxidised. When copper ions $\text{Cu}^{2+}$ become copper metal, they have gained electrons, so they are reduced.

Redox: the two always go together

Electrons cannot just disappear. If one substance loses electrons (is oxidised), another must gain them (is reduced) in the same reaction. A reaction where both oxidation and reduction happen is a redox reaction. Most reactions of metals, including displacement and corrosion, are redox reactions.

Oxidising and reducing agents

The two reactants are given names based on what they do to the other substance:

an oxidising agent oxidises the other substance, so it itself gains electrons (it is reduced). Oxygen is a common oxidising agent.
a reducing agent reduces the other substance, so it itself loses electrons (it is oxidised). Carbon and hydrogen are common reducing agents, used to extract metals.

Spotting oxidation and reduction with electrons

For the displacement reaction $\text{Mg} + \text{Cu}^{2+} \rightarrow \text{Mg}^{2+} + \text{Cu}$ , decide what is oxidised, what is reduced, and name each agent.

Step 1: Track the magnesium

Magnesium starts as a neutral atom $\text{Mg}$ and ends as $\text{Mg}^{2+}$ . It has lost two electrons.

Step 2: Apply OIL RIG to magnesium

Loss of electrons is oxidation (OIL), so magnesium is oxidised.

Step 3: Track the copper ions

The copper ions start as $\text{Cu}^{2+}$ and end as copper metal $\text{Cu}$ . They have gained two electrons.

Step 4: Apply OIL RIG and name the agents

Gain of electrons is reduction (RIG), so the copper ions are reduced. Magnesium gives the electrons, so magnesium is the reducing agent; the copper ions take the electrons, so $\text{Cu}^{2+}$ is the oxidising agent.

Examples in context

Example 1. Extracting iron in a blast furnace. Iron is extracted by reducing iron oxide with carbon. The iron oxide loses its oxygen (it is reduced) while the carbon gains oxygen (it is oxidised), so carbon acts as the reducing agent. This single redox reaction is the basis of the steel industry.

Example 2. Rusting as slow oxidation. When iron rusts it gains oxygen to form iron oxide, so rusting is the oxidation of iron. Understanding rust as oxidation explains why painting or galvanising, which keeps oxygen away, stops it.

Try this

Q1. State the meaning of oxidation in terms of electrons. [1 mark]

Cue. Oxidation is the loss of electrons (the O and L of OIL RIG).

Q2. In a reaction a substance gains oxygen. State whether it has been oxidised or reduced. [1 mark]

Cue. Gaining oxygen is oxidation, so the substance has been oxidised.

Q3. Explain why every reaction in which one substance is oxidised must also involve a substance being reduced. [2 marks]

Cue. Electrons lost by one substance must be gained by another, so if one is oxidised (loses electrons) another must be reduced (gains them).

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.

Original4 marksMagnesium burns in oxygen to form magnesium oxide:

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

. (a) State which element is oxidised. (b) Explain your answer in terms of oxygen. (c) State whether oxygen is oxidised or reduced.

Show worked answer →

(a) Magnesium is oxidised.

(b) Magnesium gains oxygen to form magnesium oxide. Gaining oxygen is oxidation, so magnesium is oxidised.

(c) Oxygen is reduced, because it is the substance that takes part by being added to the magnesium (it gains electrons from the magnesium).

What markers reward: magnesium oxidised, the link that gaining oxygen is oxidation, and oxygen being reduced.

Original4 marksIn the reaction

\text{Zn} + \text{Cu}^{2+} \rightarrow \text{Zn}^{2+} + \text{Cu}

: (a) State which species loses electrons. (b) State whether that species is oxidised or reduced. (c) Name the reducing agent.