What is a redox reaction in the context of Cambridge IGCSE Coordinated Science?

In Cambridge IGCSE Coordinated Science, a redox reaction is a type of chemical reaction that involves the transfer of electrons between two substances. It includes both oxidation, where a substance loses electrons, and reduction, where a substance gains electrons. These reactions are fundamental in understanding various chemical processes.

How can you identify oxidation and reduction in a chemical reaction?

To identify oxidation and reduction in a chemical reaction, look for changes in oxidation states of elements. Oxidation involves an increase in oxidation state, while reduction involves a decrease. Additionally, oxidation is often associated with the gain of oxygen or loss of hydrogen, whereas reduction is the loss of oxygen or gain of hydrogen.

What are some common examples of redox reactions?

Common examples of redox reactions include the rusting of iron, combustion reactions, and the reactions occurring in batteries. In the rusting process, iron reacts with oxygen to form iron oxide, involving the oxidation of iron and reduction of oxygen.

Why are redox reactions important in everyday life?

Redox reactions are crucial in everyday life as they are involved in numerous processes such as metabolism in living organisms, energy production in batteries, and industrial processes like the extraction of metals. Understanding redox reactions helps explain how these processes work and their applications in technology and biology.

What role do redox reactions play in electrochemical cells?

In electrochemical cells, redox reactions are fundamental as they convert chemical energy into electrical energy. The cell consists of two electrodes where oxidation occurs at the anode and reduction at the cathode. This flow of electrons from the anode to the cathode through an external circuit generates electricity, which is the principle behind batteries.

Why is "Saying the oxidising agent is oxidised" a common mistake in Redox?

Why it happens: Students think the oxidising agent does what its name says — gets oxidised. How to avoid it: The oxidising agent causes oxidation of something else, so it gains electrons and is reduced. Oxidising agent = gets REDUCED. Reducing agent = gets OXIDISED. Source: 0654 Examiner Report 2023

Why is "Saying 'oxygen is transferred' rather than 'electrons are transferred' in redox" a common mistake in Redox?

Why it happens: Students learn the old definition (gain/loss of oxygen) and apply it to all contexts. How to avoid it: The most complete and universal definition uses electron transfer: oxidation = loss of electrons; reduction = gain of electrons. Use this for all half-equation questions.

Why is "Classifying neutralisation as a redox reaction" a common mistake in Redox?

Why it happens: Students see a chemical reaction and assume all reactions involve redox. How to avoid it: Check oxidation states of every element: if none change, it is not redox. Neutralisation (H⁺ + OH⁻ → H₂O) involves no change in oxidation states.

Chemical ReactionsCambridge IGCSE Coordinated Sciences 0654

Redox

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Redox Reactions — Cambridge IGCSE Coordinated Science 0654

Redox reactions involve simultaneous oxidation and reduction — electron transfer. Cambridge tests identifying oxidising/reducing agents, writing half-equations, and applying redox to displacement reactions and rusting.

What you’ll learn

Mapped to the Cambridge IGCSE 0654 syllabus (2025-2027).

C7.5 — Define oxidation and reduction in terms of electron transfer.
C7.6 — Identify oxidising and reducing agents.
C7.7 — Write and balance half-equations for redox reactions.
C7.8 — Describe rusting and methods of prevention.

Oxidation and Reduction — OIL RIG

Oxidation and reduction always occur together (redox). OIL RIG: Oxidation Is Loss of electrons, Reduction Is Gain of electrons.

Oxidation: loss of electrons (or gain of oxygen, or loss of hydrogen — older definition). Reduction: gain of electrons (or loss of oxygen, or gain of hydrogen).

These ALWAYS occur together — if one species loses electrons, another must gain them.

OIL RIG mnemonic: Oxidation Is Loss, Reduction Is Gain (of electrons).

Oxidising agent (oxidant): the species that CAUSES oxidation of another. It does this by ACCEPTING electrons → so the oxidising agent is itself REDUCED.

Reducing agent (reductant): the species that CAUSES reduction of another. It does this by DONATING electrons → so the reducing agent is itself OXIDISED.

Example — Reaction of zinc with copper sulfate:

Zn + CuSO₄ → ZnSO₄ + Cu

Half-equations:

Zn → Zn²⁺ + 2e⁻ (zinc is OXIDISED — loses electrons — zinc is the reducing agent)
Cu²⁺ + 2e⁻ → Cu (copper ions are REDUCED — gain electrons — CuSO₄ is the oxidising agent)

Oxidation states: integer assigned to each atom showing the charge it would have if the compound were fully ionic. Useful for tracking oxidation/reduction in complex reactions.

OIL RIG: Oxidation = Loss of electrons. Reduction = Gain of electrons.
Oxidising agent: accepts electrons → gets reduced. Reducing agent: donates electrons → gets oxidised.
Redox: both oxidation AND reduction happen simultaneously.

Displacement Reactions and Rusting

Displacement reactions are redox reactions where a more reactive metal displaces a less reactive one. Rusting is the oxidation of iron requiring both oxygen and water.

Displacement reactions: A more reactive metal displaces a less reactive metal from its salt solution.

Example: Iron nails in copper sulfate solution:

Fe + CuSO₄ → FeSO₄ + Cu

Fe → Fe²⁺ + 2e⁻ (oxidised, reducing agent)
Cu²⁺ + 2e⁻ → Cu (reduced, oxidising agent)
Observable: brown copper coating forms on iron nails; solution goes from blue to pale green (Fe²⁺)

Only works if the metal being added is HIGHER in the reactivity series than the metal in solution.

Rusting of iron: Conditions required: BOTH oxygen AND water (neither alone causes rusting).

Iron + oxygen + water → hydrated iron(III) oxide (Fe₂O₃·nH₂O) — rust

Prevention of rusting:

Method	How it works
Painting/coating	Physical barrier: prevents O₂ and water reaching iron
Oiling/greasing	Physical barrier
Galvanising	Coat with zinc. Zinc oxidises preferentially (sacrificial protection even if coating scratched)
Sacrificial (cathodic) protection	Attach a more reactive metal (e.g. Mg or Zn block) to iron structure — more reactive metal oxidises preferentially
Alloying	Steel with chromium → stainless steel (Cr₂O₃ layer protects surface)

Displacement: more reactive metal + salt of less reactive metal → metal deposited.
Rusting requires BOTH O₂ AND water (neither alone). Product: hydrated iron(III) oxide.
Galvanising: zinc coating = physical barrier + sacrificial protection.

How it’s examined

Paper 4: 'Identify the oxidising agent in this reaction and explain your reasoning' (2 marks — name the species, state it gains electrons/is reduced). Write the half-equation for the oxidation of iron and the reduction of copper ions (2 marks each). 'State TWO conditions necessary for rusting' (2 marks — water + oxygen). 'Explain why attaching zinc blocks to the hull of a steel ship protects it from rusting' (3 marks — zinc more reactive, oxidises preferentially, provides sacrificial protection even if scratched). OIL RIG is tested in almost every exam paper.

Sources: Cambridge IGCSE Coordinated Sciences 0654 syllabus 2025-2027 (C7); 0654 Examiner Reports 2022-2024. Last reviewed 2026-05-14.

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Step-by-step worked examples — Redox

Step-by-step solutions to past-paper-style questions on redox, written exactly the way a tutor would explain them at the board.

1Identifying oxidation and reduction in a displacement reaction
Extended
Question
In the reaction $\text{Mg} + \text{CuSO}_4 \rightarrow \text{MgSO}_4 + \text{Cu}$ , identify what is oxidised and what is reduced. State the oxidising and reducing agents.
Step-by-step solution
1. Step 1
  Track oxidation states. Mg: 0 (element) → Mg²⁺ in MgSO₄: +2. Magnesium loses 2 electrons → oxidised.
  $\text{Mg} \rightarrow \text{Mg}^{2+} + 2e^- \quad (\text{oxidation})$
2. Step 2
  Cu²⁺ in CuSO₄: +2 → Cu: 0. Copper gains 2 electrons → reduced.
  $\text{Cu}^{2+} + 2e^- \rightarrow \text{Cu} \quad (\text{reduction})$
3. Step 3
  Oxidising agent: the species that causes oxidation of another; it is itself reduced → CuSO₄ (Cu²⁺) is the oxidising agent.
4. Step 4
  Reducing agent: the species that causes reduction of another; it is itself oxidised → Mg is the reducing agent.
Answer
Mg is oxidised (0 → +2); Cu²⁺ is reduced (+2 → 0). Oxidising agent: Cu²⁺/CuSO₄; reducing agent: Mg.
Examiner tip
Oxidising agent = is itself reduced. Reducing agent = is itself oxidised. This is the opposite of what students expect and is a common exam trap.
2Half-equation for iron(II) to iron(III) oxidation
Extended
Question
Write the half-equation for the oxidation of iron(II) ions to iron(III) ions.
Step-by-step solution
1. Step 1
  Fe²⁺ loses 1 electron to form Fe³⁺ → this is oxidation (loss of electrons).
  $\text{Fe}^{2+} \rightarrow \text{Fe}^{3+} + e^-$
2. Step 2
  Check: charge on left = +2; charge on right = +3 − 1 = +2. ✓ Balanced.
Answer
$\text{Fe}^{2+} \rightarrow \text{Fe}^{3+} + e^-$
Examiner tip
Always check that total charge is balanced on both sides of a half-equation.
3Defining oxidising agent
Core
Question
Define the term 'oxidising agent'.
Step-by-step solution
1. Step 1
  An oxidising agent is a substance that oxidises another substance — it causes another substance to lose electrons.
2. Step 2
  In doing so, the oxidising agent itself gains electrons and is therefore reduced.
Answer
An oxidising agent is a substance that oxidises another substance (causes electron loss from it) and is itself reduced (gains electrons) in the process.
4Is acid–base neutralisation a redox reaction?
Extended
Question
State whether the following reaction is a redox reaction and explain your reasoning: $\text{HCl} + \text{NaOH} \rightarrow \text{NaCl} + \text{H}_2\text{O}$
Step-by-step solution
1. Step 1
  Check the oxidation state of each element on both sides.
2. Step 2
  H: +1 in HCl and +1 in H₂O → unchanged. Na: +1 in NaOH and +1 in NaCl → unchanged. Cl: −1 in HCl and −1 in NaCl → unchanged. O: −2 in NaOH and −2 in H₂O → unchanged.
3. Step 3
  No oxidation states change → no electrons transferred → this is NOT a redox reaction.
Answer
Not a redox reaction; no oxidation states change (H stays +1, Na +1, Cl −1, O −2 throughout); no electron transfer occurs.

Key Definitions and Keywords — Redox

Definitions to memorise and the exact keywords mark schemes credit for redox answers — sharpened from recent examiner reports for the 2026 0654 sitting.

Oxidation
Examiner keyword
Loss of electrons (OIL); also: gain of oxygen or loss of hydrogen. Oxidation state increases.
Reduction
Examiner keyword
Gain of electrons (RIG); also: loss of oxygen or gain of hydrogen. Oxidation state decreases.
Oxidising agent
Examiner keyword
A substance that oxidises another substance and is itself reduced (gains electrons) in the process.
Reducing agent
Examiner keyword
A substance that reduces another substance and is itself oxidised (loses electrons) in the process.
Oxidation state
Examiner keyword
A number assigned to an element in a compound indicating the degree of oxidation; positive for metals and some non-metals, negative for non-metals in compounds. Helps identify redox changes.
OILRIG
Mnemonic: Oxidation Is Loss of electrons; Reduction Is Gain of electrons. Used to distinguish oxidation from reduction.

Common Mistakes and Misconceptions — Redox

The traps other students keep falling into on redox questions — taken from recent Cambridge IGCSE 0654 examiner reports and mark schemes — and how to avoid them.

✕Saying the oxidising agent is oxidised
0654 Examiner Report 2023
Why it happens
Students think the oxidising agent does what its name says — gets oxidised.
How to avoid it
The oxidising agent causes oxidation of something else, so it gains electrons and is reduced. Oxidising agent = gets REDUCED. Reducing agent = gets OXIDISED.
✕Saying 'oxygen is transferred' rather than 'electrons are transferred' in redox
Why it happens
Students learn the old definition (gain/loss of oxygen) and apply it to all contexts.
How to avoid it
The most complete and universal definition uses electron transfer: oxidation = loss of electrons; reduction = gain of electrons. Use this for all half-equation questions.
✕Classifying neutralisation as a redox reaction
Why it happens
Students see a chemical reaction and assume all reactions involve redox.
How to avoid it
Check oxidation states of every element: if none change, it is not redox. Neutralisation (H⁺ + OH⁻ → H₂O) involves no change in oxidation states.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

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Redox — frequently asked questions

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Redox

Short Study Notes in the form of Slides

Short Study Notes — Redox

Detailed Notes

What you’ll learn

How it’s examined

1Identifying oxidation and reduction in a displacement reaction

2Half-equation for iron(II) to iron(III) oxidation

3Defining oxidising agent

4Is acid–base neutralisation a redox reaction?

Oxidation

Reduction

Oxidising agent

Reducing agent

Oxidation state

OILRIG

✕Saying the oxidising agent is oxidised

✕Saying 'oxygen is transferred' rather than 'electrons are transferred' in redox

✕Classifying neutralisation as a redox reaction

Practice questions

Past paper style quiz

Assess your understanding

Redox — frequently asked questions