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

In Cambridge IGCSE Chemistry, a redox reaction is a type of chemical reaction where oxidation and reduction occur simultaneously. Oxidation involves the loss of electrons, while reduction involves the gain of electrons. These reactions are crucial for understanding processes such as combustion, respiration, and corrosion.

How can you identify a redox reaction?

A redox reaction can be identified by changes in oxidation states of the elements involved. If an element's oxidation state increases, it is oxidized, and if it decreases, it is reduced. Additionally, the presence of electron transfer between reactants is a key indicator of a redox reaction.

What are some common examples of redox reactions?

Common examples of redox reactions include the rusting of iron, where iron is oxidized and oxygen is reduced, and the reaction between hydrogen and oxygen to form water, where hydrogen is oxidized and oxygen is reduced. These examples illustrate the practical importance of redox reactions in everyday life.

Why are redox reactions important in chemical processes?

Redox reactions are fundamental to many chemical processes because they involve energy transfer through electron movement. They are essential in biological systems for energy production, such as in cellular respiration, and in industrial processes like metal extraction and electroplating.

What role do oxidizing and reducing agents play in redox reactions?

In redox reactions, an oxidizing agent is a substance that gains electrons and is reduced, while a reducing agent loses electrons and is oxidized. These agents facilitate the transfer of electrons, driving the chemical reaction forward. Understanding their roles is crucial for predicting the direction and outcome of redox reactions.

Why is "Inverting OIL RIG" a common mistake in Redox?

Why it happens: Memory slip. How to avoid it: OIL = Oxidation Is Loss. RIG = Reduction Is Gain. Source: 0620/42 — recurring

Why is "Calling the species that GETS oxidised the oxidising agent" a common mistake in Redox?

Why it happens: Naming feels backwards. How to avoid it: An oxidising AGENT does the oxidising — so it itself gets REDUCED.

Why is "Using H = +1 in metal hydrides (e.g. NaH)" a common mistake in Redox?

Why it happens: Generalising the rule. How to avoid it: In hydrides H is -1. The general rule is 'H = +1' applies in most other cases.

Why is "Using O = −2 in peroxides" a common mistake in Redox?

Why it happens: Generalising. How to avoid it: In H_2O_2, O is -1 (peroxide).

Chemical ReactionsCambridge IGCSE Chemistry (0620)

Redox

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Redox Reactions — Cambridge IGCSE 0620 Chemistry Extended (2026)

Reduction and oxidation happen together. Three lenses: oxygen, hydrogen, and (Extended) electrons / oxidation numbers. Plus the redox tests.

What you’ll learn

Mapped to the Cambridge IGCSE 0620 syllabus (2026-2028).

8.2 — Define oxidation and reduction in terms of oxygen, hydrogen, electrons.
8.2 — Identify oxidising and reducing agents in a reaction.
8.2 — Use oxidation numbers to identify what is oxidised/reduced (Extended).
8.2 — Describe tests for oxidising and reducing agents.

Three definitions of oxidation/reduction

Gain or loss of oxygen, hydrogen, or (best) electrons.

Definition 1: Oxygen.

Oxidation = GAIN of oxygen.
Reduction = LOSS of oxygen.

Examples: $\text{C} + \text{O}_{2} \to \text{CO}_{2}$ — carbon is OXIDISED. $\text{Fe}_{2}\text{O}_{3} + 3\text{CO} \to 2\text{Fe} + 3\text{CO}_{2}$ — iron oxide is REDUCED to iron.

Definition 2: Hydrogen.

Oxidation = LOSS of hydrogen.
Reduction = GAIN of hydrogen.

Examples: $\text{H}_{2}\text{S} + \text{Cl}_{2} \to 2\text{HCl} + \text{S}$ — H₂S is OXIDISED (loses H). Cl₂ is REDUCED (gains H).

Definition 3: Electrons (best — works for any redox).

OIL: Oxidation Is Loss of electrons.
RIG: Reduction Is Gain of electrons.

Examples:

$\text{Mg} \to \text{Mg}^{2+} + 2\text{e}^{-}$ . Mg is OXIDISED (loses 2 e⁻).
$\text{Cl}_{2} + 2\text{e}^{-} \to 2\text{Cl}^{-}$ . Cl₂ is REDUCED (gains 2 e⁻).

Why electrons are the BEST definition. Some reactions don't involve oxygen or hydrogen — but they still involve electron transfer. Definition 3 covers all cases.

Worked. $\text{Zn} + \text{CuSO}_{4} \to \text{ZnSO}_{4} + \text{Cu}$ .

Zn → Zn²⁺ + 2e⁻ (loses electrons → oxidised).
Cu²⁺ + 2e⁻ → Cu (gains electrons → reduced).
Zn is the reducing AGENT. Cu²⁺ is the oxidising AGENT.

Oxygen: oxidise = gain O.
Hydrogen: oxidise = lose H.
Electrons: OIL RIG.
Electron definition is the most general.

Oxidising and reducing agents

Oxidising agent: reduced (gains e⁻). Reducing agent: oxidised (loses e⁻). They swap.

Oxidising agent. A substance that OXIDISES another substance. To oxidise something else, it must gain electrons (be reduced) itself.

Strong oxidising agents: chlorine (Cl₂), oxygen, fluorine, KMnO₄ (in acid), $\text{H}_{2}\text{O}_{2}$ , $\text{HNO}_{3}$ .

Reducing agent. A substance that REDUCES another substance. To reduce something else, it must lose electrons (be oxidised) itself.

Strong reducing agents: alkali metals, hydrogen, carbon, KI, sulfur dioxide.

Worked. $\text{Cl}_{2} + 2\text{KI} \to 2\text{KCl} + \text{I}_{2}$ .

Cl₂ → Cl⁻: gains electrons → REDUCED. Cl₂ is the OXIDISING AGENT.
2I⁻ → I₂: loses electrons → OXIDISED. I⁻ (in KI) is the REDUCING AGENT.

Tip. Each agent does the OPPOSITE of its name to itself. Oxidising agent is REDUCED; reducing agent is OXIDISED.

Oxidising agent: reduced itself.
Reducing agent: oxidised itself.
Strong oxidisers: Cl₂, O₂, KMnO₄, H₂O₂.
Strong reducers: alkali metals, H₂, C, KI.

Oxidation numbers (Extended)

Track electrons by assigning each atom an oxidation number. Increase = oxidised. Decrease = reduced.

Oxidation number rules.

Pure element (e.g. $\text{O}_{2}$ , $\text{Fe}$ ): oxidation number 0.
Monatomic ion: equal to its charge (e.g. Na⁺ has oxidation no. $+1$ , O²⁻ has $-2$ ).
Hydrogen: usually $+1$ (with non-metals); $-1$ in metal hydrides (e.g. NaH).
Oxygen: usually $-2$ (with most elements); $-1$ in peroxides ( $\text{H}_{2}\text{O}_{2}$ ); $+2$ in $\text{OF}_{2}$ .
Group 1 metals: $+1$ . Group 2: $+2$ . Aluminium: $+3$ .
Sum of oxidation numbers in a neutral compound = 0; in an ion = ion's charge.

Worked: assign oxidation numbers in MnO₄⁻.

O is $-2$ each (×4 = $-8$ ).
Sum = $-1$ (the charge of the ion).
Mn must be: $-1 - (-8) = +7$ .

Identifying oxidation/reduction.

Oxidation: oxidation number INCREASES.
Reduction: oxidation number DECREASES.

Worked. $\text{MnO}_{4}^{-} + \ldots \to \text{Mn}^{2+}$ .

Mn: $+7 \to +2$ . Decrease → REDUCED.
MnO₄⁻ is the OXIDISING AGENT (it accepts electrons).

Worked. $2\text{Fe}^{2+} \to 2\text{Fe}^{3+} + 2\text{e}^{-}$ .

Fe: $+2 \to +3$ . Increase → OXIDISED.

Naming compounds. Roman numerals in compound names give the oxidation number of the metal: e.g. iron(III) chloride means $\text{Fe}^{3+}$ (oxidation number $+3$ ).

Pure element: 0.
O usually $-2$ , H usually $+1$ .
Sum = 0 (neutral) or ion charge.
Increase: oxidised. Decrease: reduced.

Tests for oxidising and reducing agents

KI paper goes brown for oxidisers; acidified KMnO₄ goes colourless for reducers.

Test for an OXIDISING agent.

Use moist potassium iodide ( $\text{KI}$ ) paper.
An oxidising agent will OXIDISE I⁻ to I₂.
I₂ is brown-purple.
Result: white KI paper turns BROWN/PURPLE.

Test for a REDUCING agent.

Use acidified potassium manganate(VII) ( $\text{KMnO}_{4}$ ) — purple solution.
A reducing agent will REDUCE MnO₄⁻ ( $+7$ ) to Mn²⁺ ( $+2$ ).
Mn²⁺ is colourless.
Result: purple turns COLOURLESS.

Worked qualitative. A student adds chlorine water to a strip of moist KI paper. Paper turns brown. Conclusion: chlorine acts as an oxidising agent.

Worked qualitative. A student adds SO₂ gas to acidified KMnO₄. Purple fades to colourless. Conclusion: SO₂ acts as a reducing agent.

Cambridge tip. Always specify the FULL test: reagent, observation, and what the colour change tells you.

Oxidiser: KI paper turns BROWN (I₂ formed).
Reducer: KMnO₄ goes from PURPLE to COLOURLESS.
Quote both reagent and observation.

How it’s examined

Redox is examined every Paper 4 (6-8 marks): identify oxidising/reducing agents, half-equations, oxidation numbers, redox tests. Examiner reports flag students confusing 'oxidiser' (substance that does the oxidising) with 'oxidised' (substance that gets oxidised).

Sources: Cambridge IGCSE Chemistry 0620 syllabus 2026-2028 (8.2); 0620/42 Oct/Nov 2024 — Q13 (oxidation numbers, agents); 0620 Examiner Reports 2022-2024. Last reviewed 2026-05-07.

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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.

1Identify oxidation and reduction
Core• OIL RIG
Question
In the reaction $\text{Mg} + \text{Cu}^{2+} \to \text{Mg}^{2+} + \text{Cu}$ , identify what is oxidised and reduced.
Step-by-step solution
1. Step 1
  Mg loses 2 electrons → oxidised.
  $\text{Mg} \to \text{Mg}^{2+} + 2e^{-}$
2. Step 2
  Cu²⁺ gains 2 electrons → reduced.
  $\text{Cu}^{2+} + 2e^{-} \to \text{Cu}$
Answer
Mg oxidised; Cu²⁺ reduced.
2Find oxidation state
Extended• Adapted from 0620/42 May/Jun 2024 Q17• oxidation state
Question
Find the oxidation state of Mn in $\text{KMnO}_{4}$ .
Step-by-step solution
1. Step 1
  K = +1, O = −2 each.
  $+1 + Mn + 4(-2) = 0$
2. Step 2
  Solve.
  $Mn = +7$
Answer
$+7$
3Identify oxidising and reducing agents
Extended• agents
Question
$\text{Cl}_{2} + 2\text{KBr} \to 2\text{KCl} + \text{Br}_{2}$ . Identify the oxidising agent.
Step-by-step solution
1. Step 1
  Cl: 0 → −1 (gained electrons → reduced).
2. Step 2
  Br: −1 → 0 (lost electrons → oxidised).
3. Step 3
  Oxidising agent = species that GETS reduced.
Answer
$\text{Cl}_{2}$ is the oxidising agent.
4Test for an oxidising agent
Extended• test
Question
Describe a test for an oxidising agent.
Step-by-step solution
1. Step 1
  Add aqueous potassium iodide.
2. Step 2
  Oxidising agent oxidises $\text{I}^{-}$ to $\text{I}_{2}$ → solution turns brown/yellow.
Answer
Add aqueous KI — turns yellow/brown if an oxidising agent is present (iodine forms).

Key Formulae — Redox

The formulae you need to memorise for redox on the Cambridge IGCSE 0620 paper, with every variable defined in plain English and a note on when to use it.

OIL RIG
$\text{Oxidation Is Loss (of electrons), Reduction Is Gain}$
When to use
Quick mnemonic for redox identification.
Oxidation-state rules (key ones)
$\text{Element: 0; H: +1; O: -2; Group I: +1; Group II: +2; halide ions: -1}$
When to use
Computing oxidation states in formulae.

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 0620 sitting.

Oxidation
Examiner keyword
Loss of electrons (or increase in oxidation state).
Reduction
Examiner keyword
Gain of electrons (or decrease in oxidation state).
Oxidising agent
Examiner keyword
A substance that oxidises another → it itself is REDUCED.
Reducing agent
Examiner keyword
A substance that reduces another → it itself is OXIDISED.
Oxidation state
Examiner keyword
Charge an atom would have if all bonds were ionic. Tracks electron 'ownership' in a compound.

Common Mistakes and Misconceptions — Redox

The traps other students keep falling into on redox questions — taken from recent Cambridge IGCSE 0620 examiner reports and mark schemes — and how to avoid them.

✕Inverting OIL RIG
0620/42 — recurring
Why it happens
Memory slip.
How to avoid it
OIL = Oxidation Is Loss. RIG = Reduction Is Gain.
✕Calling the species that GETS oxidised the oxidising agent
Why it happens
Naming feels backwards.
How to avoid it
An oxidising AGENT does the oxidising — so it itself gets REDUCED.
✕Using H = +1 in metal hydrides (e.g. NaH)
Why it happens
Generalising the rule.
How to avoid it
In hydrides H is $-1$ . The general rule is 'H = +1' applies in most other cases.
✕Using O = −2 in peroxides
Why it happens
Generalising.
How to avoid it
In $\text{H}_{2}\text{O}_{2}$ , O is $-1$ (peroxide).

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

Detailed Study Notes

What you’ll learn

How it’s examined

1Identify oxidation and reduction

2Find oxidation state

3Identify oxidising and reducing agents

4Test for an oxidising agent

OIL RIG

Oxidation-state rules (key ones)

Oxidation

Reduction

Oxidising agent

Reducing agent

Oxidation state

✕Inverting OIL RIG

✕Calling the species that GETS oxidised the oxidising agent

✕Using H = +1 in metal hydrides (e.g. NaH)

✕Using O = −2 in peroxides

Practice questions

Past paper style quiz

Assess your understanding

Video lesson

Redox — frequently asked questions