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Detailed notes on Chemical Reactions for Cambridge IGCSE Coordinated Science, covering key concepts, explanations, examples, and exam-focused revision points.
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.
Mapped to the Cambridge IGCSE 0654 syllabus (2025-2027).
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:
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.
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
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) |
Verbatim phrases and definitions Cambridge mark schemes credit.
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.
Step-by-step solutions to past-paper-style questions on redox, written exactly the way a tutor would explain them at the board.
Question
In the reaction Mg+CuSO4→MgSO4+Cu, identify what is oxidised and what is reduced. State the oxidising and reducing agents.
Step-by-step solution
Step 1
Track oxidation states. Mg: 0 (element) → Mg²⁺ in MgSO₄: +2. Magnesium loses 2 electrons → oxidised.
Mg→Mg2++2e−(oxidation)
Step 2
Cu²⁺ in CuSO₄: +2 → Cu: 0. Copper gains 2 electrons → reduced.
Cu2++2e−→Cu(reduction)
Step 3
Oxidising agent: the species that causes oxidation of another; it is itself reduced → CuSO₄ (Cu²⁺) is the oxidising agent.
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.
Question
Write the half-equation for the oxidation of iron(II) ions to iron(III) ions.
Step-by-step solution
Step 1
Fe²⁺ loses 1 electron to form Fe³⁺ → this is oxidation (loss of electrons).
Fe2+→Fe3++e−
Step 2
Check: charge on left = +2; charge on right = +3 − 1 = +2. ✓ Balanced.
Answer
Fe2+→Fe3++e−
Examiner tip
Always check that total charge is balanced on both sides of a half-equation.
Question
Define the term 'oxidising agent'.
Step-by-step solution
Step 1
An oxidising agent is a substance that oxidises another substance — it causes another substance to lose electrons.
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.
Question
State whether the following reaction is a redox reaction and explain your reasoning: HCl+NaOH→NaCl+H2O
Step-by-step solution
Step 1
Check the oxidation state of each element on both sides.
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.
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.
Definitions to memorise and the exact keywords mark schemes credit for redox answers — sharpened from recent examiner reports for the 2026 0654 sitting.
Loss of electrons (OIL); also: gain of oxygen or loss of hydrogen. Oxidation state increases.
Gain of electrons (RIG); also: loss of oxygen or gain of hydrogen. Oxidation state decreases.
A substance that oxidises another substance and is itself reduced (gains electrons) in the process.
A substance that reduces another substance and is itself oxidised (loses electrons) in the process.
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.
Mnemonic: Oxidation Is Loss of electrons; Reduction Is Gain of electrons. Used to distinguish oxidation from reduction.
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.
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.
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 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.
The things students keep getting wrong in this sub-topic, answered.