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Next exam on November 14 – 15, 2026:
All concepts

Redox Reactions

氧化还原反应

Recognising redox by change in oxidation number and electron transfer, identifying oxidising and reducing agents, and how redox overlaps the four basic reaction types.

15 real-exam questions · 4 sittings17 in mock papers51 practice

Formulas

  • Electron conservation in redox

    Law
    nlost=ngainedn_{\text{lost}}=n_{\text{gained}}

    Applies when：Total electrons gained by the oxidant equal those lost by the reductant — the basis for balancing redox equations

How the real exam tests it

Questions from real CSCA papers that test this concept. Work each one out before you open the answer.

  1. CSCA Chemistry December 2025 Past Paper
    Which of the following is commonly used as an oxidizing agent? ( )
    • A. Iron
    • B. Carbon monoxide
    • C. Potassium permanganate
    • D. Hydrogen gas
    Show answer & solution

    Answer: C

    Answer: C In KMnO₄, Mn is in the +7 oxidation state and is very easily reduced, so potassium permanganate is a common strong oxidizing agent. Iron, carbon monoxide and hydrogen are reducing agents.
    See it in the full CSCA Chemistry December 2025 Past Paper
  2. CSCA Chemistry April 2026 Past Paper
    Among the following reactions, the one that is a double displacement (metathesis) reaction is ( ).
    • A. Hydrogen burning in oxygen
    • B. Thermal decomposition of sodium bicarbonate
    • C. Zinc reacting with dilute sulfuric acid
    • D. Hydrochloric acid reacting with sodium hydroxide
    Show answer & solution

    Answer: D

    Answer: D Hydrochloric acid reacting with sodium hydroxide is a neutralization, i.e., a double displacement reaction.
    Take the full CSCA Chemistry April 2026 Past Paper — timed & graded
  3. CSCA Chemistry March 2026 Past Paper
    Which reaction is not a basic type but is a redox reaction?
    • A．2Na+2H2O=2NaOH+H2↑2 \mathrm { Na } + 2 \mathrm { H } _ { 2 } \mathrm { O } = 2 \mathrm { NaOH } + \mathrm { H } _ { 2 } \uparrow
    • B．CO+CuO→Cu+CO2\mathrm { CO } + \mathrm { CuO } \rightarrow \mathrm { Cu } + \mathrm { CO } _ { 2 }
    • C．Fe2O3+6HCl=2FeCl3+3H2O\mathrm { Fe } _ { 2 } \mathrm { O } _ { 3 } + 6 \mathrm { HCl } = 2 \mathrm { FeCl } _ { 3 } + 3 \mathrm { H } _ { 2 } \mathrm { O }
    • D．2H2O2→2H2O+O2↑2 \mathrm { H } _ { 2 } \mathrm { O } _ { 2 } \rightarrow 2 \mathrm { H } _ { 2 } \mathrm { O } + \mathrm { O } _ { 2 } \uparrow
    Show answer & solution

    Answer: B

    CO+CuO→Cu+CO2\mathrm { CO } + \mathrm { CuO } \rightarrow \mathrm { Cu } + \mathrm { CO } _ { 2 } : two compounds react (not basic type); C(+2→+4)\mathrm { C } ( + 2 \rightarrow + 4 ) oxidized, Cu(+2→0)\mathrm { Cu } ( + 2 \rightarrow 0 ) reduced = redox. Answer: B.
    Take the full CSCA Chemistry March 2026 Past Paper — timed & graded

Real-exam evidence

Recorded past-paper appearances are listed below; online verification is labelled separately.

15 questions across 4 sittings1 of 4 sittings available to verify free online

17 questions across 5 mock papers — our own papers, so a coverage signal rather than real-exam evidence

51 related practice questions availablePractise free: Redox Reactions

Sitting evidence comes from the CSCA past papers on record; questions whose original paper is missing are excluded, and sprint papers are deliberately not counted.

Glossary

redox reaction

Related concepts

Ionic Reactions and Ion Coexistence