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Playbook

Chemical Reactions

30 q · 10% HARD. Redox classification (oxidation = loss of e⁻ / OIL RIG mnemonic), reaction-type identification (combination, decomposition, displacement, double-displacement), specific reactions (precipitation, electrolysis, lime water + CO₂), thermal decomposition products, and endo/exothermic recognition.

questions in the bank
30
tagged HARD
10%
subtopic(s)
6
worked examples
2

When you’ll see it

A redox identification (oxidation/reduction/reducing agent), a reaction-type classification (combination/decomposition/displacement), a specific reaction product (lime water + CO₂), or an endo/exothermic recognition.

How this chapter is tested

30 q in 10 years · 10% HARD — the chapter's hottest subtopic is Redox (10 q at 20% HARD). LEO RGO: Loss of Electrons = Oxidation; Reduction = Gain of e⁻. Equivalently OIL RIG (Oxidation Is Loss, Reduction Is Gain). A species OXIDISED has its oxidation state INCREASE. A species REDUCED has its oxidation state DECREASE. The REDUCING AGENT gets oxidised (it does the reducing by giving up its e⁻). In Fe + 2HCl → FeCl₂ + H₂, Fe goes 0 → +2 (oxidised, so Fe = reducing agent); H goes +1 → 0 (reduced, so HCl = oxidising agent).

Reaction Types (7 q · 14% HARD): COMBINATION A + B → AB (CaO + H₂O → Ca(OH)₂ slaking of lime). DECOMPOSITION AB → A + B (2H₂O → 2H₂ + O₂ electrolysis; CaCO₃ → CaO + CO₂ on heating). DISPLACEMENT A + BC → AC + B (more reactive displaces less; Zn + CuSO₄ → ZnSO₄ + Cu). DOUBLE-DISPLACEMENT AB + CD → AD + CB (often precipitation: AgNO₃ + NaCl → AgCl↓ + NaNO₃). Specific reactions: CO₂ + Ca(OH)₂ → CaCO₃↓ + H₂O (lime water TURNS MILKY — classic test for CO₂). Rusting = oxidation (Fe → Fe₂O₃·xH₂O). Combustion = oxidation + heat release.

Endo vs Exo (3 q): EXOTHERMIC = releases heat (combustion, respiration, neutralisation, most combinations). ENDOTHERMIC = absorbs heat (decomposition of CaCO₃, photosynthesis, dissolving NH₄Cl in water). Reaction direction: ΔH negative for exo, positive for endo. Most reactions on a paper that need a 'condition' (heat, electrolysis, sunlight) are endothermic.

The sub-skills

The rules and habits that decide whether you get a question right.

  • Redox via oxidation-state change

    Assign oxidation states to all atoms before + after. The atom that INCREASES is OXIDISED → its source is the REDUCING agent. The atom that DECREASES is REDUCED → its source is the OXIDISING agent. Free elements have ox-state 0.

  • Reaction-type identification

    Combination: A + B → AB. Decomposition: AB → A + B (thermal/electric/photochemical). Displacement: A + BC → AC + B (reactivity series decides direction). Double-displacement: AB + CD → AD + CB (often precipitation or acid-base).

  • Specific test reactions

    CO₂ + lime water Ca(OH)₂ → milky CaCO₃ ↓. NaCl + AgNO₃ → white AgCl ↓ (test for chloride). SO₂ + acidified K₂Cr₂O₇ → green Cr³⁺ (test for SO₂, reducing). Cu²⁺ + NH₃ excess → deep blue [Cu(NH₃)₄]²⁺.

  • Endo vs exothermic recognition

    Exothermic: combustion (all fuels), respiration, neutralisation, rusting, most combinations, dissolving NaOH in water. Endothermic: photosynthesis, decomposition by heat (CaCO₃, KClO₃), dissolving NH₄Cl/KNO₃ in water, melting/boiling.

2 worked examples from the bank

Real past-year questions illustrating the playbook. Click to reveal options + solution.

Example 1Chemical ReactionsHARD
Consider the following reaction-property pairs: I. Fe+2HCl→FeCl2+H2\text{Fe} + 2\text{HCl} \to \text{FeCl}_2 + \text{H}_2 | Fe is reductant II. Zn+CuSO4→ZnSO4+Cu\text{Zn} + \text{CuSO}_4 \to \text{ZnSO}_4 + \text{Cu} | Cu is being reduced III. Br2+2I−→2Br−+I2\text{Br}_2 + 2\text{I}^- \to 2\text{Br}^- + \text{I}_2 | Br2\text{Br}_2 is reductant IV. CH4+2O2→CO2+2H2O\text{CH}_4 + 2\text{O}_2 \to \text{CO}_2 + 2\text{H}_2\text{O} | C atom in CH4\text{CH}_4 is oxidized Which pairs are correct?

[Q90 · Apr · 2026]

Example 2Chemical ReactionsMODERATE
Which of the following are oxidation reactions? I. Rusting of iron II. Burning of fuel III. Rancidity of oils and fats IV. Browning of fruits

[Q80 · Apr · 2026]

Traps to expect

Distractor shapes specific to this chapter. The page-wide Traps section covers the bank-level patterns.

  • Reducing agent IS the one reduced

    Wrong. The REDUCING agent is the species that gets OXIDISED (it reduces the OTHER thing by giving up its own e⁻). Equivalently: oxidising agent gets REDUCED. Easy to flip — write out e⁻ flow explicitly before assigning.

  • Decomposition always endothermic

    Most decompositions ARE endothermic (need heat/electricity/light to break bonds), but exceptions exist (decomposition of explosives like H₂O₂ catalysed by MnO₂ is exothermic). Don't apply the rule blindly.

  • Rusting is 'just a physical change'

    Rusting is CHEMICAL — new substance (Fe₂O₃·xH₂O) is formed, and the iron is permanently altered. Wrong option labels it physical because 'it's just iron + air.'

Drill every chemical reactions question

30 questions from the bank, scoped to 6 bundled subtopics.

Drill the 30 questions

Related playbooks

Often paired with this one — drill these next if you found the worked examples above tractable.