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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.

NDA · Apr 2026 · Q90Hard

Example 1 · Chemical Reactions · Redox: Oxidation, Reduction and Reducing Agents

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?
NDA · Apr 2026 · Q80Moderate

Example 2 · Chemical Reactions · Redox: Oxidation, Reduction and Reducing Agents

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

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.