Addition Reaction – IGCSE Chemistry Definition
IGCSE Chemistry definition of addition reaction: a reaction where atoms add across a C=C double bond. Covers examples, tests for unsaturation, and 0620 tips.
Published by IGCSEChemistry.com.my
Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
Addition reactions are the characteristic reactions of alkenes and are tested through the bromine water test, hydrogenation, and hydration. The concept is simple — atoms add across the double bond — but candidates often lose marks by not showing the double bond opening.
The 0620 definition
An addition reaction is a reaction in which atoms or groups of atoms are added across a carbon–carbon double bond (C=C), producing a single product with no atoms lost.
The double bond opens (becomes a single bond) to accommodate the new atoms.
Key addition reactions of ethene
1. Addition of bromine (test for unsaturation)
CH₂=CH₂ + Br₂ → CH₂BrCH₂Br (1,2-dibromoethane)
Observation: bromine water changes from orange-brown to colourless.
This is the standard test to distinguish alkenes (unsaturated) from alkanes (saturated).
2. Addition of hydrogen (hydrogenation)
CH₂=CH₂ + H₂ → CH₃CH₃ (ethane)
Conditions: nickel catalyst, 150 °C. Used industrially to convert unsaturated vegetable oils to saturated fats (margarine production).
3. Addition of steam (hydration)
CH₂=CH₂ + H₂O → CH₃CH₂OH (ethanol)
Conditions: phosphoric acid catalyst, 300 °C, 60 atm pressure. This is the industrial method for producing ethanol.
4. Addition of hydrogen halide
CH₂=CH₂ + HBr → CH₃CH₂Br (bromoethane)
Addition vs substitution
| Feature | Addition | Substitution |
|---|---|---|
| Reactant type | Unsaturated (alkenes) | Saturated (alkanes) |
| Products | One product only | Two products |
| Double bond | Opens → single bond | No double bond involved |
| Example | CH₂=CH₂ + Br₂ → CH₂BrCH₂Br | CH₄ + Br₂ → CH₃Br + HBr |
Worked exam question
Ethene reacts with bromine water. (a) State the type of reaction. [1] (b) State the observation. [1] (c) Explain why ethane does not react with bromine water in the same way. [1]
Mark scheme
(a) Addition [1]
(b) Bromine water is decolourised / changes from orange/brown to colourless [1]
(c) Ethane is saturated / has no C=C double bond / has only single bonds, so addition cannot occur [1]
Common exam mistakes
- Saying bromine water turns “clear” — colourless is the correct term. The solution was already clear (transparent); it changes colour from orange-brown to colourless.
- Confusing addition with substitution — alkenes undergo addition, alkanes undergo substitution.
- Forgetting the conditions for hydration (steam addition) — phosphoric acid catalyst is required.
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