Who this is for: Cambridge IGCSE Coordinated Science (0654) students who want alkenes — unsaturated hydrocarbons, the bromine test and addition reactions — to become reliable marks instead of confusing them with alkanes.
What query it owns: how to understand and revise alkenes in Cambridge IGCSE Coordinated Science.
Why this is safe: this page owns the alkenes revision-guide angle, while Tutopiya’s Alkenes subtopic page owns the learning resource and the free Alkenes quiz owns the practice.

Alkenes are unsaturated hydrocarbons containing at least one carbon–carbon double bond (C=C). Cambridge IGCSE Coordinated Science (0654) expects you to name ethene and propene, state the general formula CₙH₂ₙ, use the bromine water test to distinguish alkenes from alkanes, and describe addition reactions. This guide covers the syllabus definitions, test procedures examiners reward, and the question types that appear every year.

Key takeaways

• Alkenes are unsaturated — they contain a C=C double bond; general formula CₙH₂ₙ.
• Key examples: ethene (C₂H₄), propene (C₃H₆), butene (C₄H₈).
• Bromine water test: alkenes decolourise orange bromine water; alkanes do not.
• Alkenes undergo addition reactions — atoms add across the C=C bond without losing other atoms.
• Ethene + steam → ethanol and ethene polymerisation → poly(ethene) are high-value exam reactions.

What are alkenes in Cambridge IGCSE Coordinated Science?

The C=C double bond is the functional group of alkenes. One bond is strong (sigma), the other is weaker (pi) — making alkenes more reactive than alkanes. The pi bond breaks during addition reactions, allowing atoms to attach to each carbon. Ethene is manufactured by cracking larger alkanes and is used to make ethanol and plastics. Examiners frequently test the bromine water distinction and addition equations.

Read the full notes on Tutopiya’s Alkenes subtopic page before attempting questions.

The core ideas you must master

Idea	What it means	How the exam uses it
Unsaturated	Contains C=C double bond	”State why ethene is unsaturated.”
General formula	CₙH₂ₙ	”Give the formula of the alkene with 3 carbons.”
Bromine test	Decolourises bromine water	”Describe a test to distinguish ethene from ethane.”
Addition reaction	Atoms add across C=C	”Write the equation for ethene + hydrogen.”
Polymerisation	Many ethene → poly(ethene)	“Describe the polymerisation of ethene.”

The first three alkenes

Alkene	Formula	Structural feature	Key use
Ethene	C₂H₄	H₂C=CH₂	Making ethanol and poly(ethene)
Propene	C₃H₆	3-carbon chain with one C=C	Making poly(propene)
Butene	C₄H₈	4-carbon chain with one C=C	Fuel component

The bromine water test — step by step

1. Add bromine water (orange/brown) to the test substance.
2. Alkene present: bromine adds across C=C → colourless solution (decolourised).
3. Alkane only: no reaction → orange colour remains.
4. This is the standard test for unsaturation at IGCSE level.

Addition reactions of ethene

Reaction	Conditions	Equation	Product
With hydrogen	Nickel catalyst, heat	C₂H₄ + H₂ → C₂H₆	Ethane
With steam	Phosphoric acid catalyst, 300 °C	C₂H₄ + H₂O → C₂H₅OH	Ethanol
With bromine	Room temperature	C₂H₄ + Br₂ → C₂H₄Br₂	Dibromoethane
Polymerisation	High pressure, catalyst	nC₂H₄ → –(C₂H₄)ₙ–	Poly(ethene)

Alkenes vs alkanes — comparison

Property	Alkanes	Alkenes
Bond type	C–C single only (saturated)	C=C double bond (unsaturated)
General formula	CₙH₂ₙ₊₂	CₙH₂ₙ
Bromine water	No reaction (stays orange)	Decolourises
Typical reaction	Combustion, substitution	Addition
Reactivity	Unreactive	More reactive

Alkenes in past-paper wording

Command word	What the question wants	Typical stem
Describe test	Bromine water procedure	”Describe a test for an alkene.”
Explain	Why alkene decolourises bromine	”Explain the result with ethene.”
Write equation	Addition reaction	”Write the equation for ethene + steam.”
State	General formula or unsaturation	”State why alkenes are unsaturated.”
Draw	Displayed formula	”Draw the structure of propene.”

Worked exam-style stems

1. “Describe a test to show that a hydrocarbon is an alkene.” Add bromine water; if the orange colour is decolourised, the compound is an alkene (unsaturated). Reward: bromine water + decolourises + alkene/unsaturated.
2. “Write a word equation for the reaction of ethene with steam.” Ethene + steam → ethanol. Reward: correct reactants and product.
3. “Explain why ethene is more reactive than ethane.” Ethene has a C=C double bond; the pi bond is weaker and breaks easily in addition reactions, whereas ethane has only strong single bonds. Reward: C=C double bond + pi bond breaks + addition.

Practise on the Alkenes quiz.

How alkenes connect to the syllabus

Alkenes link to Alkanes (contrast saturated vs unsaturated), Alcohols (ethene + steam → ethanol) and Fuels (cracking alkanes to alkenes). The Cambridge IGCSE Coordinated Science resource hub links every Organic Chemistry subtopic.

Common mistakes students make

• Confusing ethene (C₂H₄) with ethane (C₂H₆).
• Saying the bromine test works on alkanes (it tests for unsaturation — alkenes only).
• Using general formula CₙH₂ₙ₊₂ for alkenes (correct: CₙH₂ₙ).
• Describing addition as substitution (addition: atoms add across C=C; substitution: one atom replaces another).
• Forgetting catalyst/conditions for ethene + steam (phosphoric acid, ~300 °C).

When you need more support

If alkene and bromine-test questions keep costing marks, work through the Alkenes quiz, then get help from a Cambridge IGCSE Coordinated Science tutor.

Frequently asked questions

What is the general formula of alkenes? CₙH₂ₙ.

How do you test for an alkene? Add bromine water; the orange colour is decolourised if an alkene (unsaturated compound) is present.

What is an addition reaction? A reaction in which atoms add across the C=C double bond without any other atoms being lost.

How do I revise alkenes effectively? Learn the general formula, bromine test, key addition equations and contrast with alkanes, then take the quiz.

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