Who this is for: Cambridge IGCSE Chemistry (0620) students who want alkenes — unsaturated hydrocarbons, the bromine water test and addition reactions — to become reliable distinguishing answers instead of vague “double bond” statements.
What query it owns: how to understand and revise alkenes in Cambridge IGCSE Chemistry.
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 C=C double bond. Cambridge IGCSE Chemistry (0620) tests the general formula (CₙH₂ₙ), the bromine water test, addition reactions with hydrogen, steam and halogens, and the link to polymerisation. Because alkenes are more reactive than alkanes, examiners frequently ask you to compare the two series. This guide covers the core reactions, test conditions and past-paper wording.

Key takeaways

• Alkenes have general formula CₙH₂ₙ and contain at least one C=C double bond (unsaturated).
• Bromine water test: alkenes decolourise orange bromine water; alkanes do not.
• Addition reactions break the double bond and add atoms — e.g. ethene + H₂ → ethane (nickel catalyst, heat).
• Ethene + steam → ethanol (phosphoric acid catalyst, 300 °C, 60 atm).
• Ethene polymerises to poly(ethene) — the monomer is the alkene repeat unit.

What are alkenes in Cambridge IGCSE Chemistry?

Alkenes form a homologous series with one C=C double bond. Ethene (C₂H₄) and propene (C₃H₆) are key examples produced by cracking. The syllabus requires you to describe addition reactions, use the bromine water test to distinguish alkenes from alkanes, and link ethene to polymer manufacture.

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

The core ideas you must master

Feature / reaction	Alkene detail	Conditions / products
General formula	CₙH₂ₙ	Ethene C₂H₄, propene C₃H₆
Bromine water test	Decolourises orange solution	Distinguishes alkene from alkane
Addition of hydrogen	Alkene → alkane	Nickel catalyst, ~150 °C
Addition of steam	Ethene → ethanol	H₃PO₄ catalyst, 300 °C, 60 atm
Addition of bromine	Decolourises; dibromo product	Room temperature, no catalyst
Polymerisation	Many ethene → poly(ethene)	High pressure and catalyst

Alkanes vs alkenes — comparison table

Property	Alkanes	Alkenes
Bond type	C–C single only (saturated)	C=C present (unsaturated)
General formula	CₙH₂ₙ₊₂	CₙH₂ₙ
Bromine water	No change	Decolourises
Typical reaction	Substitution (Cl₂, sunlight)	Addition (H₂, Br₂, steam)
Reactivity	Relatively unreactive	More reactive

Alkenes in past-paper wording: command words that matter

Command word	What the question wants	Typical alkenes stem
Describe	Test or reaction details	”Describe the test for an alkene.”
State	Product or condition	”State the product when ethene reacts with steam.”
Explain	Why alkenes are more reactive	”Explain why alkenes undergo addition reactions.”
Draw	Structural or displayed formula	”Draw the structure of propene.”
Complete	Equation or equation scheme	”Complete the equation for the hydration of ethene.”

Worked exam-style stems (how to answer the wording)

1. “Describe the test for an alkene.” Add bromine water (orange). An alkene decolourises the solution. Reward: reagent named + colour change.
2. “Complete: C₂H₄ + H₂O → …” C₂H₅OH (ethanol). Conditions: phosphoric acid catalyst, 300 °C, 60 atm. Reward: correct product + conditions if asked.
3. “Explain why ethene can be polymerised but ethane cannot.” Ethene has a C=C double bond that opens to form long chains; ethane has only single bonds and lacks the unsaturated site needed for addition polymerisation. Reward: double bond opens + ethane saturated.

Test yourself with the Alkenes quiz once you have worked through a few examples.

How alkenes connect to the rest of Organic Chemistry

Alkenes link to Alkanes (addition of hydrogen), Alcohols (hydration of ethene), Fuels (cracking produces alkenes) and Polymers. The Cambridge IGCSE Chemistry resource hub links all units.

Common mistakes students make

• Saying alkanes decolourise bromine water — only alkenes do.
• Writing substitution for alkene reactions — alkenes undergo addition.
• Forgetting conditions for hydration (catalyst, temperature, pressure).
• Drawing propene with the double bond on the wrong carbon in displayed formulae.
• Confusing polymerisation (addition) with cracking (breaking long chains).

When you need more support

If bromine water tests and addition equations keep costing marks, work through the Alkenes quiz to find the gap, then get focused help from a Cambridge IGCSE Chemistry tutor.

Frequently asked questions

What is the test for an alkene? Bromine water turns from orange to colourless — the alkene decolourises it.

What is the product when ethene reacts with steam? Ethanol (C₂H₅OH), using a phosphoric acid catalyst at 300 °C and 60 atm.

Why are alkenes more reactive than alkanes? The C=C double bond is easier to break than C–C single bonds, allowing addition reactions.

How do I revise alkenes effectively? Learn the bromine water test, three addition reactions and ethene polymerisation, then take the Alkenes quiz.

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