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To test for alkenes, add bromine water and shake: it turns from orange to colourless if an alkene is present. Free notes and 9 questions answered.
Alkenes: Properties and Testing is part of Carbon compounds as fuels and feedstock in GCSE AQA Higher Chemistry.
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To test for alkenes, add bromine water and shake: it turns from orange to colourless if an alkene is present. Free notes and 9 questions answered.
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Use this lesson to stay inside Carbon compounds as fuels and feedstock while connecting the detail back to the wider Topic 7: Organic chemistry topic.
Read the core explanations from Alkenes: Properties and Testing before testing yourself from memory.
You've met alkanes - the saturated hydrocarbons with single bonds everywhere. But what happens when carbon atoms form a double bond? You get alkenes, and they behave completely differently.
Think of it like this: alkanes are like satisfied customers who don't want anything else. Alkenes are like hungry customers looking for something to react with. That double bond makes all the difference.
How do you tell if a mystery hydrocarbon is an alkene? You need a chemical test that only alkenes will pass. Enter bromine water - your chemical detective.
Bromine water starts orange. When you add an alkene, something dramatic happens. The orange colour disappears completely, leaving a colourless solution. It's like the alkene has 'eaten' the bromine.
Worked example: Bromine Water Test Procedure
Test an unknown hydrocarbon to see if it's an alkene
Answer: If orange bromine water turns colourless, the substance contains alkenes
This colour change happens because the alkene's double bond breaks open and grabs onto the bromine atoms. Alkanes can't do this - they just sit there, leaving the bromine water orange.
The secret lies in that double bond. While alkanes have all their electrons locked up in stable single bonds, alkenes have a double bond that's like a loaded spring - ready to react.
Picture the double bond as two lanes of traffic between carbon atoms. Under the right conditions, one of those lanes can break open to let new atoms join the party. This is called an addition reaction.
When bromine water meets an alkene, the double bond breaks and each carbon atom bonds to a bromine atom. The alkene becomes saturated (no more double bonds) and the bromine loses its colour.
Where do alkenes come from? Crude oil mainly contains alkanes, but we need alkenes for making plastics and other chemicals. The answer is cracking - breaking large alkane molecules into smaller, more useful pieces.
Think of cracking like snapping a long chain. When you break it, you get shorter pieces. Some of these pieces have broken ends that become double bonds - creating alkenes.
Worked example: Balancing Cracking Equations
Balance this cracking equation: C₈H₁₈ → C₄H₈ + ?
Answer: C₈H₁₈ → C₄H₈ + C₄H₁₀
Key pattern: Cracking always produces at least one alkene (with a double bond) plus other smaller hydrocarbons. The atoms must balance on both sides of the equation.
Examiners love testing alkenes because there are specific details you must get right. Miss these key points and you'll lose clear and systematic marks.
Common exam mistake: Students often write 'bromine water changes colour' without specifying from what to what. Always write the complete colour change for full marks.
Master these concepts and you'll tackle any alkene question with confidence. The key is understanding that double bond - it's the difference between a sleepy alkane and a reactive alkene ready for action.
Exam precision note: In Alkenes: Properties and Testing - Cracking and alkenes - Carbon compounds as fuels and feedstock, always identify the key chemical idea first, then link it to evidence, data, or an equation. A high-scoring GCSE answer names the particles involved, states what changes, and justifies the conclusion using correct scientific language. Where calculations are used, show each step with units and keep significant figures sensible. Finally, check that terminology matches AQA wording so your answer is unambiguous and mark-scheme friendly.
AQA GCSE Chemistry distinguishes between Combined Science and Chemistry-only detail here. All students need to know that alkenes are unsaturated hydrocarbons with the general formula CₙH₂ₙ and that bromine water turns from orange to colourless with an alkene. Chemistry-only students may also be asked to recognise and use simple alkene names such as ethene and propene. This note makes the specification boundary explicit.
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-19.
Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
The test for alkenes is to add bromine water and shake: bromine water turns from orange to colourless if an alkene is present. With an alkane there is no change, so the bromine water stays orange.
Alkenes are more reactive than alkanes because their C=C double bond can break open so new atoms can join, in an addition reaction. Alkanes have only stable single bonds, so they cannot react in this way.
Alkanes are saturated hydrocarbons with only single bonds, while alkenes are unsaturated hydrocarbons with at least one C=C double bond and the general formula CₙH₂ₙ. Alkenes turn bromine water colourless, but alkanes leave it orange.
Bromine water is orange, or orange/yellow, in colour. In the test for alkenes, it turns from orange to colourless, while with an alkane it stays orange.
Unsaturated means a hydrocarbon contains at least one carbon-carbon double bond (C=C), as alkenes do. The double bond can break open so new atoms join, which is why alkenes react with bromine water but saturated alkanes don't.
Bromine water goes colourless with an alkene because the double bond breaks and each carbon atom bonds to a bromine atom. The alkene becomes saturated and the bromine loses its orange colour.
To balance a cracking equation, work out what is left after taking the atoms in the known product away from the starting hydrocarbon. For C₈H₁₈ → C₄H₈ + ?, the remaining atoms are 4 carbons and 10 hydrogens, so the missing product is C₄H₁₀ (butane).
When bromine water is added to an alkane, there is no change: the bromine water stays orange. Alkanes have all their electrons locked up in stable single bonds, so they can't react with the bromine like alkenes do.
Cracking always produces at least one alkene, with a double bond, plus other smaller hydrocarbons. For example, C₈H₁₈ → C₄H₈ + C₄H₁₀ gives an alkene and butane, an alkane, with the atoms balanced on both sides.