Alkenes and the aqueous bromine test
Identify the carbon–carbon double bond and interpret a controlled saturated-versus-unsaturated hydrocarbon comparison.
Exam scope and assessment notes
Core and Extended · Chemistry 0971, 2026–2028. Detailed substitution and addition product structures are taught in the Extended lessons. Cambridge states that IGCSE 0971 has the same subject content as IGCSE 0620, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Define hydrocarbon and saturation from carbon–carbon bonding.
- Separate a test observation from the conclusion it supports.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which bond identifies the stated alkene family?
What you will learn
- Describe alkenes as unsaturated hydrocarbons with a carbon–carbon double covalent bond.
- Use aqueous bromine observations to distinguish the stated saturated and unsaturated hydrocarbon samples.
An alkene contains the carbon double bond
Alkenes contain a double carbon–carbon covalent bond and are unsaturated hydrocarbons. Ethene CH₂=CH₂ is the simplest alkene in this syllabus family: each carbon has the double bond plus two single C–H bonds. Propene CH₃CH=CH₂ has three carbons and one C=C bond. The word unsaturated describes carbon–carbon bonding, not a compound's colour or ability to dissolve.
Keep both criteria when classifying: carbon and hydrogen only, and a C=C bond. An oxygen-containing structure is not a hydrocarbon even if it contains a carbon double bond. A carbon–oxygen double bond in ethanoic acid is not the required carbon–carbon double bond. The simple acyclic, one-double-bond alkene general formula CₙH₂ₙ supports atom counting but does not by itself display the connection.
Compare the observable bromine colour
In the stated school test without ultraviolet irradiation, an unsaturated hydrocarbon decolourises aqueous bromine: its orange-brown colour disappears. A saturated hydrocarbon does not decolourise it under these conditions, so the colour remains. Name both the reagent, aqueous bromine, and the observed colour change; saying only reacts is an incomplete test description.
The result distinguishes the supplied hydrocarbon alternatives under controlled conditions. It does not identify a unique molecule or establish that an unknown mixture is pure, because other substances can also affect bromine. Keep UV out of the comparison: photochemical alkane substitution is a different set of conditions. Product structures and addition reagents are developed in the Extended lesson; this test interpretation is not a home experiment instruction.
Pause and explain
Within the stated hydrocarbon comparison without UV, which observation supports unsaturation?
Worked example
Two known hydrocarbon alternatives are ethane and ethene. Without UV, sample A decolourises aqueous bromine and sample B retains its orange-brown colour. Identify them and justify the comparison.
Show the worked solution
- Ethene has a C=C bond and is unsaturated; ethane has single carbon–carbon bonding and is saturated.
- Aqueous bromine loses its orange-brown colour with the unsaturated alternative under the stated test conditions.
- Therefore A is ethene and B ethane within this two-sample comparison; the observation alone does not identify every possible unknown.
Answer A ethene, B ethane in the stated comparison; ethene decolourises aqueous bromine and ethane retains its colour without UV.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe the bonding in ethene and give its aqueous-bromine test result without UV.
Give me a hint
Name composition, the C=C bond and the colour observation.
Compare my reasoning
- CH₂=CH₂ contains only carbon and hydrogen, so it is a hydrocarbon.
- Its double carbon–carbon covalent bond makes it unsaturated.
- It decolourises orange-brown aqueous bromine under the stated test conditions.
Ethene is an unsaturated hydrocarbon with C=C; orange-brown aqueous bromine is decolourised.
Look for these in your work
- The double bond and hydrocarbon composition are identified.
- The reagent and colour change are stated.
Known alternatives propane and propene are tested with aqueous bromine without UV. X keeps the colour and Y removes it. Assign X and Y with a bonding explanation.
Give me a hint
Write CH₃CH₂CH₃ and CH₃CH=CH₂ before assigning observations.
Compare my reasoning
- Propane has only single carbon–carbon bonds and is saturated.
- Propene has a C=C bond and is unsaturated.
- X is propane because bromine colour remains; Y is propene because bromine is decolourised.
X propane retains orange-brown colour; Y propene decolourises aqueous bromine because it has C=C.
Look for these in your work
- Assignments use the supplied alternatives.
- Bonding is connected to each observation.
An unknown mixture decolourises aqueous bromine. A student concludes it is pure ethene. Assess what the observation establishes and what it cannot establish.
Give me a hint
A test of a chemical feature is not a unique molecule or purity measurement.
Compare my reasoning
- The colour observation can support unsaturation when interpreted within the stated hydrocarbon context.
- Different unsaturated hydrocarbons can decolourise bromine, and other mixture components can also interfere.
- The observation alone cannot identify ethene uniquely or prove purity; further evidence is required.
Decolourisation can support unsaturation in the stated context; it does not uniquely identify ethene or prove a mixture pure.
Look for these in your work
- A supported feature is distinguished from a unique identity.
- The mixture and purity limitations are explicit.
Common mistakes
- Using a carbon–oxygen double bond as the alkene definition.
- Reporting only reacts without the reagent and observable colour change.
- Claiming decolourisation uniquely proves ethene rather than unsaturation in the supplied hydrocarbon comparison.
- Adding UV to the standard comparison and ignoring changed alkane conditions.
What can you explain now?
What observation does an unsaturated hydrocarbon give with aqueous bromine in the stated test?
Compare with the explanation
The orange-brown aqueous bromine is decolourised.
A saturated hydrocarbon retains the bromine colour under the stated non-UV conditions.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Sketch ethene and propene, then state the reagent and opposite colour observations for saturated and unsaturated hydrocarbon alternatives.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.