A student has three unlabelled bottles. Each bottle contains one of the following substances: ethanoic acid, ethene and butane. The student adds a sample from each bottle to separate tubes of bromine water. Only one sample decolourises the bromine water. The student then tests the remaining two samples with universal indicator solution. Explain how the results of both tests allow the student to identify all three substances.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
1. The sample that decolourises the bromine water is ethene.
2. Ethene is an alkene; the C=C double bond reacts with Br₂, removing the colour.
3. The sample that turns the universal indicator red‑orange (acidic) is ethanoic acid.
4. Ethanoic acid is a weak acid; it protonates the indicator, giving a low pH.
5. The remaining sample, which leaves the indicator green (neutral) and does not decolourise bromine water, is butane.
6. Butane is a saturated alkane; it has no double bond and does not react with Br₂, so it remains colourless and neutral.
2. Ethene is an alkene; the C=C double bond reacts with Br₂, removing the colour.
3. The sample that turns the universal indicator red‑orange (acidic) is ethanoic acid.
4. Ethanoic acid is a weak acid; it protonates the indicator, giving a low pH.
5. The remaining sample, which leaves the indicator green (neutral) and does not decolourise bromine water, is butane.
6. Butane is a saturated alkane; it has no double bond and does not react with Br₂, so it remains colourless and neutral.
Examiner tips
- State the identity of the bromine‑water decolouriser first (ethene).
- Explain the reaction of alkenes with Br₂.
- Use the colour change of the universal indicator to identify the acid.
- Show that the remaining sample is neutral and non‑reactive, i.e. butane.
Common mistakes
- Confusing ethanoic acid with an alkane; forgetting the acid indicator colour.
- Failing to explain why alkenes react with Br₂.
Mark scheme (5 marks)
- The sample that decolourises bromine water is ethene / the alkene
- Ethene / alkenes decolourise bromine water because they are unsaturated / have a carbon–carbon double bond (that reacts with bromine)
- The sample that turns universal indicator acidic / red-orange / gives a low pH is ethanoic acid
- The remaining sample that does not change / leaves universal indicator green / neutral is butane
- Butane does not decolourise bromine water because it is saturated / has no double bond / is an alkane
Key terms in this question
bromine water · decolourise · universal indicator
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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