A student is given four unlabelled liquids. The liquids are ethanol, ethanoic acid, propene and octane. The student uses two chemical tests to identify each liquid. Describe the two tests, the observations for each liquid, and explain why each observation occurs.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The student has access to bromine water and sodium carbonate solution.
Model answer (5 marks)
1. Bromine water test – add a few drops of bromine water to each liquid.
• Propene: the orange colour disappears, giving a colourless solution.
• Ethanol, ethanoic acid, octane: the orange colour remains unchanged.
Explanation: Propene contains a C=C double bond; bromine adds across the double bond, forming a dibrominated alkane and decolourising the solution. The other liquids are saturated or non‑reactive, so no bromine is consumed.
2. Sodium carbonate test – add a small amount of sodium carbonate solution to each liquid.
• Ethanoic acid: effervescence (bubbles of CO₂) is observed.
• Ethanol, propene, octane: no effervescence; the solutions remain clear.
Explanation: Ethanoic acid reacts with carbonate to give sodium acetate, CO₂ gas and water: CH₃COOH + Na₂CO₃ → CH₃COONa + CO₂↑ + H₂O. The other liquids are not acids and do not react with carbonate.
These two tests together allow the student to identify propene (bromine test) and ethanoic acid (carbonate test) and distinguish them from ethanol and octane.
• Propene: the orange colour disappears, giving a colourless solution.
• Ethanol, ethanoic acid, octane: the orange colour remains unchanged.
Explanation: Propene contains a C=C double bond; bromine adds across the double bond, forming a dibrominated alkane and decolourising the solution. The other liquids are saturated or non‑reactive, so no bromine is consumed.
2. Sodium carbonate test – add a small amount of sodium carbonate solution to each liquid.
• Ethanoic acid: effervescence (bubbles of CO₂) is observed.
• Ethanol, propene, octane: no effervescence; the solutions remain clear.
Explanation: Ethanoic acid reacts with carbonate to give sodium acetate, CO₂ gas and water: CH₃COOH + Na₂CO₃ → CH₃COONa + CO₂↑ + H₂O. The other liquids are not acids and do not react with carbonate.
These two tests together allow the student to identify propene (bromine test) and ethanoic acid (carbonate test) and distinguish them from ethanol and octane.
Examiner tips
- Use the exact terms ‘decolourises’ and ‘effervescence’ as the mark scheme expects.
- Show the reaction for the carbonate test to justify the bubbles.
- Explain the reason for each observation – unsaturation for bromine, acidity for carbonate.
Common mistakes
- Claiming that octane reacts with bromine water – it does not.
- Forgetting to mention that ethanol shows no reaction with either test.
- Writing ‘bubbles’ instead of ‘effervescence’ for the carbonate test.
Mark scheme (5 marks)
- Bromine water decolourises / turns from orange to colourless with propene (alkene)
- Bromine water remains orange with ethanol, ethanoic acid and octane (no reaction / not decolourised)
- Propene decolourises bromine water because it is unsaturated / has a carbon–carbon double bond that reacts with bromine
- Sodium carbonate solution produces bubbles / effervescence with ethanoic acid (carboxylic acid reacts with carbonate to produce carbon dioxide)
- Ethanol and octane show no reaction / no bubbles with sodium carbonate solution, allowing ethanoic acid to be distinguished from the remaining two liquids
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
More Organic analysis questions
- A chemist has three unlabelled bottles. Each bottle contains one of the followin…
- A student has three unlabelled bottles. Each bottle contains one of the followin…
- A student tests four organic compounds using bromine water and a lit splint. Com…
- A student has two unlabelled test tubes. One contains an alkane and the other co…