A student has a mixture containing two organic compounds: ethanol and ethanoic acid. The student wants to identify each compound using simple chemical tests. Describe the chemical tests the student could carry out to identify ethanol and ethanoic acid, stating the reagents used and the positive result for each test.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
1. Test for ethanoic acid: add a few drops of sodium carbonate (or sodium bicarbonate) solution to the sample.
2. Positive result: effervescence – bubbles of CO₂ are released.
3. Test for ethanol: add a few drops of acidified potassium dichromate (orange) solution to the sample.
4. Positive result: the orange solution turns green.
5. Additional confirmation for ethanoic acid: it turns damp blue litmus paper red (or universal indicator to a red colour), showing it is acidic.
2. Positive result: effervescence – bubbles of CO₂ are released.
3. Test for ethanol: add a few drops of acidified potassium dichromate (orange) solution to the sample.
4. Positive result: the orange solution turns green.
5. Additional confirmation for ethanoic acid: it turns damp blue litmus paper red (or universal indicator to a red colour), showing it is acidic.
Examiner tips
- Use the correct reagents – sodium carbonate for acid, acidified K₂Cr₂O₇ for alcohol.
- State the colour change or effervescence clearly – these are the marks awarded.
- Mention the pH indication if time allows – it shows understanding of acidity.
Common mistakes
- Mixing up the reagents (e.g. using potassium dichromate for the acid test).
- Failing to mention the colour change or effervescence as the positive result.
- Not recognising that the acid test is a simple CO₂ evolution test.
Mark scheme (5 marks)
- Test for ethanoic acid: add sodium carbonate (or sodium hydrogencarbonate) solution
- Positive result for ethanoic acid test: effervescence / bubbles of gas produced (carbon dioxide given off)
- Test for ethanol: add acidified potassium dichromate(VI) (orange solution)
- Positive result for ethanol: orange solution turns green
- Ethanoic acid turns damp blue litmus paper / universal indicator red / has pH less than 7 (acidic), OR ethanoic acid reacts with ethanol in the presence of a catalyst (sulfuric acid) to form a sweet-smelling ester, identifying it as a carboxylic acid
Key terms in this question
Related
- All Eduqas A-Level Chemistry revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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