A student wants to separate a mixture of two liquids, ethanol and water, to obtain a sample of pure ethanol. Explain why simple distillation would not be suitable for this purpose and describe how fractional distillation could be used instead to obtain pure ethanol.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Ethanol boils at 78 °C and water boils at 100 °C. Both liquids are miscible, meaning they mix completely together.
Model answer (5 marks)
Simple distillation cannot separate ethanol and water because their boiling points differ by only 22 °C, so the vapour contains both components and the condensate is a mixture.
In fractional distillation the mixture is heated in a fractionating column. The vapour rises and repeatedly condenses and re‑vapours, enriching the component with the lower boiling point (ethanol). The ethanol vapour reaches the top of the column first, is cooled in a condenser and collected as a liquid. The higher boiling component (water) remains lower in the column, allowing the two liquids to be separated into distinct fractions.
In fractional distillation the mixture is heated in a fractionating column. The vapour rises and repeatedly condenses and re‑vapours, enriching the component with the lower boiling point (ethanol). The ethanol vapour reaches the top of the column first, is cooled in a condenser and collected as a liquid. The higher boiling component (water) remains lower in the column, allowing the two liquids to be separated into distinct fractions.
Examiner tips
- State the small boiling‑point difference and its effect on simple distillation; describe the role of the fractionating column and the enrichment of the lower‑boiling component; mention condensation and collection of ethanol; keep points concise and use exam terminology.
Common mistakes
- Claiming simple distillation works because the boiling points are different; confusing the positions of ethanol and water in the column; omitting the condensation step in fractional distillation.
Mark scheme (5 marks)
- Simple distillation is not suitable because the boiling points of ethanol and water are too close together (only 22 °C apart) / the liquids cannot be separated completely by simple distillation because both vapourise at similar temperatures.
- In fractional distillation, the mixture is heated in a fractionating column.
- The vapour rises up the fractionating column and the liquid with the lowest boiling point (ethanol) reaches the top first / condenses lower in the column for the less volatile liquid.
- The ethanol vapour is cooled / condensed in a condenser and collected as a liquid.
- The two liquids can be separated into different fractions because they have different boiling points.
Key terms in this question
simple distillation · fractional distillation
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain and Describe" question →
- Decode the mark scheme abbreviations →
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