A food scientist wants to separate a mixture of two liquids — ethanol (boiling point 78 °C) and water (boiling point 100 °C) — to obtain pure samples of each liquid. Explain why simple distillation would not be suitable for this separation and describe how fractional distillation could be used instead.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Simple distillation is unsuitable because the boiling points of ethanol (78 °C) and water (100 °C) are close; a single distillation column would not give a clear separation and the two liquids would co‑distil.
Fractional distillation can separate them. The mixture is heated in a distillation flask and the vapour rises through a fractionating column. The column provides many theoretical plates, allowing the vapour to equilibrate repeatedly. The lower boiling component, ethanol, evaporates first and rises to the top of the column, where it is condensed in a condenser and collected as a pure liquid. The higher boiling component, water, remains in the flask or condenses lower in the column, so the two liquids are collected separately.
Fractional distillation can separate them. The mixture is heated in a distillation flask and the vapour rises through a fractionating column. The column provides many theoretical plates, allowing the vapour to equilibrate repeatedly. The lower boiling component, ethanol, evaporates first and rises to the top of the column, where it is condensed in a condenser and collected as a pure liquid. The higher boiling component, water, remains in the flask or condenses lower in the column, so the two liquids are collected separately.
Examiner tips
- Mention the close boiling points to justify why simple distillation fails. Explain the role of the fractionating column and the repeated vapour–liquid equilibria. State that ethanol is collected first, water second. Use correct units and terminology (boiling point, vapour, condensate).
Common mistakes
- Claiming simple distillation works for any two liquids. Forgetting to mention the fractionating column or the repeated equilibria. Using the wrong order of collection (water before ethanol).
Mark scheme (4 marks)
- Simple distillation is not suitable because both liquids have different but relatively close boiling points / simple distillation cannot separate two liquids from each other (it can only separate a liquid from a solution/dissolved solid).
- Fractional distillation works because the two liquids have different boiling points.
- The mixture is heated in a flask and passes through a fractionating column; the liquid with the lowest boiling point (ethanol, 78 °C) evaporates/rises first.
- The vapour is then condensed (cooled) and collected as a pure liquid, leaving the other liquid (water) behind / the fractions are collected separately.
Key terms in this question
simple distillation · fractional distillation · boiling point · fraction
Related
- All Edexcel GCSE Chemistry (1CH0) revision notes →
- How to answer a "Explain and Describe" question →
- Decode the mark scheme abbreviations →
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