Separation of mixtures
3 marks · NormalExplain why fractional distillation is able to separate ethanol from water, but simple distillation cannot produce pure ethanol from the mixture.
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Also accept: Fractional distillation uses a column to allow repeated condensation and evaporation; simple distillation does not separate liquids that boil close together fully.
Common mistakes: assuming simple distillation gives pure ethanol
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Marking points
E1 · 1 mark · Explains ethanol and water are miscible with different boiling points
E2 · 1 mark · States fractional distillation uses a fractionating column for better separation
E3 · 1 mark · Explains simple distillation cannot separate two liquids with similar boiling points effectively
Worked solution
Ethanol and water are completely miscible, but have different boiling points (ethanol at 78°C, water at 100°C). Fractional distillation uses a fractionating column that provides a large surface area for the vapours to condense and re-evaporate. This helps keep the less volatile component (water) from passing into the condenser, so almost pure ethanol is collected first. Simple distillation does not separate the liquids as effectively because both vapours can travel together if their boiling points are not far apart.