Separation of mixtures
4 marks · NormalA student wants to separate a mixture of ethanol, water, and some insoluble sand. Describe the sequence of separation techniques they should use to obtain pure ethanol.
Explain why each technique is used at each stage.
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(a) — · must show filtration to remove sand, fractional distillation of filtrate, ethanol collected first
(b) — · must show filtration removes insolubles, fractional distillation separates miscible liquids by boiling point
Also accept: Accept correct method sequence even if written as a list
Common mistakes: Using crystallisation for ethanol; Reversing filtration and distillation
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Marking points
M1 (a) · 1 mark · States filtration (first) to remove insoluble sand
M2 (a) · 1 mark · States (fractional) distillation next to separate ethanol and water
A1 (a) · 1 mark · Mentions ethanol collected as first fraction due to lower boiling point
E1 (b) · 1 mark · Gives correct reason for each technique: filtration for insoluble solid, fractional distillation for miscible liquids by boiling point difference
Worked solution
a) First, filter the mixture to remove insoluble sand. Collect the filtrate, which contains ethanol and water. Then, use fractional distillation on the filtrate: heat it, and collect the ethanol, which is the first fraction to boil because ethanol has a lower boiling point than water.
b) Filtration removes the insoluble sand; fractional distillation separates the two mixed liquids by their different boiling points.