Filtration, Crystallisation and Distillation
Practise Filtration, Crystallisation and Distillation for GCSE Chemistry with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Filtration separates an insoluble solid, crystallisation recovers a dissolved solid, and distillation separates a liquid from a solution by boiling point.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
Three separation techniques must be described in detail, and each has a required practical method.
Filtration uses filter paper in a funnel. The insoluble solid, called the residue, is trapped by the paper while the liquid, called the filtrate, passes through. Crystallisation heats a solution in an evaporating basin until crystals begin to form, then leaves it to cool so crystals grow as the remaining solvent evaporates.
Distillation heats a solution so the solvent evaporates, then cools the vapour in a condenser so it turns back to liquid and is collected. The dissolved solid is left behind in the flask. In fractional distillation, a fractionating column allows liquids with different boiling points to separate.
Revision notes
Filtration
Pour the mixture through filter paper in a funnel.
The insoluble solid is trapped as the residue; the liquid passing through is the filtrate. This works only when the solid does not dissolve.
Crystallisation
Heat the solution in an evaporating basin so the solvent evaporates.
Stop heating when crystals begin to form at the edges, then leave to cool. Crystals grow as the remaining solvent evaporates. Filter and dry the crystals. Heating too strongly can decompose the solid.
Distillation
Simple distillation: heat the solution so the solvent evaporates. The vapour passes into a condenser where it cools and turns back to liquid, and is collected. The solute remains in the flask.
Fractional distillation: a fractionating column allows liquids with different boiling points to separate, with the lowest boiling point liquid collected first.
Key points
- The residue is the solid trapped by the filter paper.
- The filtrate is the liquid that passes through.
- Crystallisation evaporates the solvent to form crystals.
- Stop heating when crystals first appear.
- In distillation the vapour is cooled in a condenser.
- Fractional distillation separates liquids by boiling point.
Worked examples
Example 1
Name the solid trapped by the filter paper and the liquid that passes through. [2 marks]
Model answer
The solid is the residue (1 mark) and the liquid is the filtrate (1 mark).
Example 2
Describe how you would obtain pure water from salty water. [3 marks]
Model answer
Heat the salty water so the water evaporates (1 mark). Pass the vapour into a condenser where it is cooled and turns back into liquid (1 mark). Collect the pure water; the salt remains behind in the flask (1 mark).
Example 3
Explain why heating should stop when crystals first appear during crystallisation. [2 marks]
Model answer
Continuing to heat strongly could decompose the solid (1 mark), so the solution is left to cool instead, allowing crystals to grow as the remaining solvent evaporates slowly (1 mark).
Common mistakes
Confusing residue and filtrate.
Residue is the solid left in the paper; filtrate is the liquid collected.
Forgetting the condenser in distillation.
Without cooling the vapour, the liquid cannot be collected.
Evaporating to dryness in crystallisation.
Stop when crystals first appear, or the solid may decompose.
Saying distillation separates an insoluble solid.
That is filtration. Distillation separates a liquid from a solution.
Exam tips
- Learn the terms residue and filtrate precisely.
- Include the condenser when describing distillation.
- Explain why heating stops early in crystallisation.
- Describe methods as a numbered sequence of steps.
Key terms
- Residue
- The insoluble solid trapped by filter paper.
- Filtrate
- The liquid that passes through the filter paper.
- Condenser
- Apparatus that cools vapour back into a liquid.
- Solvent
- The liquid in which a solute is dissolved.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.