Mixtures and Separating Techniques
Master Mixtures and Separating Techniques 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. A mixture isn't chemically bonded, so its parts can be separated by physical methods such as filtration, distillation and chromatography.
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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
Mixtures can be separated by physical processes, because the substances in them are not chemically bonded. Which method to use depends on the properties of the substances involved.
Filtration separates an insoluble solid from a liquid. Crystallisation separates a soluble solid from its solution. Simple distillation separates a liquid from a solution, and fractional distillation separates two or more liquids with different boiling points.
Chromatography separates substances based on how strongly they are attracted to the stationary phase compared with how soluble they are in the mobile phase. None of these methods involves a chemical reaction, so no new substances are made.
Revision notes
Choosing a method
Filtration: an insoluble solid from a liquid. Crystallisation: a soluble solid from its solution.
Simple distillation: a liquid from a solution, keeping the liquid. Fractional distillation: liquids with different boiling points. Chromatography: substances with different solubilities.
Why no reaction occurs
The substances in a mixture are not chemically bonded together.
Separating them therefore requires only physical processes, and no new substances are formed. Each substance keeps its own chemical properties throughout.
Chromatography
Paper chromatography uses a stationary phase, the paper, and a mobile phase, the solvent.
Substances more attracted to the paper move less; those more soluble in the solvent move further. This produces separate spots, allowing a mixture to be identified.
Key points
- Mixtures are separated by physical processes.
- Filtration separates an insoluble solid from a liquid.
- Crystallisation separates a soluble solid from solution.
- Distillation separates liquids by boiling point.
- Chromatography separates by solubility and attraction.
- No new substances are formed during separation.
Worked examples
Example 1
Name the method used to separate sand from water and explain your choice. [2 marks]
Model answer
Filtration (1 mark), because sand is insoluble in water so it is trapped by the filter paper while the water passes through (1 mark).
Example 2
Explain why fractional distillation is used rather than simple distillation to separate ethanol from water. [2 marks]
Model answer
Ethanol and water are both liquids with different but fairly close boiling points (1 mark), so a fractionating column is needed to separate them effectively rather than simple distillation (1 mark).
Example 3
Explain why no new substances are formed when a mixture is separated. [2 marks]
Model answer
The substances in a mixture are not chemically bonded together (1 mark), so separating them requires only physical processes and does not involve any chemical reaction (1 mark).
Common mistakes
Using filtration for a soluble solid.
A dissolved solid passes through the filter paper. Crystallisation is needed instead.
Confusing simple and fractional distillation.
Simple separates a liquid from a solution; fractional separates liquids from each other.
Saying separation makes new substances.
No chemical reaction occurs, so no new substances are formed.
Not justifying the choice of method.
Say why the property of the substance makes that method suitable.
Exam tips
- Match the method to the property — solubility or boiling point.
- Always justify your choice of separation technique.
- Remember physical processes make no new substances.
- Learn the stationary and mobile phases in chromatography.
Key terms
- Filtration
- Separating an insoluble solid from a liquid.
- Crystallisation
- Separating a soluble solid from its solution by evaporation.
- Fractional distillation
- Separating liquids with different boiling points.
- Chromatography
- Separating substances by differing solubility.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.