Separation Techniques

This lesson covers the key separation techniques you need to know for the Edexcel GCSE Combined Science specification (1SC0): filtration, evaporation and crystallisation, simple distillation, fractional distillation and chromatography. You need to understand when to use each method, how it works, and the scientific principles behind it.

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Choosing the Right Technique

The method used to separate a mixture depends on the physical properties of the components.

graph TD
    A["What type of<br/>mixture is it?"] --> B{"Insoluble solid<br/>in a liquid?"}
    B -->|Yes| C["Filtration"]
    B -->|No| D{"Dissolved solid<br/>in a solution?"}
    D -->|"Want the solid"| E["Evaporation /<br/>Crystallisation"]
    D -->|"Want the solvent"| F["Simple Distillation"]
    D -->|No| G{"Two or more liquids<br/>with different BPs?"}
    G -->|Yes| H["Fractional<br/>Distillation"]
    G -->|No| I{"Dissolved substances<br/>to identify?"}
    I -->|Yes| J["Chromatography"]

    style A fill:#2c3e50,color:#fff
    style B fill:#2980b9,color:#fff
    style C fill:#27ae60,color:#fff
    style D fill:#2980b9,color:#fff
    style E fill:#27ae60,color:#fff
    style F fill:#27ae60,color:#fff
    style G fill:#2980b9,color:#fff
    style H fill:#27ae60,color:#fff
    style I fill:#2980b9,color:#fff
    style J fill:#27ae60,color:#fff

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1. Filtration

Filtration separates an insoluble solid from a liquid.

Method

1. Fold filter paper and place it in a funnel.
2. Place the funnel over a conical flask or beaker.
3. Pour the mixture through the filter paper.
4. The liquid passes through the tiny holes in the paper — this is called the filtrate.
5. The solid is too large to pass through and remains on the paper — this is called the residue.

Example Uses

• Separating sand from water.
• Separating insoluble salts after a precipitation reaction.

Term	Definition
Filtrate	The liquid that passes through the filter paper
Residue	The solid that remains on the filter paper

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2. Evaporation and Crystallisation

Evaporation is used to obtain a dissolved solid from a solution by heating.

Method

1. Pour the solution into an evaporating basin.
2. Heat gently using a Bunsen burner or water bath.
3. Heat until crystals start to form at the edges of the solution — then stop heating.
4. Leave the solution to cool slowly — crystals will form as the remaining water evaporates.
5. Filter the crystals and pat dry with filter paper.

Exam Tip: Do NOT heat the solution to dryness — this can cause the crystals to spit and can decompose some compounds. Always stop heating when crystals start to appear, then leave to cool and crystallise.

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3. Simple Distillation

Simple distillation separates a solvent from a solution, collecting the pure solvent.

Method

1. Heat the solution in a flask — the solvent evaporates first (lowest boiling point).
2. The vapour rises into the condenser — a tube surrounded by cold water.
3. The cold water cools the vapour, causing it to condense back into a liquid.
4. The pure liquid (the distillate) drips into a collection beaker.
5. The dissolved solid remains in the flask.

Apparatus

Component	Purpose
Round-bottomed flask	Holds the solution being heated
Thermometer	Monitors the temperature to ensure the correct substance is evaporating
Condenser (Liebig condenser)	Cools the vapour so it condenses back into a liquid
Cold water jacket	Surrounds the condenser, providing cooling
Collection beaker	Collects the distillate

Example Uses

• Obtaining pure water from salt water.
• Obtaining pure water from ink.

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4. Fractional Distillation

Fractional distillation separates a mixture of liquids that have different boiling points.

Method

1. Heat the mixture in a flask — the liquid with the lowest boiling point evaporates first.
2. The vapour rises up a fractionating column (packed with glass beads or other material).
3. The fractionating column has a temperature gradient — it is hottest at the bottom and coolest at the top.
4. Vapours that are not hot enough condense on the glass beads and drip back down.
5. Only the vapour with the correct boiling point reaches the top and passes into the condenser.
6. This vapour condenses and is collected.
7. Once the first liquid has been collected, the temperature rises and the next liquid evaporates.

Difference from Simple Distillation

Feature	Simple Distillation	Fractional Distillation
Used for	Solvent from a solution	Separating two or more liquids
Fractionating column	Not used	Used
Number of substances collected	One	Two or more

Example Uses

• Separating ethanol from water (e.g. in alcoholic drinks production).
• Separating crude oil into fractions (covered in organic chemistry).
• Separating liquid air into nitrogen and oxygen.

Exam Tip: The key to fractional distillation is that liquids have different boiling points. The liquid with the lowest boiling point evaporates and is collected first.

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5. Chromatography

Chromatography separates and identifies dissolved substances in a mixture based on how they move through a stationary phase.

Method (Paper Chromatography)

1. Draw a pencil line (the baseline or origin) near the bottom of chromatography paper.
2. Place a spot of the mixture on the line.
3. Stand the paper in a solvent (e.g. water) — the solvent level must be below the pencil line.
4. The solvent rises up the paper by capillary action, carrying the dissolved substances with it.
5. Different substances have different solubilities in the solvent, so they travel different distances.
6. Remove the paper and mark the solvent front with a pencil before it dries.

Key Terms

Term	Definition
Stationary phase	The chromatography paper — it does not move
Mobile phase	The solvent — it moves up the paper
Baseline / Origin	The pencil line where the sample is placed
Solvent front	The furthest point reached by the solvent

Why Pencil?

The baseline is drawn in pencil because graphite is insoluble in the solvent. If pen was used, the ink would dissolve and move up the paper, contaminating the results.

Exam Tip: Always state that the solvent level must be below the baseline — otherwise the spots would dissolve directly into the solvent instead of being carried up the paper.

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Summary

• Filtration — separates an insoluble solid from a liquid (residue and filtrate).
• Evaporation / Crystallisation — obtains a dissolved solid from a solution by gentle heating.
• Simple distillation — separates and collects a solvent from a solution.
• Fractional distillation — separates a mixture of liquids with different boiling points using a fractionating column.
• Chromatography — separates dissolved substances based on how far they travel through a stationary phase.
• The choice of technique depends on the physical properties of the substances in the mixture.

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