Mixtures and Separation Techniques

This lesson covers mixtures and the key separation techniques required by AQA GCSE Combined Science Trilogy (8464, Chemistry 4.1.1). A mixture consists of two or more substances that are not chemically bonded together. Because the substances retain their individual properties, they can be separated by physical methods.

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What Is a Mixture?

A mixture contains two or more substances that are not chemically combined. The components of a mixture:

• Retain their individual properties (unlike compounds).
• Can be separated by physical methods — no chemical reactions are needed.
• Can be present in any proportion — there is no fixed ratio.

Examples of mixtures include air (a mixture of gases), sea water (salt dissolved in water) and rock (a mixture of different minerals).

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Separation Techniques Decision Tree

graph TD
    A["Mixture to Separate"] --> B{"Solid in a liquid?"}
    B -- "Insoluble solid" --> C["Filtration"]
    B -- "Dissolved solid" --> D{"Want the solid or the liquid?"}
    D -- "Want the solid" --> E["Evaporation / Crystallisation"]
    D -- "Want the liquid" --> F["Simple Distillation"]
    A --> G{"Two miscible liquids?"}
    G -- "Yes" --> H["Fractional Distillation"]
    A --> I{"Dissolved coloured substances?"}
    I -- "Yes" --> J["Chromatography"]

    style A fill:#2c3e50,color:#fff
    style C fill:#27ae60,color:#fff
    style E fill:#2980b9,color:#fff
    style F fill:#8e44ad,color:#fff
    style H fill:#e67e22,color:#fff
    style J fill:#c0392b,color:#fff

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Filtration

Filtration separates an insoluble solid from a liquid. The mixture is poured through filter paper in a funnel:

• The residue (insoluble solid) remains on the filter paper.
• The filtrate (liquid) passes through into the collecting vessel.

Example: Separating sand from salty water — the sand stays on the filter paper and the salt solution passes through.

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Evaporation and Crystallisation

Evaporation is used to obtain a dissolved solid from a solution by heating until all the water evaporates.

Crystallisation is a gentler process:

1. Heat the solution to evaporate some of the water.
2. Stop heating when crystals begin to form at the edge of the evaporating basin.
3. Leave to cool slowly — larger, well-formed crystals grow as the solution cools.

Exam Tip: Crystallisation produces better-quality crystals than evaporation because slow cooling allows a regular crystal lattice to form.

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Simple Distillation

Simple distillation separates a solvent from a solution. It recovers the liquid from the mixture:

1. The solution is heated in a flask until the solvent boils and evaporates.
2. The vapour travels into the condenser, where it is cooled by cold water and condenses back to a liquid.
3. The pure liquid (distillate) is collected in a separate container.

Example: Obtaining pure water from salty water.

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Fractional Distillation

Fractional distillation separates a mixture of miscible liquids (liquids that mix) with different boiling points:

1. The mixture is heated in a flask fitted with a fractionating column.
2. The liquid with the lowest boiling point evaporates first and rises up the column.
3. The column has a temperature gradient — cooler at the top, hotter at the bottom.
4. Vapour condenses and re-evaporates multiple times in the column, improving separation.
5. Each liquid is collected at the top of the column at its boiling point.

Example: Separating ethanol (bp 78 °C) from water (bp 100 °C).

Exam Tip (AQA 8464): You must explain why fractional distillation works — it relies on the different boiling points of the liquids. The fractionating column provides a surface for repeated condensation and evaporation.

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Chromatography

Paper chromatography separates mixtures of dissolved substances (often dyes or colourings). It works because different substances have different solubilities in the solvent (mobile phase) and different attractions to the paper (stationary phase).

Method

1. Draw a pencil baseline near the bottom of the chromatography paper (pencil because it is insoluble and will not dissolve).
2. Place a spot of the mixture on the baseline.
3. Place the paper in a solvent (the mobile phase) so the solvent level is below the baseline.
4. As the solvent rises through the paper, different substances travel different distances.
5. Remove the paper and mark the solvent front before it dries.

Rf Values

The Rf value identifies a substance:

$R_f = \frac{\text{distance moved by substance}}{\text{distance moved by solvent front}}$Rf​=distance moved by solvent frontdistance moved by substance​

• Rf values are always between 0 and 1.
• Each substance has a unique Rf value in a given solvent and conditions.
• A single spot on a chromatogram means the substance is pure; multiple spots mean it is a mixture.

Worked Example

A dye travels 3.6 cm and the solvent front travels 9.0 cm:

$R_f = \frac{3.6}{9.0} = 0.40$Rf​=9.03.6​=0.40

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Pure Substances and Formulations

In chemistry, a pure substance contains only one type of element or compound. A pure substance has:

• A sharp, specific melting point.
• A sharp, specific boiling point.

An impure substance (a mixture) melts and boils over a range of temperatures.

A formulation is a mixture that has been designed as a useful product with specific proportions of components. Examples include medicines, paints, cleaning products and fuels.

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Common Mistakes

Mistake	Correction
Drawing the baseline in pen	Use pencil — ink dissolves in the solvent and ruins the chromatogram
Saying Rf has units	Rf is a ratio — it has no units
Confusing filtration and evaporation	Filtration removes insoluble solids; evaporation recovers dissolved solids
Forgetting that "pure" in everyday language differs from "pure" in chemistry	In chemistry, "pure" means only one substance — "pure orange juice" is actually a mixture

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Summary

• A mixture contains two or more substances not chemically bonded together.
• Filtration — separates an insoluble solid from a liquid.
• Evaporation / Crystallisation — obtains a dissolved solid from a solution.
• Simple distillation — recovers the solvent from a solution.
• Fractional distillation — separates miscible liquids with different boiling points.
• Chromatography — separates and identifies dissolved substances using $R_f$Rf​ values.
• A pure substance has a single, sharp melting and boiling point.
• A formulation is a designed mixture with precise proportions.

Exam Tip (AQA 8464): If asked to describe a separation technique, state the technique, explain the step-by-step procedure, and explain why it works (link to the physical properties of the substances being separated).

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