Mixtures and Separation Techniques

This lesson covers mixtures and the key separation techniques you need to know for AQA GCSE Chemistry specification (5.1.1). A mixture consists of two or more substances that are not chemically bonded together. Because the substances in a mixture are not bonded, they retain their individual properties and can be separated by physical methods.

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What Is a Mixture?

A mixture is made up of two or more elements or compounds that are not chemically combined. The components of a mixture:

• Retain their own chemical properties.
• Can be separated by physical methods (no chemical reaction needed).
• Are not in fixed proportions — the composition can vary.

Feature	Mixture	Compound
Bonding	No chemical bonds between components	Atoms are chemically bonded
Separation	Physical methods (filtration, distillation, etc.)	Chemical reactions only
Properties	Each component keeps its own properties	New properties, different from the elements
Proportion	Variable composition	Fixed ratio of atoms (shown by formula)

Exam Tip: The key difference between a mixture and a compound is bonding. In a mixture, substances are just mixed together — no chemical bonds form. In a compound, different atoms are chemically bonded in fixed proportions. This distinction is frequently tested.

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Types of Mixtures

Mixtures can be classified into several types:

Type	Description	Example
Solution	A solute dissolved in a solvent	Salt dissolved in water
Suspension	Insoluble particles dispersed in a liquid	Sand in water
Alloy	A mixture of two or more metals (or a metal and a non-metal)	Bronze (copper and tin)
Emulsion	Tiny droplets of one liquid dispersed in another	Milk, mayonnaise

Solutions — Key Terms

• Solute — the substance that dissolves (e.g. salt, sugar).
• Solvent — the liquid that does the dissolving (e.g. water).
• Solution — the mixture formed when a solute dissolves in a solvent.
• Saturated solution — a solution in which no more solute can dissolve at that temperature.
• Soluble — a substance that dissolves in a given solvent.
• Insoluble — a substance that does not dissolve in a given solvent.

Exam Tip: Water is the most common solvent but not the only one. Some substances are insoluble in water but soluble in other solvents such as ethanol or propanone. AQA may ask about choosing an appropriate solvent for a chromatography experiment.

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Separation Techniques

You must know the following separation techniques, including when each is used and how it works.

1. Filtration

Filtration separates an insoluble solid from a liquid. The mixture is poured through filter paper in a funnel. The liquid passes through (the filtrate) and the solid remains on the paper (the residue).

Used for: Separating sand from water, removing precipitates from solutions.

2. Evaporation

Evaporation separates a dissolved solid from a solution by heating. The solvent evaporates, leaving the solid behind in the evaporating basin.

Used for: Obtaining salt from salt water.

Limitation: The solid may decompose if heated too strongly. The solvent is lost (not collected).

3. Crystallisation

Crystallisation is used to obtain pure, dry crystals of a soluble solid from a solution. The method involves:

1. Heat the solution gently to evaporate some of the solvent.
2. Test for saturation by dipping a cold glass rod into the solution — crystals should form on the rod.
3. Leave the saturated solution to cool slowly so that crystals form.
4. Filter to collect the crystals, then dry them between sheets of filter paper.

Advantage over evaporation: Crystals are well-formed and pure; the solid does not decompose from excessive heat.

4. Simple Distillation

Simple distillation separates a solvent from a solution. The solution is heated in a flask; the solvent evaporates, travels through a condenser where it cools and condenses back into a liquid, and is collected in a separate container.

Used for: Obtaining pure water from salt water.

Limitation: Cannot effectively separate two liquids with similar boiling points.

5. Fractional Distillation

Fractional distillation separates a mixture of miscible liquids (liquids that mix together) with different boiling points. A fractionating column is used, packed with glass beads to provide a temperature gradient.

How it works:

1. The mixture is heated in a flask.
2. Vapours rise up the fractionating column.
3. The liquid with the lowest boiling point evaporates first and reaches the top of the column.
4. It passes into the condenser, cools, and is collected.
5. The temperature then rises and the next liquid evaporates and is collected.

Used for: Separating ethanol from water, separating crude oil into fractions.

6. Chromatography

Chromatography separates the components of a mixture of soluble substances based on their different solubilities in a particular solvent (the mobile phase). It can also be used to identify unknown substances.

Method:

1. Draw a pencil baseline on chromatography paper (pencil is insoluble so it will not dissolve and move).
2. Place small spots of the substances to be tested on the baseline.
3. Lower the paper into a solvent (the mobile phase), ensuring the baseline is above the solvent level.
4. The solvent travels up the paper by capillary action, carrying the dissolved substances with it.
5. Different substances travel different distances depending on their solubility in the mobile phase.

Interpreting results:

• A pure substance produces a single spot.
• A mixture produces two or more spots.
• Substances can be identified by comparing with known reference spots run alongside.

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Calculating Rf Values

The Rf value (retention factor) is a ratio that helps identify substances in chromatography.

Rf = distance moved by the substance / distance moved by the solvent front

Rule	Detail
Rf is always between 0 and 1	A substance cannot travel further than the solvent
Rf has no units	It is a ratio
Rf is specific to a substance	In a given solvent under given conditions
Measurements from the baseline	Always measure from the pencil line to the centre of the spot

Exam Tip: When calculating Rf values, always measure from the pencil baseline (not the bottom of the paper) to the centre of the spot. Show your working clearly and give your answer to 2 decimal places. Remember Rf has no units.

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Separation Techniques — Decision Diagram

graph TD
    A["Mixture to Separate"] --> B{"Is the solid dissolved?"}
    B -- "No (insoluble)" --> C["Filtration"]
    B -- "Yes (soluble)" --> D{"Do you want the solid or the liquid?"}
    D -- "Solid" --> E{"Need pure crystals?"}
    D -- "Liquid / solvent" --> F["Simple Distillation"]
    E -- "Yes" --> G["Crystallisation"]
    E -- "No" --> H["Evaporation"]
    A --> I{"Two miscible liquids?"}
    I -- "Yes" --> J["Fractional Distillation"]
    A --> K{"Mixture of soluble dyes / inks?"}
    K -- "Yes" --> L["Chromatography"]

    style A fill:#2c3e50,color:#fff
    style C fill:#27ae60,color:#fff
    style F fill:#2980b9,color:#fff
    style G fill:#8e44ad,color:#fff
    style H fill:#e67e22,color:#fff
    style J fill:#e74c3c,color:#fff
    style L fill:#f39c12,color:#fff

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Formulations