Mixtures and Pure Substances

This lesson covers mixtures, pure substances and the key differences between them, as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand what makes a substance pure in chemistry, how mixtures differ from compounds, and how to use melting and boiling points to test for purity.

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Pure Substances in Chemistry

In everyday language, "pure" often means "nothing added" (e.g. "pure orange juice"). In chemistry, the word pure has a very specific meaning:

A pure substance is one that consists of only a single element or a single compound. It is not mixed with anything else.

Examples of Pure Substances

Substance	Type	Why It Is Pure
Distilled water	Compound (H₂O)	Contains only water molecules
Pure iron	Element (Fe)	Contains only iron atoms
Pure sodium chloride	Compound (NaCl)	Contains only sodium chloride

Testing for Purity — Melting and Boiling Points

A pure substance has a sharp, fixed melting point and a sharp, fixed boiling point.

• Pure water melts at exactly 0 °C and boils at exactly 100 °C (at standard atmospheric pressure).
• If a substance is impure (a mixture), its melting point will be lower than expected and its boiling point will be higher than expected. The substance will also melt and boil over a range of temperatures rather than at a single fixed temperature.

Exam Tip: In the exam, if you are given melting or boiling point data and asked whether a substance is pure, look for two things: (1) Does it match the known value for the substance? (2) Does it melt/boil at a sharp, fixed temperature or over a range? A range indicates a mixture.

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What Is a Mixture?

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

• Retain their own individual properties.
• Can be present in any proportion (the ratio is not fixed).
• Can be separated by physical methods (not chemical reactions).

Common Examples of Mixtures

Mixture	Components
Air	Nitrogen, oxygen, argon, carbon dioxide, water vapour and other gases
Sea water	Water, sodium chloride and many other dissolved salts
Rock	Different minerals mixed together
Alloys	Two or more metals (or a metal and a non-metal) mixed together, e.g. bronze = copper + tin
Crude oil	A mixture of many different hydrocarbons

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Comparing Mixtures and Compounds

Feature	Compound	Mixture
Chemical bonds	Elements are chemically bonded	Substances are NOT chemically bonded
Fixed ratio	Always a fixed ratio of atoms	Can be in any proportion
Properties	Different from the constituent elements	Each substance retains its own properties
Separation	Only by chemical reactions	By physical methods
Melting/boiling point	Sharp, fixed values	Melts/boils over a range

Exam Tip: A classic exam question asks you to explain the difference between a compound and a mixture. Always mention that in a compound the elements are chemically bonded in fixed proportions, whereas in a mixture the substances are not chemically bonded and can vary in proportion.

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Methods of Separating Mixtures

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

Overview of Separation Techniques

Method	Used to Separate	Principle
Filtration	Insoluble solid from a liquid	Solid particles are too large to pass through the filter paper
Evaporation	Dissolved solid from a solution	Solvent evaporates, leaving the solid behind
Simple distillation	Solvent from a solution	Solvent evaporates and is then condensed and collected
Fractional distillation	Liquids with different boiling points	Liquids evaporate at different temperatures
Chromatography	Dissolved substances that move at different rates	Components travel different distances through the stationary phase

Filtration

Used to separate an insoluble solid from a liquid. For example, separating sand from water.

1. Pour the mixture through filter paper in a funnel.
2. The liquid passes through — this is the filtrate.
3. The solid stays on the filter paper — this is the residue.

Evaporation and Crystallisation

Used to obtain a dissolved solid from a solution. For example, obtaining salt from salt water.

1. Gently heat the solution so the water evaporates.
2. Stop heating when crystals start to form at the edge.
3. Leave the solution to cool — crystals form as the remaining water evaporates slowly.

Simple Distillation

Used to separate a solvent from a solution, collecting the solvent. For example, obtaining pure water from salt water.

1. Heat the solution — the solvent evaporates first.
2. The vapour travels into a condenser where it is cooled.
3. The condensed liquid (the distillate) is collected in a separate container.

Chromatography

Used to separate and identify dissolved substances in a mixture, such as the dyes in an ink.

1. Draw a pencil line near the bottom of chromatography paper (pencil is insoluble so it will not dissolve).
2. Place a spot of the mixture on the line.
3. Stand the paper in a solvent (the solvent level must be below the pencil line).
4. The solvent rises up the paper, carrying the dissolved substances with it.
5. Different substances travel different distances — more soluble substances travel further.

graph TD
    A["Mixture"] --> B{"Is one component<br/>an insoluble solid?"}
    B -->|Yes| C["Filtration"]
    B -->|No| D{"Do you want<br/>the dissolved solid?"}
    D -->|Yes| E["Evaporation /<br/>Crystallisation"]
    D -->|No| F{"Do you want<br/>the solvent?"}
    F -->|Yes| G["Simple Distillation"]
    F -->|No| H{"Are there liquids with<br/>different boiling points?"}
    H -->|Yes| I["Fractional<br/>Distillation"]
    H -->|No| 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:#2980b9,color:#fff
    style G fill:#27ae60,color:#fff
    style H fill:#2980b9,color:#fff
    style I fill:#27ae60,color:#fff
    style J fill:#27ae60,color:#fff

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Interpreting Melting and Boiling Point Data

Pure Substance vs Mixture — Heating Curves

When you heat a pure substance, the temperature stays constant during a change of state (e.g. at the melting point or boiling point). On a heating curve, you see a flat plateau.

When you heat a mixture, the temperature rises gradually during the change of state. There is no sharp plateau — instead the line curves gently.