Pure Substances and Mixtures

This lesson covers the difference between pure substances and mixtures, including how melting and boiling points can be used to distinguish between them, as required by the AQA GCSE Chemistry specification (8.1.1). Understanding purity in chemistry is essential for chemical analysis — a chemist's definition of "pure" is very different from the everyday use of the word. You need to be able to interpret melting point data and explain how it relates to purity.

What Is a Pure Substance?

In chemistry, a pure substance is one that consists of only one type of element or one type of compound. This is different from everyday language, where "pure" often means "natural" or "uncontaminated."

Term	Everyday Meaning	Chemistry Meaning
Pure	Natural, clean, uncontaminated	Contains only one element or compound
Pure water	Clean drinking water (may contain dissolved minerals)	Water containing only H\u2082O molecules and nothing else
Pure orange juice	Juice with no additives	Would mean only one chemical compound (not possible for real juice)

Examples of Pure Substances

Pure water — contains only H\u2082O molecules
Pure iron — contains only Fe atoms
Pure sodium chloride — contains only NaCl (in the form of Na\u207a and Cl\u207b ions in a lattice)
Pure gold (24 carat) — contains only Au atoms

Exam Tip: If a question asks about "purity," always use the chemistry definition: a pure substance contains only one element or one type of compound. Do not confuse this with the everyday meaning of "clean" or "natural."

What Is a Mixture?

A mixture consists of two or more elements or compounds that are not chemically combined. The substances in a mixture retain their own chemical properties and can be separated by physical methods.

Key Properties of Mixtures

The components are not chemically bonded together.
The components retain their individual properties.
Mixtures can be separated by physical methods such as filtration, distillation, chromatography, and evaporation.
The composition of a mixture is variable — you can have different proportions of each component.

Property	Pure Substance	Mixture
Composition	Fixed	Variable
Melting point	Sharp / fixed	Range (melts over a range of temperatures)
Boiling point	Sharp / fixed	Range (boils over a range of temperatures)
Separation	Cannot be separated by physical methods	Can be separated by physical methods

Exam Tip: In the exam, you might be given data showing a substance melting over a range of temperatures (e.g., 95 \u00b0C to 102 \u00b0C). This indicates the substance is impure (a mixture). A pure substance would have a single, sharp melting point.

Melting and Boiling Points of Pure Substances

A pure substance has a sharp melting point and a sharp boiling point. This means the substance changes state at one specific temperature.

For example:

Pure water melts at exactly 0 \u00b0C and boils at exactly 100 \u00b0C (at standard atmospheric pressure).
Pure ethanol boils at exactly 78.4 \u00b0C.

How Impurities Affect Melting and Boiling Points

Adding an impurity to a pure substance:

Lowers the melting point (the substance starts melting at a lower temperature).
Raises the boiling point (the substance boils at a higher temperature).
Causes the substance to melt or boil over a range of temperatures rather than at a single, fixed temperature.

This is why road salt (sodium chloride) is spread on icy roads in winter — it lowers the melting point of ice, so it melts at temperatures below 0 \u00b0C.

Exam Tip: If asked how to determine whether a substance is pure, explain that you would measure its melting point. If it melts sharply at a specific temperature that matches the known melting point from a data book, it is pure. If it melts over a range, it contains impurities.

Using Melting Point Data to Assess Purity

You may be given melting point data and asked to determine whether a substance is pure. Here is an example:

Sample	Observed Melting Point	Known Melting Point of Pure Substance
A	80.5 \u00b0C (sharp)	80.5 \u00b0C
B	75 \u00b0C to 80 \u00b0C (range)	80.5 \u00b0C
C	80.5 \u00b0C (sharp)	115 \u00b0C

Sample A is likely pure — its melting point is sharp and matches the known value.
Sample B is impure — it melts over a range and starts below the known value.
Sample C is pure but is a different substance — it has a sharp melting point but it does not match the expected value.

Classifying Pure Substances and Mixtures

graph TD
    A["Is it made of only one type of element or compound?"] -->|Yes| B["Pure Substance"]
    A -->|No| C["Mixture"]
    B --> D["Sharp, fixed melting and boiling points"]
    C --> E["Melts and boils over a range of temperatures"]
    C --> F["Can be separated by physical methods"]
    F --> G["Filtration"]
    F --> H["Distillation"]
    F --> I["Chromatography"]
    F --> J["Evaporation"]

    style A fill:#3498db,color:#fff
    style B fill:#27ae60,color:#fff
    style C fill:#e74c3c,color:#fff

Separation Techniques Overview

Different types of mixtures require different separation techniques:

Technique	Used to Separate	Example
Filtration	An insoluble solid from a liquid	Sand from water
Evaporation	A dissolved solid from a solution	Salt from salt water
Simple distillation	A solvent from a solution	Water from salt water
Fractional distillation	Miscible liquids with different boiling points	Ethanol from water
Chromatography	Dissolved substances with different solubilities	Dyes in ink

Common Exam Questions

Explain the difference between a pure substance and a mixture.
- A pure substance contains only one element or compound; a mixture contains two or more elements or compounds not chemically combined.
How can you tell if a sample is pure?
- Measure its melting point; a pure substance has a sharp melting point that matches the known value from a data book.
Why does salt lower the melting point of ice?
- Salt is an impurity; adding an impurity to a pure substance lowers its melting point.

Summary

In chemistry, a pure substance contains only one element or one compound.
A mixture contains two or more substances that are not chemically combined.
Pure substances have sharp, fixed melting and boiling points.
Impurities lower the melting point and raise the boiling point, causing them to occur over a range of temperatures.
Mixtures can be separated by physical methods (filtration, distillation, chromatography, evaporation).
You can assess purity by comparing the observed melting point with the known value from a data book.

Exam Tip: When describing how to test for purity, always mention three things: (1) measure the melting point, (2) compare it with the known value from a data book, and (3) check whether it is a sharp melting point or occurs over a range. This structured answer will gain full marks.