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.

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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."

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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.

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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.

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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.

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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

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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

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Common Exam Questions

1. 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.

2. 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.

3. Why does salt lower the melting point of ice?

   • Salt is an impurity; adding an impurity to a pure substance lowers its melting point.

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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.

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Worked Example: Distinguishing Pure Substances from Mixtures Using a Cooling Curve

A technician is given three unlabelled samples (A, B, C) and asked to classify each as a pure substance or a mixture. She melts each sample, then allows it to cool while recording temperature against time.

Time (s)	Sample A (°C)	Sample B (°C)	Sample C (°C)
0	120	120	120
60	95	102	100
120	80	80	80
180	80	74	80
240	80	68	80
300	65	60	80
360	50	50	65

Analysis. Sample A plateaus briefly at 80 °C before continuing to fall: a short horizontal section typical of a pure substance freezing at a single, sharp temperature. Sample B shows no plateau — the temperature falls continuously through a range of values, the signature of a mixture solidifying as the most-soluble components crystallise first. Sample C holds at 80 °C for a longer time, suggesting it is a pure substance with a higher latent heat of fusion (or a larger mass). The shape of the cooling curve, not simply the final temperature, reveals purity.

Common mistake: Students often confuse a low melting point with impurity. A low melting point is only evidence of impurity when it is lower than the data-book value for that substance. Pure ice melts at 0 °C; pure mercury melts at -39 °C — both are pure despite having very different values.

Formulation vs impurity: a subtle distinction

Scenario	Is it pure?	Is it a mixture?	Does it have a sharp melting point?
Distilled water	Yes (one compound)	No	Yes, 0 °C
Sea water	No	Yes	No — melts across a range
24-carat gold	Yes (one element)	No	Yes, 1064 °C
18-carat gold (alloy)	No	Yes (formulation)	No — melts across a range
Pure paracetamol crystals	Yes (one compound)	No	Yes, 169 °C
Paracetamol tablet	No	Yes (formulation)	No — contains binders and fillers

Answering at different grade levels

Exam-style question (4 marks): A student claims that bottled mineral water is a pure substance because the label says "100 % pure natural spring water". Evaluate this claim using chemical evidence.

Grade 4–5 answer: The student is wrong. Mineral water is not a pure substance because it contains dissolved minerals as well as water, so it is a mixture. Pure water has a sharp melting point of 0 °C.

Grade 8–9 answer: The student has confused the everyday meaning of "pure" (clean, untainted) with the chemistry definition. A pure substance contains only one element or one compound; mineral water contains H₂O together with dissolved Ca²⁺, Mg²⁺, HCO₃⁻ and SO₄²⁻ ions, so it is a mixture (specifically, a formulation, because the mineral composition is controlled). Chemically, this can be demonstrated by measuring the freezing point: pure water freezes sharply at 0 °C, but mineral water will begin to freeze below 0 °C and continue to freeze across a range of temperatures as the ionic concentration of the remaining liquid increases. Evaporation to dryness would also leave a residue of mineral salts, which a pure substance would not.

AQA alignment: This content is aligned with AQA GCSE Chemistry (8462) specification section 5.8 Chemical analysis — specifically 5.8.1.1 Pure substances. Assessed on Paper 2.

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Extension: Why Impurities Change Melting Points

At molecular level, a pure substance freezes into a regular crystal lattice in which every particle is in an ordered position with optimal attractive forces to its neighbours. Breaking that lattice requires the same amount of energy per particle throughout the sample, which is why pure substances melt at a single temperature — all the bonds break at once. When an impurity is introduced, foreign particles disrupt the regular lattice: they do not fit neatly into the crystal structure, so parts of the lattice have weaker attractions and melt at a lower temperature. As melting proceeds, the composition of the remaining solid changes, so the temperature creeps up through a range. Chemists exploit this to identify unknown solids: if an unknown substance is mixed with a known pure reference, and the mixture still melts sharply at the reference value, the two are almost certainly the same compound (a "mixed melting point" experiment).

Exam Tip: The phrase "melts over a range" is often the piece of evidence most closely tied to impurity. If a data table shows two temperatures for the start and end of melting, always describe the gap between them as a range and conclude that the substance is not pure.