AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
In chemistry, a pure substance is a single element or compound, not mixed…
Full specification wording
Pure substances: In chemistry, a pure substance is a single element or compound, not mixed with any other substance.
In everyday language, 'pure' often means clean or natural, but in chemistry it has a precise meaning: a pure substance is a single element or a single compound, with no other substances mixed in.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 1 of 5
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
In everyday language, 'pure' often means clean or natural, but in chemistry it has a precise meaning: a pure substance is a single element or a single compound, with no other substances mixed in. A pure element contains only atoms of that element, such as pure copper or pure oxygen. A pure compound contains only one compound, such as pure water (H₂O) or pure carbon dioxide (CO₂). If two or more different elements or compounds are present together and not chemically combined, the material is a mixture, even if it looks uniform. For example, air is a mixture of nitrogen, oxygen, argon and carbon dioxide, so it is not a pure substance. Similarly, salty water is a mixture of water and sodium chloride. The key test is whether more than one substance is present.
What earns the marks
- A pure substance in chemistry is a single element or a single compound.
- A pure substance is not mixed with any other substance.
- A mixture contains two or more different elements or compounds that are not chemically combined.
- Air and salty water are examples of mixtures, not pure substances.
- The chemical definition of pure differs from the everyday meaning of clean or natural.
Common mistakes and how to put them right
Using the everyday meaning of pure to mean clean or natural.
Put it right: In chemistry, pure means a single element or compound only.
Thinking that a clear or colourless liquid is automatically pure.
Put it right: Clarity does not show purity.
Classifying air as a pure substance because it looks like one substance.
Put it right: Air is a mixture of gases.
Exam tips
- Define pure substance using the words 'single element or compound' and 'not mixed with any other substance'.
- When deciding if something is pure, list the substances present; if there is more than one, it is a mixture.
- Use examples such as pure water versus salty water to show the difference between a pure substance and a mixture.
Try the lesson
The start of the Pure substances: In chemistry, a pure substance is a single element or compound, not mixed with any other substance. lesson, made from checked teaching notes for this specification point.
Have a go first
A sealed bottle contains only distilled water, H₂O, with nothing else added. In chemistry, how should the distilled water be described?
Listen
In everyday language, 'pure' often means clean or natural, but in chemistry it has a precise meaning: a pure substance is a single element or a single compound, with no other substances mixed in. A pure element contains only atoms of that element, such as pure copper or pure oxygen. A pure compound contains only one compound, such as pure water (H₂O) or pure carbon dioxide (CO₂). If two or more different elements or compounds are present together and not chemically combined, the material is a mixture, even if it looks uniform. For example, air is a mixture of nitrogen, oxygen, argon and carbon dioxide, so it is not a pure substance. Similarly, salty water is a mixture of water and sodium chloride. The key test is whether more than one substance is present. A strong answer earns marks for these points. A pure substance in chemistry is a single element or a single compound. A pure substance is not mixed with any other substance. A mixture contains two or more different elements or compounds that are not chemically combined. Air and salty water are examples of mixtures, not pure substances. Exam tip: Define pure substance using the words 'single element or compound' and 'not mixed with any other substance'.
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Key words
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Pure substance
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Mixture
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Everyday meaning of pure
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See every definition
- Pure substance
- A single element or compound, not mixed with any other substance.
- Mixture
- Two or more different elements or compounds present together and not chemically combined, even if it looks uniform.
- Everyday meaning of pure
- Often means clean or natural, which differs from the chemical definition of pure.
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