Revision notes for AQA GCSE Chemistry Pure substances. Open the guide for explanations and worked examples. Written against the AQA GCSE Chemistry (8462) specification, so the content matches what's examinable rather than general Chemistry background.
Pure substances
What you'll learn
- What pure substance means in chemistry, compared with everyday language.
- How elements, compounds and mixtures fit into the idea of purity.
- Why pure substances have specific melting points and boiling points.
- How to use melting point and boiling point data to decide whether a sample is pure or impure.
The key idea of purity in chemistry
In chemistry, we use the word pure in a very precise way. It does not just mean “natural”, “healthy”, “clean” or “good quality”.
A material is chemically pure only if it contains one substance and nothing else.
Definition
Pure substance
In chemistry, a pure substance is a single element or a single compound, not mixed with any other substance.
So:
- pure copper is a pure substance because it contains only copper atoms
- pure water is a pure substance because it contains only water molecules, H₂O
- salty water is not pure because it contains water and dissolved salt
- air is not pure because it is a mixture of gases
Elements, compounds and mixtures
Before you can judge purity, you need to be clear about three basic words.
Element
An element is a substance made from only one type of atom. For example, oxygen, copper and carbon are elements.
Compound
A compound is made from two or more different elements chemically joined together. For example, water is a compound made from hydrogen and oxygen.
Mixture
A mixture contains two or more substances that are not chemically joined together. The substances are just mixed.
For example, seawater is a mixture because it contains water, salts and other dissolved substances.
Key Idea
Pure does not mean simple
A pure substance can be an element or a compound. A compound can still be pure if it is the only substance present.
Everyday meaning vs chemistry meaning
In everyday language, “pure” can mean that nothing has been added to something, or that it is in its natural state.
For example, “pure milk” might mean milk with no added water or preservatives. But in chemistry, milk is not a pure substance. It contains water, fats, proteins, sugars, minerals and other substances, so it is a mixture.
Common Mistake
Pure milk is not chemically pure
In chemistry, “pure milk” is still a mixture because it contains many different substances. Everyday purity and chemical purity are not the same thing.
Melting point
The melting point is the temperature at which a solid changes into a liquid.
For a pure substance, melting happens at a specific temperature. This is often called a sharp melting point.
Definition
Melting point
The melting point of a substance is the temperature at which it changes from solid to liquid.
For example, pure water melts at 0 °C under normal laboratory conditions.
If a solid is impure, it usually melts over a range of temperatures instead of at one sharp temperature. For example, it may start melting at 58 °C and finish melting at 65 °C.
That range is evidence that the sample is a mixture.
Boiling point
The boiling point is the temperature at which a liquid changes into a gas throughout the liquid.
For a pure substance, boiling happens at a specific temperature. For example, pure water boils at 100 °C under normal laboratory pressure.
Definition
Boiling point
The boiling point of a substance is the temperature at which it changes from liquid to gas throughout the liquid.
A mixture usually boils over a range of temperatures because the substances in the mixture may have different boiling points.
The diagram below shows the important difference: pure substances have flat sections at fixed temperatures, while mixtures change state over a range.
Key Idea
Sharp point or range?
A pure substance melts and boils at a specific temperature. A mixture usually melts or boils over a range of temperatures.
Why mixtures melt and boil over a range
A pure substance contains only one type of particle or one chemical substance. Its particles have the same structure and the same forces between them, so the change of state happens at a fixed temperature.
A mixture contains more than one substance. Different substances may have different melting points and boiling points. They can also interfere with each other, so the sample does not change state neatly at one temperature.
This is why melting point and boiling point data can be used to test purity.
Tip
Quick exam shortcut
If the data says a sample melts or boils over a range, it is very likely to be impure or a mixture.
Using melting point data to test purity
When using melting point data, look for two things:
- Does the sample melt at one sharp temperature?
- Does that temperature match the known melting point of the substance?
If the answer to both is yes, the sample is likely to be pure.
If the sample melts over a range, it is impure.
Example
Deciding whether a solid is pure
A student tests a solid. It starts to melt at 48 °C and is fully melted at 55 °C. A data book says the pure substance melts at 52 °C. Decide whether the sample is pure.
- Compare the observed melting behaviour with the expected behaviour for a pure substance. A pure substance should melt sharply at 52 °C, not across several degrees.
- The sample melts from 48 °C to 55 °C, so it melts over a range of temperatures.
- Therefore, the sample is impure. It may contain the named substance, but it also contains at least one other substance.
Using boiling point data to identify a substance
Boiling point data can also help you identify a pure liquid.
Suppose a liquid boils sharply at 78 °C. You compare this with data for known substances:
| Substance | Boiling point |
|---|---|
| Ethanol | 78 °C |
| Water | 100 °C |
| Propanone | 56 °C |
A sharp boiling point of 78 °C suggests the liquid is pure ethanol.
Example
Identifying a liquid from boiling point data
A colourless liquid boils sharply at 100 °C. The data book gives these boiling points: ethanol 78 °C, water 100 °C and propanone 56 °C. Identify the liquid.
- Check whether the liquid boils sharply or over a range. It boils sharply, so it could be a pure substance.
- Compare the measured boiling point, 100 °C, with the data book values.
- The value matches water, so the liquid is most likely pure water.
Be careful with pressure
Boiling point depends on pressure. The standard boiling point of water is 100 °C at normal atmospheric pressure. If the pressure is different, the boiling point can change.
At GCSE, questions normally give you the data you need, or they assume normal laboratory conditions.
Common Mistake
Compare like with like
When comparing boiling points, make sure the values are measured under the same conditions. In GCSE questions, use the data given in the question unless told otherwise.
What “impure” means
An impure substance is not necessarily dirty or dangerous. It just means there is more than one substance present.
For example:
- copper mixed with zinc forms brass, which is useful but not chemically pure copper
- water with dissolved salt is impure water
- air is a useful mixture, but not a pure substance
Definition
Impure substance
An impure substance contains more than one substance, so it is a mixture.
How this links to the bigger topic
This section belongs to Purity, formulations and chromatography.
Later, you will meet:
- formulations, which are useful mixtures made with specific amounts of substances
- chromatography, which separates and identifies substances in mixtures
For now, the big skill is recognising that melting point and boiling point data can show whether something is pure or mixed.
Exam technique
In the exam
- Use the chemistry definition of pure: one element or one compound only.
- If a sample melts or boils over a range of temperatures, describe it as impure or a mixture.
- If a sample has a sharp melting point or boiling point that matches data, say it is likely to be that pure substance.
Self review
Check yourself
- Why is pure water a pure substance, but seawater is not?
- A solid melts from 121 °C to 128 °C. What does that suggest about its purity?
- Why is “pure orange juice” not chemically pure?
Purity, formulations and chromatography
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