Pure substances — AQA GCSE Combined Science
Test yourself on Pure substances with AQA GCSE practice questions.
7 days Premium · Then free forever · No card, no charge
Pure substances explained
In everyday language, 'pure' often means clean or natural, but in chemistry it has a precise meaning: a pure substance consists of only one element or one compound, with nothing else mixed in.
Read the full explanation
A pure element contains only atoms of that element, such as a block of copper or a sample of oxygen. A pure compound contains only one compound, such as distilled water (H₂O) or pure sodium chloride. Importantly, a pure substance can contain more than one type of atom, provided those atoms are chemically bonded in a fixed ratio as a single compound. If two or more different elements or compounds are present but not chemically bonded together, the sample is a mixture, even if it looks uniform, such as salty water or air.
Pure elements and compounds melt and boil at specific temperatures. Melting point and boiling point data can be used to distinguish pure substances from mixtures.
Every pure element and pure compound has a sharp, fixed melting point and boiling point at a given pressure. For example, pure water melts at 0 °C and boils at 100 °C at standard atmospheric pressure, while pure iron melts at about 1538 °C. A mixture usually melts and boils over a range of temperatures rather than at one exact value, because its components have different melting and boiling points. This means melting point and boiling point data can be used to test purity: a sample melting sharply at the expected temperature is likely pure, whereas a sample melting over a range, or at a lower temperature than expected, is likely impure or a mixture. Impurities generally lower the melting point and raise the boiling point of a substance.
In everyday language, a pure substance can mean a substance that has had nothing added to it, so it is unadulterated and in its natural state, eg pure milk.
In everyday language, 'pure' describes something with nothing added, unadulterated and in its natural state, such as pure milk or pure orange juice. This everyday meaning is about absence of additives, not chemical composition. Chemists use 'pure' differently: a pure substance contains only one element or compound, with no other substances mixed in. So pure milk is not chemically pure because it is a mixture of water, fats, proteins, sugars and minerals. A bottle labelled 'pure water' may still contain dissolved ions, making it a mixture in chemical terms. To decide which meaning applies, ask what is being claimed: if the claim is about no additives or natural state, it is the everyday meaning; if it is about a single substance with a sharp melting point, it is the chemical meaning.
Students should be able to use melting point and boiling point data to distinguish pure from impure substances.
A pure substance melts at a specific temperature and boils at a specific temperature. Impurities lower the melting point and raise the boiling point, and they also spread the melting and boiling ranges over several degrees. To distinguish pure from impure, compare the measured melting point or boiling point with the known value for that substance. A pure sample melts sharply at the known temperature, for example pure ice melts at 0 °C. An impure sample melts over a range, starting below the known value, for example salty ice may begin melting at −5 °C and finish at 0 °C. Similarly, pure water boils at 100 °C, while salty water boils above 100 °C. A sharp melting point close to the known value indicates purity; a lower, broader melting range indicates impurities.
Your focus
- Define a pure substance as a single element or compound not mixed with any other substance.
- Classify given examples as pure elements, pure compounds or mixtures.
- Explain why a compound such as water is pure even though it contains two elements.
Show all 12 objectives
- Describe how melting point and boiling point data can be used to identify pure substances.
- Interpret temperature data to decide whether a sample is pure or a mixture.
- Explain how impurities affect the melting point and boiling point of a substance.
- State the everyday meaning of a pure substance as unadulterated and in its natural state.
- Give an example such as pure milk and explain why it is not chemically pure.
- Distinguish between everyday and chemical meanings of pure in a given context.
- Describe how melting point and boiling point data can distinguish pure from impure substances.
- Explain that impurities lower the melting point and raise the boiling point and cause a range.
- Apply known melting or boiling point values to judge whether a sample is pure.
Pure substances exam tips
Marking Points
- States that a pure substance contains only one element or only one compound.
- Recognises that a pure compound may contain different types of atom chemically bonded together, for example H₂O or CO₂.
- Distinguishes a pure substance from a mixture, where two or more substances are present but not chemically combined.
- Applies the definition to unfamiliar examples, such as classifying pure iron as an element and pure ethanol as a compound.
- Explains that a mixture can contain a pure element or compound alongside other substances, so the mixture itself is not pure.
- States that pure elements and compounds have specific, sharp melting and boiling points.
- Explains that mixtures melt and boil over a range of temperatures rather than at one fixed value.
- Uses given melting point or boiling point data to decide whether a sample is pure or a mixture.
- Explains that impurities lower the melting point and raise the boiling point of a substance.
- Compares a measured melting or boiling point with a known value to judge purity.
- Everyday meaning: a substance that has had nothing added to it, so it is unadulterated and in its natural state.
- Example: pure milk means milk with no additives, not a single chemical substance.
- Chemical meaning: a pure substance contains only one element or compound, with no other substances mixed in.
- Pure milk is a mixture of water, fats, proteins, sugars and minerals, so it is not chemically pure.
- The context of the claim determines which meaning of 'pure' is intended.
- Pure substances have a specific sharp melting point and boiling point.
- Impurities lower the melting point and raise the boiling point.
- Impurities also cause melting and boiling to occur over a range of temperatures rather than sharply.
- Compare measured values with known values for the substance to judge purity.
- A sharp melting point at the known value indicates a pure substance; a lower, broader range indicates impurities.
Examiner Tips
- 💡Read the question carefully to see whether it asks about a pure element, a pure compound or a mixture before answering.
- 💡When justifying purity, name the single substance present and state that no other substance is mixed with it.
- 💡Use particle diagrams or formulae to support your answer, for example showing H₂O as one compound rather than a mixture of hydrogen and oxygen.
- 💡Quote the actual temperature values from the data when explaining whether a sample is pure.
- 💡Use the phrase 'sharp melting point' or 'melts over a range' to show you understand the difference between pure substances and mixtures.
- 💡If comparing two samples, state which has the sharper or more expected value and link this to purity.
- 💡Read the context of the word 'pure' before deciding which meaning is intended.
- 💡Use the milk example to show you understand the everyday meaning is about no additives.
- 💡Contrast everyday and chemical meanings explicitly in your answer to show understanding.
- 💡Quote the known melting or boiling point and compare it with the measured value.
- 💡Mention both the value and the sharpness or range when judging purity.
- 💡Use the words 'lower' and 'broader' for impure melting behaviour to gain credit.
Common Mistakes
- Thinking 'pure' means clean, safe or natural; correction: in chemistry it means only one element or compound is present.
- Believing a pure substance must contain only one type of atom; correction: a pure compound contains different atoms bonded in a fixed ratio.
- Classifying air or salty water as pure because they look clear; correction: they are mixtures of two or more substances.
- Assuming any liquid that boils is pure; correction: a mixture boils over a range of temperatures, not at one sharp value.
- Thinking impurities raise the melting point; correction: impurities usually lower the melting point and widen the melting range.
- Ignoring the effect of pressure; correction: melting and boiling points are quoted at a stated pressure, usually standard atmospheric pressure.
- Assuming 'pure' always means chemically pure; correction: in everyday language it can mean unadulterated or natural, as with pure milk.
- Thinking pure milk is a single substance; correction: milk is a mixture of water, fats, proteins, sugars and minerals.
- Treating the two meanings as interchangeable; correction: identify whether the claim is about additives or about chemical composition.
- Thinking impurities raise the melting point; correction: impurities lower the melting point and raise the boiling point.
- Ignoring the range and using only the start or end temperature; correction: a pure substance melts sharply at one temperature, while an impure one melts over a range.
- Assuming any melting point below the known value means a different substance; correction: a lower, broader melting range indicates impurities in the same substance.