AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
Melting point and boiling point data can be used to distinguish pure…
Full specification wording
Pure substances: Melting point and boiling point data can be used to distinguish pure substances from mixtures.
A pure substance in the chemical sense contains only one substance, so it melts and boils at single sharp temperatures.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 3 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A pure substance in the chemical sense contains only one substance, so it melts and boils at single sharp temperatures. An impurity lowers the melting point and raises the boiling point, and it spreads melting and boiling over a range. For example, pure ice melts at 0 °C and pure water boils at 100 °C at normal atmospheric pressure, whereas salty water melts below 0 °C and boils above 100 °C. A mixture such as ethanol and water boils over a range of temperatures rather than at one fixed value. To use data, compare the measured melting or boiling point with the known value for the expected pure substance: a close match to the sharp known value indicates purity, while a lower melting point, a higher boiling point or a wide range indicates an impurity.
What earns the marks
- A pure substance has a fixed, sharp melting point and boiling point at a stated pressure.
- An impurity lowers the melting point and raises the boiling point of a substance.
- An impurity causes melting and boiling to occur over a range of temperatures rather than at one value.
- Comparing measured melting or boiling point data with known values for the pure substance allows a judgement about purity.
- A measured value close to the known sharp value indicates a pure substance; a lower melting point, higher boiling point or wide range indicates an impure substance or mixture.
Common mistakes and how to put them right
Thinking that any liquid that looks clear is pure.
Put it right: Clarity is not evidence of purity, so the melting or boiling point must be compared with the known value.
Assuming an impurity always raises the melting point.
Put it right: An impurity lowers the melting point and raises the boiling point.
Treating a wide melting range as normal for a pure substance.
Put it right: A pure substance melts sharply at one temperature, so a range indicates an impurity.
Exam tips
- Quote the measured value and the known value for the pure substance, then state whether they match closely.
- State the direction of change: impurity lowers melting point and raises boiling point.
- Use the phrase sharp melting point or boiling point when describing a pure substance, and range when describing an impure one.
Try the lesson
The start of the Pure substances: Melting point and boiling point data can be used to distinguish pure substances from mixtures. lesson, made from checked teaching notes for this specification point.
Have a go first
A student heats a sample of a solid. The solid melts sharply at 80 °C. What does this sharp melting point suggest about the sample?
Listen
A pure substance in the chemical sense contains only one substance, so it melts and boils at single sharp temperatures. An impurity lowers the melting point and raises the boiling point, and it spreads melting and boiling over a range. For example, pure ice melts at 0 °C and pure water boils at 100 °C at normal atmospheric pressure, whereas salty water melts below 0 °C and boils above 100 °C. A mixture such as ethanol and water boils over a range of temperatures rather than at one fixed value. To use data, compare the measured melting or boiling point with the known value for the expected pure substance: a close match to the sharp known value indicates purity, while a lower melting point, a higher boiling point or a wide range indicates an impurity. A strong answer earns marks for these points. A pure substance has a fixed, sharp melting point and boiling point at a stated pressure. An impurity lowers the melting point and raises the boiling point of a substance. An impurity causes melting and boiling to occur over a range of temperatures rather than at one value. Comparing measured melting or boiling point data with known values for the pure substance allows a judgement about purity. Exam tip: Quote the measured value and the known value for the pure substance, then state whether they match closely.
See it
Key words
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Purity and impurities
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Melting and boiling point data
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Comparison with known values
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Effect of impurities on physical properties
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See every definition
- Purity and impurities
- A pure substance contains only one substance, while an impurity is something else present that lowers the melting point and raises the boiling point.
- Melting and boiling point data
- Measurements of the temperatures at which a substance melts and boils, which for a pure substance are single sharp temperatures at a stated pressure.
- Comparison with known values
- Checking a measured melting or boiling point against the known value for the expected pure substance, where a close match to the sharp known value indicates purity.
- Effect of impurities on physical properties
- An impurity lowers the melting point and raises the boiling point, and it spreads melting and boiling over a range of temperatures rather than at one value.
Label the diagram
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