AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
Students should be able to use melting point and boiling point data to…
Full specification wording
Pure substances: Students should be able to use melting point and boiling point data to distinguish pure from impure substances.
To distinguish a pure substance from an impure one, compare its measured melting point and boiling point with the known values for the pure substance at the same pressure.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 5 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
To distinguish a pure substance from an impure one, compare its measured melting point and boiling point with the known values for the pure substance at the same pressure. A pure substance melts and boils sharply at those known values. An impure substance melts below the known melting point and boils above the known boiling point, and both changes happen over a range of temperatures.
For example: Pure water melts at 0 °C and boils at 100 °C at normal atmospheric pressure; adding salt lowers the melting point below 0 °C and raises the boiling point above 100 °C. In an answer, quote the measured value, quote the known value, state the direction of any difference and conclude whether the sample is pure or impure.
What earns the marks
- Compare the measured melting point or boiling point with the known value for the pure substance.
- A measured value equal or very close to the known value indicates a pure substance.
- A melting point below the known value indicates an impurity.
- A boiling point above the known value indicates an impurity.
- A melting or boiling range rather than a sharp value indicates an impure substance or mixture.
Common mistakes and how to put them right
Judging purity from appearance alone.
Put it right: Appearance is unreliable, so melting or boiling point data must be compared with known values.
Saying an impurity raises the melting point.
Put it right: An impurity lowers the melting point and raises the boiling point.
Ignoring the width of the range.
Put it right: A pure substance changes state sharply, so a wide range indicates an impurity.
Exam tips
- Write both the measured value and the known value before drawing a conclusion.
- State the direction of the difference, for example lower than the known melting point.
- Finish with a clear conclusion: pure or impure, with the reason.
Try the lesson
The start of the Pure substances: Students should be able to use melting point and boiling point data to distinguish pure from impure substances. lesson, made from checked teaching notes for this specification point.
Have a go first
A solid sample melts sharply at 80 °C. What does this sharp melting point show about the sample?
Listen
To distinguish a pure substance from an impure one, compare its measured melting point and boiling point with the known values for the pure substance at the same pressure. A pure substance melts and boils sharply at those known values. An impure substance melts below the known melting point and boils above the known boiling point, and both changes happen over a range of temperatures. For example, pure water melts at 0 °C and boils at 100 °C at normal atmospheric pressure; adding salt lowers the melting point below 0 °C and raises the boiling point above 100 °C. In an answer, quote the measured value, quote the known value, state the direction of any difference and conclude whether the sample is pure or impure. A strong answer earns marks for these points. Compare the measured melting point or boiling point with the known value for the pure substance. A measured value equal or very close to the known value indicates a pure substance. A melting point below the known value indicates an impurity. A boiling point above the known value indicates an impurity. Exam tip: Write both the measured value and the known value before drawing a conclusion.
See it
Key words
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Comparing measured and known values
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Melting point depression
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Boiling point elevation
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Evidence-based conclusions
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See every definition
- Comparing measured and known values
- A measured melting point or boiling point is set against the known value for the pure substance at the same pressure.
- Melting point depression
- An impure substance melts below the known melting point, and the change happens over a range of temperatures.
- Boiling point elevation
- An impure substance boils above the known boiling point, and the change happens over a range of temperatures.
- Evidence-based conclusions
- The measured value and the known value are quoted, the direction of any difference is stated, and the sample is concluded to be pure or impure.
Label the diagram
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