A student wants to find out whether a sample of aspirin tablets is pure. She measures the melting point of the aspirin powder and finds that it melts over a range of temperatures rather than at a single sharp temperature. Explain what this result tells the student about the aspirin sample, and describe one further test she could carry out to find out how many substances are present in the sample.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The melting point of a pure substance is a single, sharp temperature. The fact that the aspirin powder melts over a range of temperatures shows that the sample is not pure – it is a mixture of aspirin and one or more impurities.
To determine how many different substances are present, the student could perform paper chromatography. A pure compound will give only one spot on the chromatogram, whereas a mixture will give two or more spots, indicating the number of components.
To determine how many different substances are present, the student could perform paper chromatography. A pure compound will give only one spot on the chromatogram, whereas a mixture will give two or more spots, indicating the number of components.
Examiner tips
- Mention the sharp melting point of a pure substance and the range indicates impurity; state that paper chromatography can separate components and count spots; keep answer concise and use exam terminology.
Common mistakes
- Saying the sample is ‘impure’ without linking it to the melting point range; confusing melting point with boiling point; not specifying that paper chromatography shows multiple spots for a mixture.
Mark scheme (4 marks)
- Melting over a range of temperatures shows the aspirin sample is not pure / is a mixture
- A pure substance melts at a single/sharp specific temperature (so the range confirms impurity)
- She could carry out paper chromatography on the sample
- A pure compound produces a single spot on the chromatogram, whereas a mixture produces more than one spot, showing how many substances are present
Key terms in this question
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