A student investigates whether a sample of aspirin is pure. She measures the melting point of the aspirin sample and finds that it melts between 132 °C and 141 °C. The melting point of pure aspirin is 135 °C. Explain what the student's results tell her about the purity of the aspirin sample, and describe how melting point data can be used to identify whether a substance is pure.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student investigates whether a sample of aspirin is pure. She measures the melting point of the aspirin sample and finds that it melts between 132 °C and 141 °C. The melting point of pure aspirin is 135 °C.
Model answer (5 marks)
The sample melts over a range of 132 °C to 141 °C, rather than at a single temperature. A pure substance has a sharp, specific melting point, so this spread shows the sample is not pure – it is a mixture of aspirin and an impurity.
Melting‑point determination is used to assess purity because a pure compound melts at one temperature, whereas a mixture melts over a range. By comparing the measured range with the known melting point of the pure substance (135 °C for aspirin), a student can see whether the sample matches the pure value. If the measured melting point is sharp and equals the known value, the substance is confirmed as pure and correctly identified.
Melting‑point determination is used to assess purity because a pure compound melts at one temperature, whereas a mixture melts over a range. By comparing the measured range with the known melting point of the pure substance (135 °C for aspirin), a student can see whether the sample matches the pure value. If the measured melting point is sharp and equals the known value, the substance is confirmed as pure and correctly identified.
Examiner tips
- Use the term ‘sharp, specific melting point’ for pure substances
- Explain that a range indicates a mixture
- Show the comparison with the known 135 °C value
- State that a sharp match confirms purity
Common mistakes
- Saying the sample is pure because it contains aspirin
- Confusing the melting point range with the melting point of the pure substance
- Not recognising that a range indicates an impurity
Mark scheme (5 marks)
- A pure substance melts at a specific/sharp (single) temperature
- A mixture melts over a range of temperatures
- The aspirin sample is impure / is a mixture (because it melts over a range of temperatures)
- The measured melting point range can be compared to the known melting point of a pure substance to identify it
- If the measured melting point is sharp and matches the known value, the substance is confirmed as pure / correctly identified
Key terms in this question
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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