A scientist is testing two samples of a soluble white solid. Sample A is known to be pure aspirin. Sample B is suspected to be impure. Explain how the scientist could use melting point data to determine whether Sample B is pure or impure, and describe what results would be expected for each sample.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Aspirin is a medicine that can be produced in a laboratory. A pure substance has a sharp, specific melting point, whereas a mixture melts over a range of temperatures.
Model answer (5 marks)
A scientist can determine purity by measuring the melting point of each sample. A pure substance has a sharp, specific melting point, so Sample A (pure aspirin) will melt at a single, sharp temperature equal to the known melting point of aspirin. If Sample B is impure it will melt over a range of temperatures, usually starting at a lower temperature than the pure substance and ending near the pure melting point. By comparing the melting point range of Sample B with the sharp melting point of Sample A, the scientist can confirm whether Sample B is pure or contains impurities.
Examiner tips
- Measure the melting point of both samples and note any range; a sharp point indicates purity.
- Compare Sample B’s range to the known melting point of pure aspirin to identify impurities.
- Use a calibrated melting point apparatus and record temperatures accurately.
- Mention that impurities lower the onset temperature and broaden the range.
Common mistakes
- Failing to recognise that a sharp melting point indicates purity.
- Confusing the melting point of the pure substance with the onset temperature of the impure sample.
- Not comparing Sample B’s data to the known value for pure aspirin.
Mark scheme (5 marks)
- A pure substance melts at a specific/sharp temperature (not a range)
- Sample A (pure aspirin) would melt at a single, sharp temperature
- A mixture / impure substance melts over a range of temperatures
- Sample B (if impure) would melt over a range of temperatures (lower than the true melting point of pure aspirin)
- The melting point of Sample B can be compared to the known melting point of pure aspirin (Sample A) to confirm purity
Key terms in this question
Related
- All Eduqas A-Level Chemistry revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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