A technician is testing two samples of aspirin — one freshly prepared in a laboratory and one that has been stored for several years. Explain how melting point data could be used to determine which sample is purer, and what the results would indicate about the stored sample.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Aspirin is a white crystalline solid. A pure substance has a sharp, defined melting point. Impure substances melt over a range of temperatures lower than the expected value.
Model answer (4 marks)
A pure sample of aspirin will melt at a single, sharp temperature (≈ 135 °C). If the sample contains impurities it will melt over a temperature range, for example 130–135 °C, and the onset will be lower. Therefore, by measuring the melting point of the two samples, the one that shows a sharp, single‑point melting temperature is the purer sample. The stored sample will melt at a lower temperature and over a broader range, indicating it has become impure during storage.
Examiner tips
- Show the definition of a sharp melting point for a pure substance. Show that an impurity causes a broadened, lower melting range. State which sample is purer and why. Explain the implication for the stored sample.
Common mistakes
- Confusing melting point depression with boiling point. Failing to mention the range of temperatures for the impure sample. Not linking the lower melting point to the presence of impurities.
Mark scheme (4 marks)
- A pure substance has a sharp melting point (melts at one specific temperature)
- An impure substance melts over a range of temperatures (rather than at a single point)
- The stored aspirin sample would have a lower melting point than the pure/fresh sample
- This indicates the stored sample contains impurities / is less pure than the fresh sample
Key terms in this question
Related
- All OCR GCSE Chemistry A: Gateway Science (J248) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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