A student is given two white powder samples. One is pure aspirin and the other is an aspirin tablet that has been crushed. Explain how the student could use melting point data to determine which sample is the pure substance, and what results they would expect for each sample.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Heat both samples in a melting point apparatus and record the temperatures at which each begins to melt and at which it is completely liquid.
1. The pure aspirin will show a single, sharp melting point – a narrow temperature range (e.g. 135–136 °C).
2. The crushed tablet, which contains excipients and other additives, will melt over a broader temperature range (e.g. 130–140 °C) and the onset will be less distinct.
3. The sample that displays the sharp, single melting point is therefore the pure aspirin; the sample with the broad, multi‑temperature melt is the tablet mixture.
1. The pure aspirin will show a single, sharp melting point – a narrow temperature range (e.g. 135–136 °C).
2. The crushed tablet, which contains excipients and other additives, will melt over a broader temperature range (e.g. 130–140 °C) and the onset will be less distinct.
3. The sample that displays the sharp, single melting point is therefore the pure aspirin; the sample with the broad, multi‑temperature melt is the tablet mixture.
Examiner tips
- Show the two temperatures (start and finish) for each sample; use the word ‘sharp’ for the pure substance and ‘range’ for the impure one.
- Explain that a pure compound has only one component, so its melting point is a single value; a mixture shows a range due to different components melting at different temperatures.
Common mistakes
- Failing to record both the onset and completion temperatures; only giving one number. Not recognising that a sharp melting point indicates purity. Confusing the melting point of the tablet with that of the pure aspirin, or vice versa.
Mark scheme (4 marks)
- Heat both samples and record the temperature(s) at which each melts
- The pure substance melts at a single/sharp temperature
- The impure sample (crushed tablet) melts over a range of temperatures
- The pure substance can be identified because a pure substance contains only one element or compound, whereas the tablet is a mixture and mixtures melt over a range of temperatures
Key terms in this question
pure substance · melting point data
Related
- All WJEC GCSE Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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