A student wants to find out whether a sample of aspirin tablets is pure. She measures the melting point of a small amount of the tablet and finds that it melts between 135 °C and 142 °C. The accepted melting point of pure aspirin is 140 °C. Explain what the student's melting point result tells her about the purity of the aspirin sample, and explain how a pure substance behaves differently when it melts.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student measures the melting point of an aspirin tablet sample and records that it melts over the range 135 °C to 142 °C. The accepted melting point of pure aspirin is 140 °C.
Model answer (4 marks)
The tablet does not melt at one sharp temperature but over a range of 135 °C to 142 °C, so it is not pure – it contains other substances.
A pure substance melts at a single, fixed temperature (140 °C for aspirin).
A pure substance melts at a single, fixed temperature (140 °C for aspirin).
Examiner tips
- State the range of temperatures and link it to impurity; mention the fixed melting point of pure aspirin; keep answer concise; use exact terminology (range, fixed, pure).
Common mistakes
- Saying the sample is pure because it contains aspirin; confusing melting point with boiling point; not explaining that a pure substance melts at one temperature.
Mark scheme (4 marks)
- The sample melts over a range of temperatures (not at a single/sharp temperature), indicating it is not pure / is a mixture.
- A pure substance has a sharp / specific / fixed melting point (melts at one exact temperature).
- Mixtures / impure substances melt over a range of temperatures (rather than at a single point).
- The sample therefore contains other substances / impurities mixed with the aspirin (it is not a pure compound).
Key terms in this question
pure substance · melting point
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