A pharmacist is checking the purity of a white powder that is supposed to be a single painkiller compound. She heats a small sample carefully and records the temperature at which it melts. Explain how the melting point data can be used to decide whether the powder is a pure substance or a mixture.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The powder is reported to melt at exactly 153 °C. A reference book states that the pure painkiller compound has a melting point of 153 °C.
Model answer (4 marks)
A pure substance melts at a single, sharp temperature. A mixture melts over a range of temperatures. The powder melts sharply at 153 °C, exactly the reference melting point of the pure painkiller. Therefore the powder is a pure substance, not a mixture.
Examiner tips
- State the definition of a pure substance’s melting point (single, sharp).
- Explain that a mixture gives a temperature range.
- Show that the measured 153 °C matches the reference value.
Mark scheme (4 marks)
- A pure substance melts at a single, sharp (specific) temperature
- A mixture melts over a range of temperatures (not at one fixed point)
- The powder melts sharply at 153 °C, which matches the reference value for the pure compound
- This indicates the powder is a pure substance (not a mixture)
Key terms in this question
pure substance · mixture · melting point data
Related
- All Eduqas GCSE Chemistry revision notes →
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