A student is given a white powder and is told it is either a pure substance or a mixture. Explain how melting point data could be used to determine whether the white powder is a pure substance or a mixture, and describe what the student would expect to observe in each case.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student is investigating an unknown white powder in a school laboratory. The student uses melting point data to evaluate the purity of the sample.
Model answer (5 marks)
A pure substance will have a sharp, well‑defined melting point. The student should observe that the powder melts at one fixed temperature, with no range of temperatures.
If the powder is a mixture, the melting will occur over a range of temperatures. The observed melting point will be lower than the expected value for the pure substance and will be broadened, indicating the presence of impurities or other components.
The student can compare the measured melting point to a known literature value. If the measured point matches the literature value and is sharp, the sample is pure. If it is lower and broad, the sample is a mixture or impure.
Thus, a sharp, single‑temperature melt indicates purity, whereas a depressed, broad melt indicates a mixture.
If the powder is a mixture, the melting will occur over a range of temperatures. The observed melting point will be lower than the expected value for the pure substance and will be broadened, indicating the presence of impurities or other components.
The student can compare the measured melting point to a known literature value. If the measured point matches the literature value and is sharp, the sample is pure. If it is lower and broad, the sample is a mixture or impure.
Thus, a sharp, single‑temperature melt indicates purity, whereas a depressed, broad melt indicates a mixture.
Examiner tips
- Use the terms "sharp" and "broad" for melting points
- Mention comparison with literature values
- Explain melting point depression for mixtures
Common mistakes
- Confusing melting point depression with boiling point depression
- Saying the mixture melts at a higher temperature
- Not specifying that the pure substance melts at a single fixed temperature
Mark scheme (5 marks)
- A pure substance has a sharp (precise/specific) melting point
- If the powder is a pure substance, it will melt at one fixed/consistent temperature (no range)
- A mixture melts over a range of temperatures (not at a single fixed point)
- The melting point of a mixture is lower than expected for a pure substance (melting point depression) OR the presence of impurities/other substances lowers/broadens the melting point
- The student could compare the measured melting point to a known/data book value to confirm identity and purity of the pure substance
Key terms in this question
melting point data · pure substance · mixture
Related
- All OCR A-Level Chemistry B: Salters (H433) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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