A perfume manufacturer claims that a particular fragrance ingredient is a pure substance. Explain how melting point data could be used to evaluate whether the fragrance ingredient is pure or a mixture, and describe what the results would show in each case.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A perfume manufacturer produces fragrance ingredients. Quality control staff need to check the purity of one ingredient before it is added to a new perfume.
Model answer (4 marks)
A pure substance has a sharp, well‑defined melting point – it melts at one specific temperature.
If the fragrance ingredient is pure, the measured melting point will be a single, narrow range and will match the known melting point of that substance.
If the ingredient is a mixture, it will melt over a range of temperatures rather than at one fixed point, and the measured melting point will be lower or different from the expected value for the pure substance.
If the fragrance ingredient is pure, the measured melting point will be a single, narrow range and will match the known melting point of that substance.
If the ingredient is a mixture, it will melt over a range of temperatures rather than at one fixed point, and the measured melting point will be lower or different from the expected value for the pure substance.
Examiner tips
- Use the word "sharp" or "precise" for a pure substance’s melting point. Show that a mixture gives a range of temperatures and a lower or altered value. Mention that the measured value should be compared with the known value for the pure ingredient.
Common mistakes
- Confusing boiling point with melting point. Failing to state that a mixture gives a temperature range. Not comparing the measured value with the expected value for the pure substance.
Mark scheme (4 marks)
- A pure substance has a sharp/precise melting point (melts at one specific temperature)
- A mixture melts over a range of temperatures (rather than at one fixed temperature)
- The measured melting point of the pure substance would match the known/expected melting point for that substance
- If the ingredient is a mixture, the melting point would be lower than/different from the expected value for the pure substance
Key terms in this question
melting point data · pure substance · mixture
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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