A student measures the melting point of a sample of naphthalene and finds that it melts between 74 °C and 81 °C. The accepted melting point of pure naphthalene is 80 °C. Explain what the student's results suggest about the sample of naphthalene, and describe how melting point data can be used to distinguish a pure substance from a mixture.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
The sample is not pure – it is an impure mixture.
The measured melting point is a range (74 °C–81 °C) rather than a single sharp temperature, which is characteristic of a mixture.
A pure substance melts at one specific temperature (80 °C for naphthalene).
Because the impure sample melts over a range, it shows that different components melt at slightly different temperatures.
The lowest temperature in the range (74 °C) is below the accepted 80 °C, confirming that impurities lower the melting point of the sample.
The measured melting point is a range (74 °C–81 °C) rather than a single sharp temperature, which is characteristic of a mixture.
A pure substance melts at one specific temperature (80 °C for naphthalene).
Because the impure sample melts over a range, it shows that different components melt at slightly different temperatures.
The lowest temperature in the range (74 °C) is below the accepted 80 °C, confirming that impurities lower the melting point of the sample.
Examiner tips
- Use the word "impure" or "mixture" to show you understand the sample is not pure.
- Explain that a pure substance has a sharp melting point, whereas a mixture melts over a range.
- Mention that the lower end of the range is below the accepted value, indicating impurity.
Common mistakes
- Saying the sample is pure because it contains naphthalene.
- Failing to mention the range of temperatures as evidence of impurity.
- Not noting that the melting point is lower than the accepted value.
Mark scheme (5 marks)
- The sample is not pure / is impure / is a mixture
- The sample melts over a range of temperatures (rather than at a single/sharp temperature), which indicates it is not pure
- A pure substance has a sharp / specific melting point (melts at one precise temperature)
- A mixture melts over a range of temperatures
- The melting point of the impure sample is lower than the accepted value for pure naphthalene, which is further evidence of impurity
Key terms in this question
melting point · pure substance · mixture
Related
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