A technician is testing two white solid substances, substance A and substance B, to find out whether each one is a pure substance or a mixture. The technician heats both substances slowly and records the temperature at which each one melts. Substance A melts completely at exactly 58 °C. Substance B begins to soften at 52 °C and is fully melted by 61 °C. Explain what the melting point data tell the technician about the purity of substance A and substance B.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Substance A has a sharp melting point – it melts completely at exactly 58 °C. A sharp, single‑temperature melting point is characteristic of a pure substance, so substance A is pure.
Substance B softens at 52 °C and is fully melted only at 61 °C, so it melts over a temperature range. A non‑sharp, broadened melting point indicates a mixture; the different components melt at different temperatures, producing a range.
Substance B softens at 52 °C and is fully melted only at 61 °C, so it melts over a temperature range. A non‑sharp, broadened melting point indicates a mixture; the different components melt at different temperatures, producing a range.
Examiner tips
- Mention the sharp vs. broadened melting point and link each to purity or mixture.
- Use the exact temperatures given to show the range for B.
- Explain that a pure compound has a single melting point, a mixture does not.
Common mistakes
- Saying “B is pure” because it melts – forgetting the range indicates a mixture.
- Using vague terms like “high purity” without linking to the melting point data.
- Not specifying the exact temperatures or the range for B.
Mark scheme (5 marks)
- Substance A has a sharp melting point (melts at one specific temperature / melts at exactly 58 °C)
- A sharp melting point indicates substance A is a pure substance (contains only one element or compound)
- Substance B melts over a range of temperatures (begins to soften at 52 °C and is not fully melted until 61 °C / does not have a sharp melting point)
- Melting over a range of temperatures indicates substance B is a mixture
- In a mixture, the substances are not chemically bonded/combined together, so each component affects the melting behaviour, causing the range
Key terms in this question
pure substance · mixture · melting point data
Related
- All Eduqas A-Level Chemistry revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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