A student obtains a sample of a white solid and wants to find out whether it is a pure substance or a mixture. Describe how the student could use melting point data to determine whether the white solid is pure, and explain what results would indicate that the solid is a pure substance.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The student has access to standard laboratory equipment and a data book containing melting points of known pure substances.
Model answer (5 marks)
1. Heat the white solid in a melting point apparatus and record the temperature at which it begins to melt.
2. Observe whether the melting occurs at a single, sharp temperature or over a range of temperatures.
3. Compare the recorded melting point with the known melting point of the expected pure substance from the data book.
4. If the solid melts sharply at the same temperature as the known value, conclude that it is a pure substance.
2. Observe whether the melting occurs at a single, sharp temperature or over a range of temperatures.
3. Compare the recorded melting point with the known melting point of the expected pure substance from the data book.
4. If the solid melts sharply at the same temperature as the known value, conclude that it is a pure substance.
Examiner tips
- Use the phrase "record the temperature at which it melts" to show understanding of the procedure. Mention that a pure substance gives a single, sharp melting point. Show the comparison step explicitly. State the conclusion clearly.
- common_mistakes
- :
- Failing to note that a pure substance melts at a single, sharp temperature. Comparing only the start of melting without checking the sharpness. Using the wrong terminology (e.g., “boiling point” instead of “melting point”).
Mark scheme (5 marks)
- Heat the solid and record the temperature at which it melts
- A pure substance melts at a single, sharp (specific) temperature
- A mixture melts over a range of temperatures
- Compare the recorded melting point to the known melting point of the expected pure substance (from a data book)
- If the solid melts sharply at the same temperature as the known value, it is a pure substance
Key terms in this question
pure substance · melting point data · mixture
Related
- All Eduqas A-Level Chemistry revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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