A pharmacist is checking the purity of two batches of a white crystalline medicine. Batch X melts sharply at 157 °C. Batch Y begins to melt at 149 °C and is not completely melted until 156 °C. Explain what the melting point data tells the pharmacist about the purity of each batch, and describe one other technique that could be used to check whether Batch Y is a pure substance or a mixture.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Batch X is a pure substance because it melts sharply at a single temperature of 157 °C, indicating a homogeneous crystal lattice.
Batch Y is a mixture or impure because its melting range from 149 °C to 156 °C shows a broad transition, typical of different components melting at different temperatures.
One other technique that could be used is paper chromatography: a pure substance will give a single spot in a chosen solvent system, whereas a mixture will separate into multiple spots.
Batch Y is a mixture or impure because its melting range from 149 °C to 156 °C shows a broad transition, typical of different components melting at different temperatures.
One other technique that could be used is paper chromatography: a pure substance will give a single spot in a chosen solvent system, whereas a mixture will separate into multiple spots.
Examiner tips
- State the melting point behaviour first, then link it to purity. Mention the range of temperatures for Batch Y. Give the technique and explain what a pure substance would look like.
- common_mistakes
- :
- Confusing a sharp melting point with a melting range. Not specifying that the range indicates a mixture. Omitting the explanation of how chromatography distinguishes pure vs. mixed substances.
Mark scheme (5 marks)
- Batch X is a pure substance
- Because Batch X has a sharp melting point (melts at a single/specific temperature)
- Batch Y is a mixture / impure
- Because Batch Y melts over a range of temperatures (not a sharp melting point)
- Paper chromatography could be used — a pure substance produces a single spot in all solvents, whereas a mixture separates into more than one spot
Key terms in this question
melting point data · pure substance · mixture
Related
- All Eduqas A-Level Chemistry revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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