A chemist is testing two liquid samples, X and Y, to find out whether each one is a pure substance or a mixture. The chemist heats both samples carefully and records the temperature at which each one boils. Sample X boils at a single, sharp temperature of 78 °C. Sample Y boils over a range of temperatures between 82 °C and 97 °C. Explain what these boiling point results tell the chemist about samples X and Y, and state one further test the chemist could carry out to confirm these conclusions.

Model answer (5 marks)

Examiner tips

• Use the exact phrases ‘single, sharp temperature’ and ‘range of temperatures’ to match the mark scheme.
• Explain the reasoning for each sample before stating the test.
• Mention the test in one clear sentence, linking it to the evidence of purity or mixture.
• Keep the answer concise – 5 points can be covered in 4–5 short sentences.

Common mistakes

• Saying ‘Sample X is pure because it boils at 78 °C’ without mentioning the ‘single, sharp temperature’ phrase.
• Confusing the test with a qualitative analysis that doesn’t distinguish purity, e.g., just saying ‘tasting’ or ‘colour change’.
• Using a test that is not relevant to purity, such as a solubility test, which does not confirm the boiling‑point evidence.