A chemist is given two samples of a liquid. Sample A is described as pure ethanol. Sample B is described as a mixture of ethanol and water. Explain how the chemist could use melting point data to decide which sample is the pure substance, and what result she would expect for each sample.

Eduqas GCSE Chemistry — 1.4 The ever-changing Earth · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

The chemist measures the melting point of each sample. A pure substance shows a sharp, specific melting point. Sample A, being pure ethanol, will melt at a single, sharp temperature (≈ –114.1 °C, the known melting point of ethanol). Sample B, a mixture of ethanol and water, will melt over a range of temperatures, not at a single point, because the components depress each other’s melting points and the mixture has a melting‑point range.

Examiner tips

  • State that the chemist will record the melting point of each sample. Mention that a pure substance gives a sharp melting point. Identify the expected result for Sample A (single temperature) and for Sample B (range).

Common mistakes

  • Confusing boiling point with melting point. Failing to note that the mixture will give a range rather than a single temperature. Giving the wrong known melting point for ethanol.

Mark scheme (4 marks)

  1. The chemist measures/records the melting point (or melting point range) of each sample
  2. A pure substance has a sharp (specific/precise) melting point
  3. Sample A (pure ethanol) would melt at one specific/sharp temperature (accept: would match the known melting point of ethanol)
  4. Sample B (the mixture) would melt over a range of temperatures (rather than at a sharp point)

Key terms in this question

pure substance · melting point data · mixture

Related

More The ever-changing Earth questions

▶ Try answering this question with AI marking (free) →