A student carries out tests on a sample of tap water to find out whether it is pure water. Describe the tests the student could carry out and explain how the results would show whether the water is pure.

Eduqas GCSE Chemistry — C8 Chemical analysis · Describe · 4 marks · View as Markdown

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

Model answer (4 marks)

1. Measure the boiling point of the tap water sample.
2. Pure water boils at 100 °C; a higher boiling point shows dissolved impurities.
3. Measure the freezing (melting) point of the sample.
4. Pure water freezes at 0 °C; a lower freezing point indicates impurities.

Examiner tips

  • Use the correct units (°C) and state the reference values for pure water.
  • Show the logical link between a change in boiling/freezing point and the presence of solutes.
  • Keep the answer concise – one sentence per point.
  • Mention that both tests are simple and commonly used for purity checks.

Common mistakes

  • Confusing boiling point with melting point or vice versa.
  • Failing to give the exact reference temperatures for pure water.
  • Using incorrect units or omitting the °C symbol.

Mark scheme (4 marks)

  1. The student could measure the boiling point of the sample
  2. Pure water boils at exactly 100 °C; a boiling point above 100 °C indicates impurities are present
  3. The student could measure the melting point (freezing point) of the sample
  4. Pure water melts/freezes at exactly 0 °C; a melting point below 0 °C indicates impurities are present

Key terms in this question

pure water

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