A teacher heats a sample of candle wax and measures the temperature at which it melts. The candle wax melts over a range of temperatures from 52 °C to 61 °C. The teacher then heats a sample of naphthalene and finds that it melts sharply at 80 °C. Explain what these melting point results tell us about whether candle wax and naphthalene are pure substances.

Eduqas GCSE Chemistry — 1.2 Atomic structure and the periodic table · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Candle wax is not a pure substance – it is a mixture. It melts over a range of temperatures (52 °C to 61 °C) rather than at a single, sharp temperature.

Naphthalene is a pure substance. It melts sharply at one fixed temperature (80 °C).

Examiner tips

  • Use the words "pure substance" and "mixture" exactly as the mark scheme. Show the temperature ranges and the single temperature. Keep the answer short and to the point. Include the reason for each conclusion. Use UK spelling. No extra words or explanations.
  • Mention that a range indicates a mixture, a sharp point indicates purity.
  • Use the exact temperature values given.
  • Avoid unnecessary sentences.

Common mistakes

  • Saying "candle wax is impure" instead of "not a pure substance / mixture".
  • Using the word "solid" or "liquid" incorrectly.
  • Giving extra details about composition or structure that are not asked.

Mark scheme (4 marks)

  1. Candle wax is not a pure substance / candle wax is a mixture
  2. Because candle wax melts over a range of temperatures (rather than at a single/sharp temperature)
  3. Naphthalene is a pure substance (a single element or compound)
  4. Because naphthalene has a sharp/specific melting point (melts at one fixed temperature)

Key terms in this question

pure substance · melting point

Related

More Atomic structure and the periodic table questions

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