A student heats a sample of candle wax and records the temperature at which it melts. He then heats a sample of naphthalene and records its melting point. Explain how the melting point data for each substance shows whether it is pure or impure.

Eduqas GCSE Chemistry — 1.1 The nature of substances and chemical reactions · Explain · 4 marks · View as Markdown

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

Candle wax melts gradually between 46 °C and 68 °C. Naphthalene melts sharply at 80 °C.

Model answer (4 marks)

A pure substance has a sharp, specific melting point – it melts at one exact temperature. Naphthalene melts at a single fixed temperature of 80 °C, showing it is pure. A mixture or impure substance melts over a range of temperatures. Candle wax melts gradually between 46 °C and 68 °C, indicating it is impure or a mixture.

Examiner tips

  • Use the term ‘sharp/specific melting point’ for pure substances
  • Show the two substances’ data and link each to purity
  • Mention that a range indicates a mixture

Common mistakes

  • Saying the wax is pure because it melts, ignoring the temperature range
  • Confusing melting point with boiling point
  • Using vague terms like ‘high’ or ‘low’ instead of specific temperatures

Mark scheme (4 marks)

  1. A pure substance has a sharp/specific melting point (melts at one exact temperature)
  2. Naphthalene melts at one fixed temperature (80 °C), so it is a pure substance
  3. A mixture/impure substance melts over a range of temperatures
  4. Candle wax melts over a range of temperatures (46–68 °C), so it is impure/a mixture

Key terms in this question

melting point data

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