When candle wax is heated, it melts and becomes a liquid. When the liquid wax is then left to cool, it solidifies back into a solid. Explain, in terms of particles and energy, what happens during the melting of the candle wax, and state why melting is described as a physical change rather than a chemical change.

Pearson Edexcel International GCSE Physics (4PH1) — 5.2 Change of state · Explain · 4 marks · View as Markdown

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

Candle wax melts at approximately 57 °C. When a candle is lit, the solid wax near the flame melts into a liquid. When the flame is blown out, the liquid wax cools and solidifies again.

Model answer (4 marks)

During melting the wax is heated until the temperature reaches the melting point of about 57 °C. The energy supplied is used to overcome the forces that keep the wax molecules in a fixed lattice. The particles are freed from their fixed positions and can move past one another, so the solid becomes a liquid. Because the energy is used for changing the state rather than increasing the kinetic energy of the particles, the temperature remains constant during the whole melting process.

Melting is a physical change because it is reversible – the liquid wax can be cooled and will solidify again, returning to its original form. In a chemical change the chemical structure of the substance is altered and the original properties cannot be restored, which is not the case for wax melting.

Examiner tips

  • Use the term ‘melting point’ and state the temperature 57 °C; explain that energy is used to break intermolecular forces, not to raise temperature; mention the temperature remains constant; justify that the change is reversible and therefore physical.
  • Include the key phrase ‘physical change’ and contrast with a chemical change by noting no new substances are formed.

Common mistakes

  • Saying the temperature rises during melting; confusing melting with evaporation; claiming the wax changes its chemical composition.

Mark scheme (4 marks)

  1. During melting, energy supplied goes towards separating / freeing the particles from their fixed positions (rather than increasing kinetic energy / temperature)
  2. The temperature remains constant / does not rise during melting because the energy is used to change state, not increase kinetic energy
  3. Melting is a physical change because the process is reversible / the wax can return to its original solid state on cooling (freezing)
  4. In a chemical change, the chemical structure of the substance changes and the original properties cannot be restored; this does not happen when wax melts

Key terms in this question

melting · physical change · chemical change

Related

More Change of state questions

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