A candle is made of wax. When a candle burns, liquid wax forms near the flame and a new white solid forms as the candle cools further from the flame. At the same time, carbon dioxide and water vapour are produced near the flame. State which of these observations represents a physical change and which represents a chemical change, and justify each answer.

Cambridge International IGCSE Chemistry (0620) — 6.1 Physical and chemical changes · Justify · 4 marks · View as Markdown

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

Model answer (4 marks)

The melting of the wax into liquid and its subsequent solidification back into a white solid are physical changes, because no new substances are formed and the process is reversible – the wax can be recovered.

The burning of the wax, which produces carbon dioxide and water vapour, is a chemical change, because new substances are formed and the process is irreversible – the CO₂ and H₂O cannot be converted back into wax.

Examiner tips

  • Use the word ‘physical’ or ‘chemical’ first, then give the justification in one sentence.
  • Mention reversibility for the physical change and irreversibility for the chemical change.
  • Keep the answer concise – 4 marks only.
  • Use UK spelling (e.g. ‘solidification’).

Common mistakes

  • Confusing the solidification of wax with a chemical change; it is still a physical change.
  • Failing to mention reversibility or irreversibility in the justification.
  • Using vague terms like ‘change’ without specifying physical or chemical.

Mark scheme (4 marks)

  1. The melting of wax (or solidification of wax) is a physical change
  2. Justification: no new substance is formed / the change is reversible / wax can be recovered
  3. The burning of wax (production of carbon dioxide and water vapour) is a chemical change
  4. Justification: new substances are formed / the change is irreversible / carbon dioxide and water cannot easily be converted back to wax

Key terms in this question

physical change · chemical change

Related

More Physical and chemical changes questions

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