A chef dissolves sugar in hot water to make a syrup. Later, the chef accidentally burns some sugar in a pan, turning it black and producing a smell of caramel. State which of these two processes is a physical change and which is a chemical change, and justify your answer for each process.

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)

1. Dissolving sugar in hot water is a physical change.
2. It is reversible – the sugar can be recovered by evaporating the water.
3. No new substance is formed; the sugar molecules remain unchanged.
4. Burning sugar is a chemical change.
5. It is irreversible – a new substance (black carbon and other compounds) is produced.
6. The original sugar molecules are altered and new chemical bonds are formed.

Examiner tips

  • Use the word "physical" or "chemical" first, then give two reasons: reversibility and formation of new substances.
  • Mention the specific evidence: sugar can be recovered vs new black solid produced.

Common mistakes

  • Confusing dissolving with caramelisation; stating that dissolving creates a new substance.
  • Forgetting to note that burning is irreversible or that a new solid is formed.

Mark scheme (4 marks)

  1. Dissolving sugar is a physical change
  2. Because dissolving is reversible / no new substance is formed / sugar can be recovered (by evaporation)
  3. Burning sugar is a chemical change
  4. Because a new substance is formed / the change is irreversible / carbon (black solid) and new compounds are produced

Key terms in this question

physical change · chemical change

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