When copper carbonate is heated strongly, it decomposes to form copper oxide and carbon dioxide. Explain why this thermal decomposition is a chemical change rather than a physical change.

Eduqas GCSE Chemistry — C4 Chemical changes · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

1. A new substance is produced – copper oxide and carbon dioxide are not present before heating.
2. The reaction is irreversible; the products cannot be converted back to copper carbonate by simple physical means.
3. The colour change from green to black shows the chemical composition has altered.
4. In a physical change no new substances are formed and the change is easily reversible, which is not the case here.

Examiner tips

  • Use the word ‘new substance’ to hit the first mark. Show that the reaction cannot be reversed by a physical process. Mention the colour change as evidence of a chemical change. Contrast with a physical change to cover the fourth point.

Common mistakes

  • Saying the change is reversible or that it is a physical change. Forgetting to state that copper oxide and CO₂ are new substances. Not linking the colour change to a chemical change.

Mark scheme (4 marks)

  1. A new substance is formed (copper oxide and/or carbon dioxide are new substances not present at the start)
  2. The change is irreversible / cannot be reversed by simple physical means
  3. The colour change (green to black) indicates a chemical change has occurred, showing the substance has altered chemically
  4. In a physical change, no new substances are formed and the change is easily reversible, which is not the case here

Key terms in this question

chemical change · physical change · thermal decomposition

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