Limestone is a sedimentary rock made mainly of calcium carbonate. When limestone is heated strongly in a kiln, it undergoes thermal decomposition. Explain what happens during the thermal decomposition of calcium carbonate, naming the products formed and describing one use of each product.

WJEC GCSE Chemistry (Wales) — 1.6 Limestone · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

When limestone (CaCO₃) is heated strongly it decomposes:
1. CaCO₃(s) → CaO(s) + CO₂(g)
2. The solid product is calcium oxide, also called quicklime.
3. The gaseous product is carbon dioxide.
4. Quicklime is used in making mortar and in the steel industry; CO₂ is used in carbonated drinks and as a refrigerant gas.

Examiner tips

  • Use the correct chemical equation, naming both products; mention the solid/gaseous state; give a specific industrial use for each product.
  • Show the decomposition reaction clearly; keep the answer concise and to the point.

Common mistakes

  • Writing CaCO₃ → CaO + CO₂ without indicating states; forgetting to name quicklime; giving a non‑specific use such as ‘building material’ instead of a clear example.

Mark scheme (4 marks)

  1. Calcium carbonate breaks down / decomposes when heated
  2. Calcium oxide (quicklime) is named as a product
  3. Carbon dioxide is named as the other product
  4. A correct use stated for either calcium oxide OR carbon dioxide

Key terms in this question

thermal decomposition · calcium carbonate

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