When copper carbonate is heated, it decomposes to form copper oxide and carbon dioxide gas. A student heats 6.2 g of copper carbonate and collects the carbon dioxide produced. After the reaction, the student finds that some solid remains in the tube. Explain why the mass of solid remaining is less than 6.2 g, and describe what the student would observe happening to the solid during heating.

Eduqas A-Level Chemistry — 3.5 Aromatic chemistry (A-Level only) · Explain · 5 marks · View as Markdown

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

Copper carbonate (CuCO₃) decomposes on heating: CuCO₃(s) → CuO(s) + CO₂(g)

Model answer (5 marks)

The mass of solid remaining is less than 6.2 g because the reaction releases CO₂ gas:

CuCO₃(s) → CuO(s) + CO₂(g)

The CO₂ leaves the tube, so it is not part of the solid mass. According to the law of conservation of mass, the total mass of all products (solid + gas) equals the initial mass of 6.2 g. The solid changes colour from green (CuCO₃) to black (CuO) and appears to shrink as the carbonate decomposes, leaving a smaller amount of solid.

Examiner tips

  • Use the reaction equation to show CO₂ is a gas and leaves the tube. Mention conservation of mass. Describe colour change and size reduction. Keep answer concise and use exam terminology.

Common mistakes

  • Forgetting that CO₂ is a gas and not part of the solid mass. Not mentioning the colour change from green to black. Writing the answer in a long paragraph instead of clear points.

Mark scheme (5 marks)

  1. Carbon dioxide gas is released/escapes into the surroundings during the reaction
  2. The law of conservation of mass states that no atoms are lost or made, so the missing mass is accounted for by the carbon dioxide gas produced
  3. The solid changes colour from green (copper carbonate) to black (copper oxide)
  4. The solid appears to shrink in size / the amount of solid decreases
  5. The carbon dioxide produced is a gas at room temperature and pressure, so it is not included in the mass of the solid product

Related

More Aromatic chemistry (A-Level only) questions

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