Copper reacts with oxygen when heated. The equation for this reaction is: 2Cu + O₂ → 2CuO. The relative atomic mass of copper is 64 and the relative atomic mass of oxygen is 16. Explain why the mass of the solid in the container increases during this reaction.

AQA GCSE Chemistry (8462) — 4.3.2 Use of amount of substance in relation to masses of pure substances · Explain · 4 marks · View as Markdown

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

A student heats copper powder in an open crucible. After the reaction, the student notices that the mass of the solid has increased.

Model answer (4 marks)

Copper is a solid and oxygen is a gas in the air. When the copper is heated the reaction 2Cu + O₂ → 2CuO occurs. No atoms are lost or created – the law of conservation of mass is obeyed. The oxygen atoms from the air combine with the copper atoms and are added to the solid. The mass of the oxygen that joins the copper is now part of the product CuO. Therefore the mass of the solid product (copper oxide) is greater than the mass of the copper alone because the mass of oxygen has been added.

Examiner tips

  • Show the reaction equation and point out O₂ is a reactant added to the solid.
  • Explain that the added oxygen mass becomes part of CuO, increasing the solid mass.
  • Use the law of conservation of mass to justify that no mass is lost.
  • Keep the answer concise – 4 marks can be earned with 3–4 short sentences.

Common mistakes

  • Failing to mention that oxygen is a reactant added from the air.
  • Confusing the mass increase with a loss of copper rather than addition of oxygen.
  • Using vague phrases like ‘more mass’ without linking to oxygen addition.

Mark scheme (4 marks)

  1. No atoms are lost or made during a chemical reaction (law of conservation of mass)
  2. Oxygen (from the air) is a reactant that combines with / is added to the copper
  3. The mass of oxygen gained is included in the mass of the product copper oxide (CuO)
  4. The mass of the solid product (copper oxide) is greater than the mass of the copper alone because the mass of oxygen has been added

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