Magnesium ribbon is burned in a open crucible in air. A student notices that the mass of the crucible and its contents increases after the reaction. Explain why the mass increases, and state what would happen to the total mass of all substances involved if the reaction took place in a sealed container.

Eduqas GCSE Chemistry — C3 Quantitative chemistry · Explain · 4 marks · View as Markdown

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

Magnesium burns in oxygen to form magnesium oxide. When burned in an open crucible, the mass of the solid increases.

Model answer (4 marks)

Magnesium reacts with oxygen from the air to give magnesium oxide.
The oxygen atoms are added to the solid, so the mass of the crucible and its contents increases.
If the reaction were carried out in a sealed container the total mass of all the substances would remain constant.
This is because atoms are neither created nor destroyed – the law of conservation of mass applies.

Examiner tips

  • Use the word ‘reacts with’ to show the chemical change. Mention that oxygen is added to the solid. State that the mass would stay the same in a sealed system. Show the conservation‑of‑mass principle explicitly.

Common mistakes

  • Saying the mass increases because the reaction is exothermic. Forgetting to mention the oxygen source. Claiming the mass would change in a sealed container.

Mark scheme (4 marks)

  1. Magnesium reacts with oxygen from the air
  2. Oxygen is added to / gained by the solid, increasing its mass
  3. In a sealed container the total mass would stay the same / remain constant
  4. Because atoms are neither created nor destroyed / law of conservation of mass

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