When magnesium is burned in air, the mass of the product is greater than the mass of the magnesium used. Explain why the mass increases, and explain why, despite this apparent change, the law of conservation of mass still holds.

AQA GCSE Chemistry (8462) — 4.3.1 Chemical measurements, conservation of mass and the quantitative interpretation of chemical equations · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Magnesium reacts with oxygen from the air to form magnesium oxide.
The oxygen gas has mass, so the mass of oxygen is added to the mass of the magnesium.
The law of conservation of mass states that no atoms are lost or made during a chemical reaction.
The total mass of the reactants (magnesium + oxygen) equals the total mass of the product (magnesium oxide).

Examiner tips

  • Use the word "reacts" to show the chemical change. Mention that oxygen is taken from the air. Show that the added mass is the mass of oxygen. State the conservation law and the equality of reactant and product masses.

Common mistakes

  • Saying the mass increases because of heat or energy. Forgetting to mention oxygen as the added mass. Claiming the law is broken instead of explaining it holds.

Mark scheme (4 marks)

  1. Magnesium reacts with oxygen (from the air)
  2. The oxygen gas has mass / the mass of oxygen is added to the mass of magnesium
  3. The law of conservation of mass states that no atoms are lost or made during a chemical reaction
  4. The total mass of reactants (magnesium + oxygen) equals the total mass of the product (magnesium oxide)

Key terms in this question

conservation of mass · product

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