Magnesium burns in oxygen to form magnesium oxide. The balanced equation for this reaction is: 2Mg + O₂ → 2MgO. A student burns a piece of magnesium ribbon in a sealed container. After the reaction is complete, the student claims that the total mass of the container and its contents has increased. Explain why the student's claim is incorrect, and describe what the law of conservation of mass tells us about the masses of the reactants and products in this reaction.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
The student’s claim is incorrect because the total mass of the sealed system does not increase. In a chemical reaction no atoms are lost or created; the atoms rearrange to form new compounds. Therefore the mass of the products must equal the mass of the reactants. In this reaction the oxygen gas is present in the sealed container and is consumed to form MgO, so no external mass is added. Consequently the total mass of magnesium plus oxygen before the reaction equals the total mass of magnesium oxide after the reaction.
Examiner tips
- State that the claim is wrong and the mass is unchanged; mention no atoms are lost or made; note oxygen is a reactant in the sealed system; show mass equality; keep answer concise and use correct terminology.
Common mistakes
- Saying the mass increases or that oxygen comes from outside the container; confusing the reaction with combustion that releases energy but not mass; omitting that the sealed container contains all reactants.
Mark scheme (5 marks)
- The student's claim is incorrect / the total mass does not increase
- No atoms are lost or made during a chemical reaction
- The mass of the products equals the mass of the reactants
- The oxygen gas is used up / included as a reactant in the sealed container, so no mass is gained from outside
- The total mass of magnesium and oxygen (reactants) equals the total mass of magnesium oxide (product)
Key terms in this question
conservation of mass · reactants · products
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