A student burns magnesium ribbon in air. The magnesium reacts with oxygen gas from the air to form magnesium oxide. Explain why the mass of the solid in the reaction vessel increases, even though the law of conservation of mass states that mass is neither created nor destroyed.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Magnesium burns brightly in air. The word equation for the reaction is: magnesium + oxygen → magnesium oxide.
Model answer (4 marks)
The mass of the solid increases because the oxygen gas from the air is a reactant that joins with the magnesium. No atoms are lost or created; the oxygen atoms combine with the magnesium atoms to form magnesium oxide. The mass of the oxygen gas is added to the mass of the magnesium, so the mass of the magnesium oxide product is greater than the mass of the magnesium alone.
Examiner tips
- Use the word ‘reactant’ to show oxygen joins magnesium
- Explain that no atoms are lost – conservation of mass
- State that the added mass comes from oxygen gas
- Mention that the product’s mass is the sum of the reactants’ masses
Common mistakes
- Saying the mass decreases or that atoms are destroyed
- Forgetting to mention that oxygen comes from the air
- Not linking the increase in mass to the addition of oxygen
Mark scheme (4 marks)
- No atoms are lost or made during a chemical reaction (conservation of mass)
- Oxygen (gas) from the air is a reactant that joins with / combines with the magnesium
- The mass of oxygen (gas) is added to the mass of the magnesium (solid)
- The mass of the magnesium oxide product is greater than the mass of magnesium alone because the mass of the oxygen gas that reacted has been gained
Key terms in this question
conservation of mass · magnesium oxide
Related
- All AQA GCSE Chemistry (8462) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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