The law of conservation of mass states that no atoms are lost or made during a chemical reaction. A student burns magnesium ribbon in air and notices that the solid product has a greater mass than the original magnesium ribbon. Explain why the mass of the product is greater than the mass of the magnesium, and explain why this observation does not break the law of conservation of mass.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
The magnesium ribbon reacts with oxygen from the air to give magnesium oxide:
Mg (s) + ½O₂ (g) → MgO (s)
The solid product contains both magnesium and oxygen atoms. The mass is greater because oxygen atoms from the air have been added to the magnesium. No atoms are lost or made – the total mass of the magnesium plus the oxygen that entered the reaction equals the mass of the magnesium oxide formed. Therefore the observation does not violate the law of conservation of mass.
Mg (s) + ½O₂ (g) → MgO (s)
The solid product contains both magnesium and oxygen atoms. The mass is greater because oxygen atoms from the air have been added to the magnesium. No atoms are lost or made – the total mass of the magnesium plus the oxygen that entered the reaction equals the mass of the magnesium oxide formed. Therefore the observation does not violate the law of conservation of mass.
Examiner tips
- State the reaction equation to show the source of the extra mass
- Explain that oxygen atoms are added, not created or destroyed
- Show that the total mass of reactants (Mg + O₂) equals the mass of product (MgO)
- Use the phrase "law of conservation of mass" to link to the principle
Common mistakes
- Failing to mention the oxygen from the air
- Confusing the mass increase with a violation of the law
- Not recognising that the law applies to the whole system, not just the magnesium
Mark scheme (5 marks)
- The magnesium reacts with oxygen (from the air) during burning
- The product (magnesium oxide) contains atoms of both magnesium and oxygen
- The mass increases because the oxygen atoms from the air have been added to the magnesium
- No atoms are lost or made — the total mass of magnesium plus oxygen used equals the mass of magnesium oxide formed
- The law is not broken because if the mass of oxygen used is included alongside the magnesium, the total mass of reactants equals the total mass of the product
Key terms in this question
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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