Magnesium reacts with oxygen to form magnesium oxide. Explain why the mass of magnesium oxide produced is greater than the mass of magnesium used in the reaction.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student burns a strip of magnesium ribbon in air and notices that the white powder formed has a greater mass than the original magnesium ribbon.
Model answer (4 marks)
Magnesium atoms react with oxygen atoms from the air.
Atoms are not created or destroyed in a chemical reaction (conservation of mass).
The product, magnesium oxide, contains both magnesium atoms and oxygen atoms.
Therefore the mass of the magnesium oxide equals the mass of the magnesium used plus the mass of the oxygen that reacted, so it is greater than the mass of the magnesium alone.
Atoms are not created or destroyed in a chemical reaction (conservation of mass).
The product, magnesium oxide, contains both magnesium atoms and oxygen atoms.
Therefore the mass of the magnesium oxide equals the mass of the magnesium used plus the mass of the oxygen that reacted, so it is greater than the mass of the magnesium alone.
Examiner tips
- Use the word "react" to show the interaction with oxygen.
- Mention conservation of mass to justify why the extra mass comes from oxygen.
- Show the final reasoning that the product mass is the sum of Mg and O masses.
- Keep the answer concise and use the exact terminology from the mark scheme.
Common mistakes
- Failing to mention that oxygen comes from the air.
- Using vague terms like "more stuff" instead of specifying oxygen atoms.
- Omitting the conservation of mass principle.
Mark scheme (4 marks)
- Magnesium atoms react with / combine with oxygen atoms (from the air)
- Atoms are not created or destroyed in a chemical reaction (conservation of mass / atoms)
- The magnesium oxide contains both magnesium atoms AND oxygen atoms
- So the mass of the product (magnesium oxide) equals the mass of magnesium PLUS the mass of oxygen that reacted
Key terms in this question
Related
- All Edexcel GCSE Chemistry (1CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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