Explain why the law of conservation of mass means that the total mass of products formed in a chemical reaction is always equal to the total mass of the reactants used.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student burns a piece of magnesium ribbon in air and notices that the white powder produced appears to have a greater mass than the original magnesium ribbon.
Model answer (4 marks)
1. In a chemical reaction atoms are not created or destroyed – they are only rearranged.
2. Bonds between atoms are broken and new bonds are formed, producing new compounds.
3. The magnesium ribbon reacts with oxygen from the air to form magnesium oxide, adding oxygen atoms to the product.
4. When the mass of the oxygen taken from the air is included, the total mass of reactants (magnesium + oxygen) equals the total mass of products (magnesium oxide).
2. Bonds between atoms are broken and new bonds are formed, producing new compounds.
3. The magnesium ribbon reacts with oxygen from the air to form magnesium oxide, adding oxygen atoms to the product.
4. When the mass of the oxygen taken from the air is included, the total mass of reactants (magnesium + oxygen) equals the total mass of products (magnesium oxide).
Examiner tips
- Use the exact phrase "atoms are neither created nor destroyed" to gain the first mark. Show the rearrangement of atoms and mention bond breaking/formation for the second mark. Explain the role of oxygen in increasing mass for the third mark. Include the oxygen mass in the final comparison to satisfy the fourth mark.
- common_mistakes
- :
- Forgetting to mention that oxygen atoms are added to the product. Treating the reaction as if mass is lost rather than gained. Using vague terms like "changes" instead of "atoms are rearranged".
Mark scheme (4 marks)
- Atoms are neither created nor destroyed during a chemical reaction
- Atoms are rearranged / bonds are broken and new bonds are formed
- The magnesium has reacted with oxygen (from the air) and gained mass because oxygen atoms have been added to the product
- If the oxygen taken from the air is included, the total mass of reactants (magnesium + oxygen) equals the total mass of products (magnesium oxide)
Key terms in this question
law of conservation of mass · reactants · products
Related
- All Eduqas GCSE Chemistry revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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