Explain what is meant by the law of conservation of mass and explain why chemical equations must be balanced.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
When magnesium burns in oxygen, the reaction can be represented by the equation: 2Mg + O₂ → 2MgO
Model answer (5 marks)
The law of conservation of mass states that no atoms are lost or made during a chemical reaction, so the mass of the products equals the mass of the reactants.
Chemical equations must be balanced because a balanced equation has the same number of atoms of each element on both sides. This reflects the fact that atoms are rearranged but not created or destroyed during a reaction.
In the given equation 2Mg + O₂ → 2MgO, there are 2 Mg atoms and 2 O atoms on each side, showing the equation is balanced.
Chemical equations must be balanced because a balanced equation has the same number of atoms of each element on both sides. This reflects the fact that atoms are rearranged but not created or destroyed during a reaction.
In the given equation 2Mg + O₂ → 2MgO, there are 2 Mg atoms and 2 O atoms on each side, showing the equation is balanced.
Examiner tips
- Use the phrase "law of conservation of mass" and explain that mass is unchanged.
- Show that the equation is balanced by counting atoms of each element.
- Link the need to balance to atoms being rearranged, not created or destroyed.
Common mistakes
- Writing the equation as Mg + O₂ → MgO, which is unbalanced.
- Failing to explain why atoms cannot be created or destroyed.
- Using the wrong spelling of "conservation" or "mass".
Mark scheme (5 marks)
- No atoms are lost or made during a chemical reaction
- The mass of the products equals the mass of the reactants
- A balanced equation has the same number of atoms of each element on both sides
- This is because atoms are rearranged but not created or destroyed during a reaction (linking conservation of mass to the need to balance)
- Correct application to the given equation: there are 2 Mg atoms and 2 O atoms on each side, showing the equation is balanced
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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