Zinc carbonate decomposes when heated to form zinc oxide and carbon dioxide gas. A student heats a sample of zinc carbonate and finds that the mass of the solid decreases during the reaction. Explain why the mass of the solid decreases, and state the law that describes what happens to the total mass of all the substances present, including the gas produced.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The equation for the reaction is: ZnCO₃(s) → ZnO(s) + CO₂(g)
Model answer (5 marks)
The mass of the solid decreases because carbon dioxide gas is released and escapes from the solid.
Carbon dioxide is a gas, so it is no longer part of the solid being weighed; it leaves the reaction vessel.
The total mass of all substances (solid + gas) remains the same – the mass of the products equals the mass of the reactants.
This is described by the law of conservation of mass.
No atoms are lost or made during the reaction; atoms are rearranged but the number of each type of atom stays the same.
Carbon dioxide is a gas, so it is no longer part of the solid being weighed; it leaves the reaction vessel.
The total mass of all substances (solid + gas) remains the same – the mass of the products equals the mass of the reactants.
This is described by the law of conservation of mass.
No atoms are lost or made during the reaction; atoms are rearranged but the number of each type of atom stays the same.
Examiner tips
- Use the word ‘released’ or ‘escapes’ to show the gas leaves the solid. Mention that the gas is no longer part of the weighed solid. State the law of conservation of mass explicitly. Show that atoms are conserved, not created or destroyed.
Common mistakes
- Saying the solid simply ‘shrinks’ without explaining the gas escape. Forgetting to mention the law of conservation of mass. Confusing the decrease in solid mass with a loss of atoms rather than a change in phase.
Mark scheme (5 marks)
- The mass of the solid decreases because carbon dioxide gas is released / escapes from the solid
- Carbon dioxide is a gas so it is no longer part of the solid being measured / it escapes into the surroundings
- The total mass of all substances (solid + gas) remains the same / the mass of products equals the mass of reactants
- This is described by the law of conservation of mass
- No atoms are lost or made during the reaction / atoms are rearranged but the number of each type of atom stays the same
Related
- All Eduqas A-Level Chemistry revision notes →
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- Decode the mark scheme abbreviations →
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