A student burns copper in air inside an open crucible. After the reaction, the student notices that the mass of the crucible and its contents has increased. Explain why the mass increases, and explain why the total mass of all substances involved has not changed.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Copper reacts with oxygen from the air to form copper oxide. Oxygen is a reactant and has mass, so the product copper oxide is heavier than the copper alone, which is why the crucible and its contents weigh more after the reaction.
During the reaction no atoms are created or destroyed – the law of conservation of mass applies. The total mass of the reactants (copper + oxygen) equals the total mass of the products (copper oxide), so the overall mass of all substances involved has not changed.
During the reaction no atoms are created or destroyed – the law of conservation of mass applies. The total mass of the reactants (copper + oxygen) equals the total mass of the products (copper oxide), so the overall mass of all substances involved has not changed.
Examiner tips
- Mention the reaction Cu + O₂ → CuO and the added mass of O₂; state the law of conservation of mass; keep the answer concise; use correct chemical symbols and terms.
Common mistakes
- Forgetting to mention that oxygen is a reactant; claiming the mass actually decreases; not recognising that the crucible is part of the system and its mass is included in the measurement.
Mark scheme (4 marks)
- Copper reacts with oxygen (from the air)
- Oxygen is a reactant / has mass that is added to the copper, so the product (copper oxide) is heavier than the copper alone
- No atoms are created or destroyed during the reaction (law of conservation of mass)
- The total mass of reactants (copper + oxygen) equals the total mass of products (copper oxide), so overall mass is conserved
Related
- All AQA GCSE Chemistry (8462) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Chemical measurements, conservation of mass and the quantitative interpretation of chemical equations questions
- A student heats calcium carbonate in an open crucible. The student notices that …
- When magnesium is burned in air, the mass of the product is greater than the mas…
- A student dissolves iron sulfide in excess dilute sulfuric acid inside a sealed …
- A student reacts zinc with dilute hydrochloric acid in an open conical flask pla…