A student is investigating how the mass of iron wool changes when it is heated in a sealed container. Before heating, the student records the mass of the sealed container and its contents. After heating, the student notices the iron wool has turned black and records the mass again. Explain why the mass of the sealed container and its contents remains the same before and after heating, and describe how this investigation demonstrates the law of conservation of mass.

OCR GCSE Chemistry A: Gateway Science (J248) — C7 Practical activity skills · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Iron wool reacts with oxygen when heated. The reaction can be represented as: iron + oxygen → iron oxide.

Model answer (4 marks)

The iron wool reacts with the oxygen that is already present in the sealed container, forming iron oxide. No atoms are created or destroyed during the reaction; the iron atoms simply combine with the oxygen atoms. Because the container is sealed, no oxygen can enter or leave the system, so the total amount of material inside remains unchanged. Consequently, the mass of the sealed container and its contents before heating equals the mass after heating. This experiment therefore demonstrates the law of conservation of mass: the total mass of reactants equals the total mass of products, showing that mass is neither created nor destroyed in a chemical reaction.

Examiner tips

  • Use the phrase "no atoms are created or destroyed" to hit the key point.
  • Mention that the container is sealed so no mass enters or leaves.
  • State that the total mass of reactants equals the total mass of products.
  • Conclude that this confirms the law of conservation of mass.

Common mistakes

  • Forgetting to note that the container is sealed and no mass enters or leaves.
  • Using vague wording such as "the mass stays the same" without explaining why.
  • Mixing up the roles of iron and oxygen in the reaction description.

Mark scheme (4 marks)

  1. No atoms are created or destroyed during a chemical reaction
  2. The oxygen used in the reaction comes from inside the sealed container, so no matter enters or leaves the system
  3. The total mass of reactants equals the total mass of products
  4. This shows the law of conservation of mass: mass is neither created nor destroyed in a chemical reaction

Key terms in this question

law of conservation of mass · sealed container

Related

More Practical activity skills questions

▶ Try answering this question with AI marking (free) →