A student investigates the rate of reaction between marble chips (calcium carbonate) and hydrochloric acid. The student repeats the experiment using the same mass of marble chips but crushes them into a powder before adding the acid. Explain why crushing the marble chips into a powder increases the rate of reaction.

Edexcel GCSE Chemistry (1CH0) — 7.1 Rates of reaction · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Crushing the marble chips increases the surface area of the reactant, so more calcium carbonate particles are exposed to the acid.
The larger surface area means more reacting particles are available on the surface.
This leads to a higher frequency of collisions between the acid molecules and the calcium carbonate.
With more frequent collisions, the number of successful collisions rises, increasing the rate of reaction.

Examiner tips

  • Use the word "surface area" and link it to "more collisions" and "successful collisions".
  • Show the causal chain: surface area → more exposed particles → more collisions → higher rate.
  • Keep each point short and use exam‑style terminology.

Common mistakes

  • Failing to mention surface area or collisions explicitly.
  • Using vague terms like "faster" without explaining why.
  • Mixing up the order of the causal chain.

Mark scheme (4 marks)

  1. Crushing the marble chips increases the surface area of the reactant
  2. There are more exposed reacting particles (on the surface)
  3. Collisions between reactant particles occur more frequently / there are more frequent collisions
  4. There are more frequent successful collisions, so the rate of reaction increases

Key terms in this question

rate of reaction

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