A student is investigating the rate of a reaction between marble chips (calcium carbonate) and hydrochloric acid by measuring the loss in mass over time. Explain how the student should carry out this investigation to obtain valid and reliable results, and describe how they would use their results to reach a conclusion.

OCR A-Level Chemistry B: Salters (H433) — 1.1 Practical skills assessed · Explain · 5 marks · View as Markdown

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

Marble chips react with hydrochloric acid to produce carbon dioxide gas, which escapes from the reaction vessel, causing a measurable loss in mass. The student wants to investigate how the concentration of hydrochloric acid affects the rate of reaction.

Model answer (5 marks)

1. Keep the mass/size of the marble chips constant for all trials.
2. Keep the temperature of the reaction vessel constant.
3. Keep the total volume of acid used the same in each trial.
4. Measure the loss in mass by placing the flask on a balance and recording the mass at regular time intervals.
5. Repeat each trial at least twice and calculate a mean loss‑in‑mass value to improve reliability.
6. Systematically change the independent variable – use at least three different concentrations of hydrochloric acid.
7. Plot the mean loss in mass against time for each concentration.
8. A steeper/faster loss in mass indicates a faster rate of reaction.
9. Conclude that a higher concentration of hydrochloric acid gives a greater rate of reaction.

Examiner tips

  • Use a clear, consistent method for weighing and timing; include a blank run to check for systematic error.
  • Show the mean calculation and explain why repetition improves reliability.
  • State the independent, dependent and controlled variables explicitly.

Common mistakes

  • Not keeping the marble chip mass or temperature constant, leading to variable rates.
  • Failing to repeat trials or calculate a mean, reducing reliability.
  • Misidentifying the dependent variable (e.g. using volume of gas instead of mass loss).

Mark scheme (5 marks)

  1. Control variables — the student must keep at least one named variable constant (e.g. mass/size of marble chips, temperature, or total volume of acid)
  2. Method of measuring loss in mass — place the flask on a balance and record mass at regular time intervals
  3. Repeat the experiment and calculate a mean to improve reliability
  4. Change the independent variable systematically — use at least three different concentrations of hydrochloric acid
  5. Conclusion — a steeper/faster loss in mass indicates a faster rate of reaction, so a higher concentration of acid gives a greater rate of reaction

Key terms in this question

valid · reliable

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