A student is investigating the rate of a reaction between marble chips and hydrochloric acid by collecting the gas produced using a gas syringe. Explain how the student should carry out a fair and reliable investigation, including how they would record and process their results.

OCR A-Level Chemistry A (H432) — 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. A student plans to compare the volume of gas collected over time at two different concentrations of hydrochloric acid.

Model answer (5 marks)

1. Keep the mass of marble chips constant for both concentrations – e.g. weigh 5.00 g of chips and use the same amount each time.
2. Use the same volume of acid for each trial (e.g. 50 mL) and keep the temperature constant.
3. Record the volume of CO₂ in the syringe at regular, stated intervals (e.g. every 30 s) until the reaction stops.
4. Repeat the whole experiment at least once more for each concentration and calculate the mean volume for each time point.
5. Identify any anomalous data points (e.g. sudden jumps or drops) and exclude them before calculating the mean.
6. Plot volume of gas (mL) on the y‑axis against time (s) on the x‑axis for each concentration; the curve for the higher concentration should rise more steeply, indicating a faster rate.

Examiner tips

  • Use precise, consistent units (mL, s) and state the interval clearly; this shows you understand the rate concept.
  • Show the mean calculation step‑by‑step – examiners reward clear arithmetic.
  • Explain why you exclude outliers – this demonstrates critical data handling.

Common mistakes

  • Changing the amount of marble chips between trials; this confuses the variable being tested.
  • Failing to repeat the experiment – no mean or error analysis.
  • Not stating the time interval or using inconsistent units in the plot.

Mark scheme (5 marks)

  1. Identify and control a named variable (e.g. mass/size of marble chips, volume of acid, temperature) to make it a fair test
  2. Read the gas syringe / volume of gas at regular, stated time intervals
  3. Repeat the experiment (at least once more) for each concentration and calculate a mean
  4. Identify and exclude anomalous results before calculating the mean
  5. Plot / describe plotting the volume of gas against time to compare the two concentrations, or state the expected trend (higher concentration gives a faster rate / greater volume collected in the same time)

Related

More Practical skills assessed questions

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