A student is investigating how the volume of carbon dioxide gas produced changes over time when yeast is mixed with glucose solution. The student collects the gas using an upturned measuring cylinder filled with water. Explain how the student should carry out a fair and accurate investigation, including how they should control variables and record results reliably.

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).

Yeast breaks down glucose in a process that releases carbon dioxide gas. A student wants to find out how the volume of carbon dioxide collected changes over the first ten minutes of the reaction.

Model answer (5 marks)

1. Set up the apparatus with the measuring cylinder inverted in a beaker of water, ensuring the cylinder is completely filled with water so that the gas is collected against a water column.
2. Add a fixed mass of yeast to a fixed volume of glucose solution of known concentration.
3. Start the timer and record the volume of CO₂ in the cylinder every minute for ten minutes.
4. Keep temperature, glucose concentration, yeast mass and initial water level constant throughout the experiment.
5. Repeat the whole procedure at least three times and calculate the mean volume for each time point to improve reliability.
6. Check for leaks by ensuring the rubber stopper is snug and that no bubbles escape from the cylinder opening.
7. Record all data in a clear table, noting the time, volume and any observations.
8. Analyse the data to show how the volume changes over time, using the mean values to draw a reliable conclusion.

Examiner tips

  • Use the exact phrase ‘measure and record the volume of gas collected at regular, equal time intervals’ to hit the first point. Show that you control one variable (e.g. temperature) and vary only one independent variable. Mention repeating the experiment and calculating a mean to demonstrate reliability. Describe the water‑filled inverted cylinder set‑up to prove accurate collection.
  • common_mistakes
  • :
  • Failing to fill the measuring cylinder with water, leading to leaks. Not keeping temperature or yeast mass constant, so the test is not fair. Recording data in a messy or incomplete table, losing marks for reliability.

Mark scheme (5 marks)

  1. Measure and record the volume of gas collected at regular, equal time intervals (e.g. every minute)
  2. Keep one variable constant / controlled, e.g. temperature, volume of glucose solution, concentration of glucose, or mass of yeast
  3. Change only one independent variable at a time (e.g. temperature or concentration of glucose) to make the test fair
  4. Repeat the experiment (at least three times) and calculate a mean volume to improve reliability / reduce the effect of anomalous results
  5. Ensure the measuring cylinder is fully filled with water at the start and that the gas is collected without leaks, so measurements are accurate

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