A student is investigating the effect of pH on the germination of seeds. Describe the steps the student should take to ensure the pH of the soil is accurately measured and controlled throughout the experiment.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
1. Calibrate the pH meter with standard buffer solutions before each set of measurements.
2. Prepare soil samples with buffer solutions of known pH to set the desired pH levels.
3. Measure the pH of each soil sample immediately after mixing and record the value.
4. Re‑measure the pH at regular intervals during the experiment to detect any drift or variation, and adjust with buffer if necessary.
2. Prepare soil samples with buffer solutions of known pH to set the desired pH levels.
3. Measure the pH of each soil sample immediately after mixing and record the value.
4. Re‑measure the pH at regular intervals during the experiment to detect any drift or variation, and adjust with buffer if necessary.
Examiner tips
- Use the exact wording ‘calibrate the pH meter’ and ‘buffer solutions’ to match the mark scheme.
- Show that you will take multiple readings over time – this demonstrates control and monitoring.
Common mistakes
- Assuming the pH stays constant without re‑measuring; examiners penalise this.
- Using only one buffer or failing to mention calibration; this loses marks.
Mark scheme (4 marks)
- The student should calibrate the pH meter
- The student should use buffer solutions to control the pH
- The student should take multiple pH readings over time
- The student should consider the potential for pH drift or variation
Related
- All OCR A-Level Biology A (H420) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →