A student is investigating the effect of salt concentration on the mass of potato cylinders using osmosis. Describe how the student should design a reliable investigation, including how variables should be controlled.

OCR GCSE Biology A: Gateway Science (J247) — B7 Practical skills · Describe · 4 marks · View as Markdown

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

A student wants to find out whether increasing the concentration of salt solution causes potato cylinders to lose mass. They have access to potato, salt solution at a range of concentrations, distilled water, a balance, and standard laboratory equipment.

Model answer (4 marks)

Identify the independent variable: concentration of salt solution.
Identify the dependent variable: change in mass of the potato cylinders.
Control the size and shape of the potato cylinders – use a standard cutter so all samples have the same surface area and mass.
Use the same volume of solution for each cylinder and keep the temperature constant.
Repeat the experiment at each concentration to improve reliability.

Examiner tips

  • Use clear, exam‑style wording – e.g. "identify the independent variable…". Mention at least one controlled variable and give a short reason. Show that you will repeat each concentration to increase reliability.

Common mistakes

  • Not naming the independent or dependent variable. Failing to state a controlled variable or its reason. Ignoring the need for repeats to improve reliability.

Mark scheme (4 marks)

  1. Identify the independent variable as the concentration of salt solution
  2. Identify the dependent variable as the change in mass of the potato cylinders
  3. State at least one controlled variable with a reason
  4. State that repeats should be carried out at each concentration to improve reliability

Key terms in this question

osmosis

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