Osmosis
Build confidence in Osmosis for GCSE Biology with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Osmosis is the diffusion of water across a partially permeable membrane; the required practical uses potato cylinders to find the concentration where mass stays the same.
Free downloads
These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
Osmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.
The membrane lets water molecules through but not the larger solute molecules, so it is the water that moves. Water moves from where there is a higher concentration of water — the dilute solution — to where there is a lower concentration of water.
The required practical investigates osmosis using potato cylinders in sugar solutions of different concentrations. Percentage change in mass is calculated so that cylinders of different starting masses can be compared fairly, which is the key reason the calculation is done at all.
Revision notes
What osmosis is
Osmosis is the movement of water across a partially permeable membrane, from a dilute solution to a more concentrated one.
The membrane allows water through but not the larger solute molecules. Like diffusion, osmosis is passive and needs no energy.
The potato practical
Cut potato cylinders of equal size, weigh each, and place them in sugar solutions of different concentrations. After a set time, dry gently and reweigh.
Cylinders in dilute solutions gain mass because water moves in; those in concentrated solutions lose mass because water moves out.
Percentage change in mass
Percentage change = (change in mass ÷ starting mass) × 100.
Using percentage change rather than raw change allows cylinders with different starting masses to be compared fairly. Where the line crosses zero on a graph, the solution has the same concentration as the potato cells.
Key points
- Osmosis is the movement of water across a partially permeable membrane.
- Water moves from a dilute to a more concentrated solution.
- The membrane lets water through but not solute molecules.
- Osmosis is passive and needs no energy.
- Percentage change = (change ÷ starting mass) × 100.
- A zero change means the concentrations are equal.
Worked examples
Example 1
Define osmosis. [2 marks]
Model answer
Osmosis is the diffusion of water from a dilute solution to a concentrated solution (1 mark) through a partially permeable membrane (1 mark).
Example 2
A potato cylinder has a starting mass of 4.0 g and a final mass of 4.6 g. Calculate the percentage change in mass. [2 marks]
Model answer
Change in mass = 4.6 − 4.0 = 0.6 g. Percentage change = (0.6 ÷ 4.0) × 100 (1 mark) = 15 % (1 mark).
Example 3
Explain why percentage change in mass is calculated rather than the change in mass alone. [2 marks]
Model answer
The potato cylinders may have slightly different starting masses (1 mark), so using percentage change allows a fair comparison between them (1 mark).
Common mistakes
Saying solute molecules move during osmosis.
Only water moves. The membrane is partially permeable and blocks the larger solute molecules.
Describing the direction backwards.
Water moves from the dilute solution to the concentrated one — from high water concentration to low.
Dividing by the final mass.
Percentage change uses the starting mass as the denominator.
Forgetting to dry the cylinders before reweighing.
Surface water would add mass and make the results invalid.
Exam tips
- Include partially permeable membrane in your definition.
- Say water moves, not solute.
- Divide by the starting mass when calculating percentage change.
- Explain percentage change in terms of fair comparison.
Key terms
- Osmosis
- Diffusion of water across a partially permeable membrane.
- Partially permeable
- Allowing some molecules through but not others.
- Dilute solution
- A solution with a high concentration of water.
- Percentage change
- The change expressed as a percentage of the starting value.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.