Osmosis Experiment Questions | IGCSE Biology Guide

How to Answer Osmosis Experiment Questions in IGCSE Biology

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Key point

Dilute solution → water enters by osmosis → mass gain. Concentrated solution → water leaves → mass loss. The x-axis crossing on a % change graph shows where the solution matches the cell concentration. Use percentage change for fair comparison.

Osmosis experiment questions are among the most predictable in IGCSE Biology. They typically involve potato cylinders placed in different sugar or salt solutions, and ask you to predict, describe, and explain the mass changes. Here is the complete approach.

The standard experiment

Potato cylinders of equal length and diameter are placed in solutions of different concentrations (e.g. 0.0 M, 0.2 M, 0.4 M, 0.6 M, 0.8 M, 1.0 M sucrose). After a set time (e.g. 30 minutes), the cylinders are removed, blotted dry, and re-measured (mass or length).

Predicting results

In dilute solution (e.g. 0.0 M): the solution has a higher water potential than the potato cells → water enters the cells by osmosis → potato gains mass and becomes turgid.

In concentrated solution (e.g. 1.0 M): the solution has a lower water potential than the potato cells → water leaves the cells by osmosis → potato loses mass and becomes flaccid/soft.

At some intermediate concentration: the water potential of the solution equals the water potential of the potato cells → no net movement of water → no mass change.

Explaining the graph

Plot percentage change in mass (y-axis) against solution concentration (x-axis). The line crosses the x-axis where there is no net water movement: this is where the solution concentration equals the concentration inside the potato cells.

Above the x-axis = mass gained (water entered by osmosis). Below the x-axis = mass lost (water left by osmosis).

Why use percentage change?

Different potato cylinders may have slightly different starting masses. Using percentage change = ((final mass − initial mass) ÷ initial mass) × 100 allows fair comparison because it accounts for these differences.

Frequently asked questions

Why do we blot the potato before re-weighing?

To remove surface water, which would give an inaccurately high mass reading. We want to measure the water inside the cells, not water sitting on the surface.