Why is the percentage change in mass used instead of the change in grams?

The pieces did not all start with the same mass

Why are the potato pieces blotted dry before each weighing?

To remove solution from the surface so only the potato is weighed

On the graph of % change against concentration, what does the point where the line crosses zero show?

The concentration that matches the inside of the potato cells

Core practical: osmosis in potato

Core practical 1.16 measures how pieces of potato change mass in sugar solutions of different concentrations. The results show osmosis in action.

GCSE · 1BI0 · 1.15, 1.16, 1.17
International GCSE · 4BI1 · 2.15, 2.16, 2.17
About 16 minutes

Practise this lesson free (6 questions)

The method

1. Cut equal cylinders of potato with a cork borer and trim them to the same length.
2. Blot each one dry and record its mass.
3. Put one cylinder in each sucrose solution, for example 0, 0.2, 0.4, 0.6, 0.8 and 1.0 mol/dm³. Leave them for the same time (at least 15 minutes) at the same temperature.
4. Remove them, blot dry in the same way and weigh again.
Control variables: size and surface area of the potato, time, temperature, volume of solution.

Percentage change in mass

Pieces start with slightly different masses, so compare the percentage change:
% change = (final mass − initial mass) ÷ initial mass × 100
A positive answer means the potato gained water: the solution was more dilute than the cell contents.
A negative answer means it lost water: the solution was more concentrated.

Using the graph

Plot % change in mass (y-axis) against concentration (x-axis) and draw a line of best fit.
Where the line crosses zero, there is no net movement of water. That concentration is about the same as the concentration inside the potato cells.

Worked example

A potato piece goes from 2.50 g to 2.20 g in 0.8 mol/dm³ sucrose. Calculate the percentage change in mass.

Change = 2.20 − 2.50 = −0.30 g
% change = −0.30 ÷ 2.50 × 100
= −12%

Answer: −12% (it lost water by osmosis)

Key idea

Blot, weigh, soak, blot, reweigh. % change = change ÷ initial mass × 100. Gain means the solution was more dilute; loss means more concentrated. Where the graph crosses zero shows the concentration of the cell contents.

Quick check

Try two quick questions

See if it has sunk in. Answers are marked straight away.

Extra practice

2 more questions

Try each one, then tap to check.

A potato piece goes from 2.50 g to 2.80 g in pure water. Calculate the percentage change in mass.
Show the answer
Answer: 12 %
Change = 2.80 − 2.50 = 0.3 g. 0.3 ÷ 2.50 × 100 = 12%
A potato piece goes from 3.20 g to 2.88 g. Calculate the percentage change in mass. Include a minus sign if it lost mass.
Show the answer
Answer: -10 %
Change = 2.88 − 3.20 = −0.32 g. −0.32 ÷ 3.20 × 100 = −10%

Check you have got it

Answer 6 quick questions with instant marking. If you get one wrong, GCSE-ready shows you why and gives you another go. It is free, and you do not need an account.

Practise this lesson free (6 questions)All GCSE and International GCSE Biology lessons

Video for this lesson coming soon. Subscribe on YouTube

Sources and references

Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.

What do exam command words like explain and evaluate mean? →