Applies to: AQA GCSE Biology and Combined Science (Required Practical 3). Edexcel, OCR and the IGCSE boards all include an osmosis practical too, so this one is worth doing whichever board you're on.

Leave a chip of raw potato in a glass of very salty water overnight and it comes out soft and bendy. Leave one in plain water and it comes out crisp and slightly heavier. Nothing you can see has been added or taken away. Water has quietly moved in or out of thousands of cells, and you can measure it with a set of kitchen scales.

What it tests

How the concentration of a salt solution affects the change in mass of potato pieces, and the point where the potato cells and the solution are balanced.

What you need

One large potato, about 40 g of table salt, five glasses or jars labelled 0, 2, 4, 6 and 8, a measuring jug, kitchen scales (ideally ones that read to 0.1 g), a knife, kitchen roll and a timer.

How to do it

Pour 100 ml of tap water into each glass and stir in 0, 2, 4, 6 and 8 g of salt to match the labels. Ask an adult to cut five potato chips the same size, about 1 cm by 1 cm by 4 cm, with the skin removed. Blot each chip with kitchen roll, weigh it and write down its start mass. Put one chip into each glass, making sure it's fully covered, and leave them for at least an hour (overnight is better). Then take them out, blot them dry in exactly the same way and weigh them again.

Work out the percentage change in mass for each chip: change in mass divided by start mass, multiplied by 100.

Safety note

Ask an adult to help with the cutting, and cut on a chopping board away from your fingers. Don't eat the potato afterwards, and wipe up any spilt salt water so nobody slips.

What you should find

The chip in plain water gains mass and feels firm, because water moves into the cells by osmosis. The chips in the stronger salt solutions lose mass and go floppy, because water moves out of the cells into the more concentrated solution. Plot percentage change in mass against salt concentration, and the point where your line crosses zero is where the solution matches the concentration inside the potato cells.

Why it matters for the exam

This practical comes up again and again. Be ready to explain why we use percentage change rather than just change in mass (the chips start at different masses, so percentage change makes the comparison fair), why every chip is blotted in the same way, and how to read the concentration where the line crosses zero on a graph. In school you'll use solutions measured in mol/dm³ and a cork borer, but the skills and the science are exactly the same.

On motivation: leaving something overnight and coming back to a result is a brilliant hook for students who struggle to get started. The waiting does half the work, and a floppy potato is far more memorable than a paragraph in a textbook.

Free download: the Practicals at Home PDF

I've put everything for this practical into one free PDF: the home method, a results table, sample results if you don't have the kit, the school lab method the exam questions are based on, exam-style questions and a full mark scheme so you can check your own answers.

Download the Osmosis in potato PDF

Watch and listen: Key Ideas

If you'd like the theory behind this practical explained in a different way, I've made a short Key Ideas video and a podcast on diffusion, osmosis and active transport. Both are short enough to fit in a bus journey or a dog walk.

• Watch the Key Ideas video: Diffusion, osmosis and active transport

• Listen to the Key Ideas podcast: Diffusion, osmosis and active transport

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