What it shows
The surface of water behaves like a thin stretched skin. Soap weakens this skin where it touches, so the stronger pull from the surrounding water drags the floating pepper outwards.
The science
Water molecules attract each other strongly, so those at the surface are pulled inwards and sideways, giving a tension that holds small things up. Pepper floats on this surface. Soap molecules spread over the water and reduce the surface tension at that place. The pull of the untouched water around it is stronger, and it drags the pepper away in all directions. A soap boat moves because soap leaks from the back and lowers the tension there, so the surface in front pulls the boat forwards.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A wide, shallow plate or tray
- Water to fill it to 1 cm depth
- Ground black pepper
- A toothpick
- Liquid soap or dishwashing liquid
- A thin piece of plastic foam or card, cut as a boat
- A paper clip
Safety
For this project: Wash hands after the experiment. Do not put soap near the eyes or mouth.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Fill the plate with water and wait until it is still.
Shake pepper lightly on the water so that it covers the surface in a thin layer.
Dip a toothpick in plain water and touch the middle of the pepper. Nothing much happens.
Dip another toothpick in soap and touch the middle of the pepper. Watch the pepper rush to the edge.
Cut a boat from foam or card with a small notch at the back.
Place the boat gently on clean water in a new plate and put a drop of soap in the notch.
Watch the boat move forward. Try boats of different shapes and time how long each takes to cross the plate.
Place a paper clip carefully on the surface using a tissue and see it float, and then add soap and see it sink.
What you should see
The pepper moves away from the soap at once, the soap boat glides across the water and the floating paper clip sinks after soap is added.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is surface tension?
The tendency of the surface of a liquid to behave like a stretched skin because of attractions between molecules.
2How does soap affect surface tension?
It lowers it.
3Why does the pepper move to the edge?
The water with higher surface tension pulls it away from the soapy part.
4Why can some insects walk on water?
Their light bodies are held up by the surface tension of water.
Take it further
Compare the time a boat runs with soap, with lemon juice and with cooking oil and find which one drives it best.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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