What it shows
The surface of water acts like a thin stretched skin that holds light grains of pepper on top. A touch of soap breaks this skin in one place, and the pepper rushes away from it.
The science
Water molecules pull on each other strongly and make a skin called surface tension. Soap molecules push in between them and weaken the pull. The stronger pull from the untouched water drags the pepper along the surface to the edges, which is why it rushes away from the soap.
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 steel plate or shallow dish
- Water
- Black pepper powder or talcum powder
- A cotton bud or a toothpick
- Dishwashing liquid
- A second cotton bud for plain tests
- A paper towel
Safety
For this project: Do not taste the water. Keep pepper away from the eyes and nose, and wash hands after using soap.
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
7 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.
Pour water into the plate until it is about one finger deep.
Shake pepper lightly over the surface to make an even layer.
Touch the middle of the pepper with a clean cotton bud dipped in plain water and watch what happens.
Dip a fresh cotton bud in dish soap and touch the middle again.
Watch the pepper shoot to the edge of the plate, and note how fast.
Touch the pepper again with the soapy bud and see that it does not move once the soap has spread.
Wash the plate, add fresh water and try again with the other tools, such as a toothpick.
What you should see
Plain water changes nothing, but a touch of soap makes the pepper dash to the plate edge at once. Once soap is spread through the water the effect stops.
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 skin-like pull on the surface of a liquid.
2What does soap do?
It weakens surface tension where it touches.
3Why does the pepper move away?
The stronger pull of the clean water drags it outward.
4Why does a needle float on water?
Surface tension supports it.
Take it further
Float a paper boat with a notch of soap at the back and see it travel.
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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