What it shows
Water drops hold together like a skin and make a dome. A coin can hold many drops before the water spills over.
The science
Water particles pull on each other, which makes the surface behave like a thin skin. This is called surface tension. It holds a drop in a dome shape on the coin. Soap weakens the pull, so the drop flattens and spills earlier.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Two coins of the same size (free)
- A dropper or a straw
- A cup of water
- A sheet of wax paper or a plastic sheet
- A drop of dish soap
- Paper and pencil
Safety
For this project: Wipe spills. Do not drink the soapy water.
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.
Place the coin flat on a plate.
Guess how many drops of water will fit on it and write your guess.
Use the dropper to add drops one at a time and count them.
Stop when the water spills over the edge and write the count.
Dry the coin and repeat with the second coin that has a drop of soap on it.
Compare the number of drops in each case.
Put drops on wax paper and see that they form little balls.
Tilt the paper and watch the drops roll.
What you should see
Plain water makes a tall dome and holds more drops. Soapy water spills sooner.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does the water form a dome?
Its surface acts like a skin.
2What does soap do to the drops?
It weakens the skin so the drop flattens.
3Why does water make balls on wax paper?
Wax does not wet, so drops keep their shape.
4Name an insect that walks on water.
A water strider.
Take it further
Try cooking oil and juice on the coin and compare the drop counts.
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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