Overflowing Coins

What you need

• A glass of water
• A pile of coins
• A saucer
• Some washing up liquid

Instructions

1. Fill the glass to the very brim with water and place on top of the saucer
2. Carefully drop a coin in to the water, edge first
3. Slowly and carefully drop more coins in to the water – how many can you add before the glass overflows?
4. Try again, but this time add a few drops of washing up liquid to the water before you start adding the coins

Results and Explanation

Water molecules are strongly attracted to each other, creating a surface tension that allows the water to bulge over the top of the glass as you add the coins. But washing up liquid reduces the surface tension so the glass overflows sooner.

These experiments have not been specifically safety tested for home use but we believe them to be safe if the instructions are followed. Adult supervision or direction is recommended as appropriate. All experiments are carried out at your own risk.