Count the drops
Drip, drip, drip. How many drops can you place on a 20¢ coin?
What you need
- 20¢ piece
- clean eye-dropper
- cup of water
- detergent
What to do
- Make sure the 20¢ piece is clean and dry. Lay the coin flat on a table.
- Partially fill the eye-dropper with water and practice using the dropper over the cup of water until you can let the drops out one at a time.
- Hold the dropper approximately one centimetre above the coin and drip the water one drop at a time onto the 20¢ piece, counting the drops.
- Note the number of drops that can be placed on the 20¢ piece before the water spills over the edge of the coin.
- Thoroughly dry the coin and do the experiment two more times. Compare the three results and then compare your results with the rest of the class.
What's happening?
Water has a high surface tension. Surface tension helps a drop of water hold its shape. If the coin is dry and flat, the water can be added until it’s almost twice as high as the coin itself.
Add a drop of detergent to the cup of water and repeat the above experiment. Are your results different this time?
Detergent lowers the surface tension of water and the soapy water cannot form a large drop on the coin.
