How many drops of water can fit on a penny? Take a guess. Go on. Right down your prediction. In the following activity, we will see how close you came to guessing correctly. The answer might surprise you.
Here's What You Need
Here's What You Do
Place the penny on a flat surface. Fill you eye dropper or pipette with water from the cup. Carefully place drops of water on the penny. Remember to count each drop. What do you notice about the water? Stop adding water when the water runs off of the penny. How did you do with your guess? Were you close?
Here are some things to test:
- Add a little dish detergent to the cup of water. How does this change the way the water behaves on the penny?
- Add salt to the cup of water. Does this change the results?
- Try a different coin. Do pennies hold the most water?
You probably managed to fit quite a few drops of water on the penny (more than you probably guessed). This is due to two forces; cohesion and adhesion. Adhesion is the force the causes the water to stick to the penny. It's one of the reasons the water doesn't just slide off the penny. Cohesion describes the force between the individual water molecules. It's as if the water molecules were holding hands with one another. Cohesion is reason that the water forms a large drop on top of the penny.
water striders can actually walk on the top of water