Penny Drops
Students will explore surface tension and how water molecules stick together to form a drop of water by dropping water onto a penny or any other small coin.
What You Need
- Pennies - a lot, each student should get a few
- Water droppers
- Cups
- Sponges or paper towels
- Penny Drops - Worksheet
Guide:
NOTE: You can use different coins as long as they are the same monetary value.
Safety Notes
Ensure you are familiar with Let's Talk Science's precautions with respect to safe outreach to youth.
What To Do
- Hand out worksheet to students
- Appoint groups of 3-4, one student in the group should be in charge of getting water, another student be in charge of wiping up the spilled water. Each student should have pennies and a dropper each.
- Students make an initial guess about how many drops of water the penny can hold.
- With the dropper, students count how many drops of water the penny can hold.
- Students repeat on different pennies.
- Ask students if there is a difference between the heads and tails side of the penny, what about the year? Whether it's American or Canadian?
- Students should record their experiment results on the worksheet.
- Brainstorm what's happening with the water and penny. Ask students if they know what is happening.
- Explain surface tension and answer the questions at the bottom of the worksheet.
- Discuss why differences in coins would change the number of drops the coin can hold.
Discovery
Surface tension is a property of the surface of a liquid that allows it to resist an external force. The water molecules hold on tightly to each other. They especially cling to each other at the surface because there is no water molecule on the other side of them to grab on to. The water droplets keep building on top of each other until a small dome of water forms on the top of the penny. Eventually the water molecules can't hold on any longer and the water dome collapses.
What's Happening?
Surface tension is a property of the surface of a liquid that allows it to resist an external force. The water molecules hold on tightly to each other. They especially cling to each other at the surface because there is no water molecule on the other side of them to grab on to. The water droplets keep building on top of each other until a small dome of water forms on the top of the penny. Eventually the water molecules can't hold on any longer and the water dome collapses.