Ever wonder why some bugs can walk on top of water? It is all because of surface tension. Surface tension is a property of liquids that is a result of cohesive forces. Translated: cohesive forces are those forces that hold things together. For example, the reason why water forms a spherical shape as is falls and also when you put a drop of water on the table is because of water’s cohesive force.
Sprinkle some pepper on water and see it float. Maybe if you’re good enough you can make a staple float on it. This week, we will see how the surface tension of water can be really pushed to the limit in a couple of simple experiments: one called “Pennies in a cup,” and the other “Drops of water on a penny.”
A tall drinking glass, a plate, a bunch of coins, water, and a medicine dropper (the plastic one for children’s meds works well).
Procedure for the Pennies in a Cup Experiment:
- Fill a glass to the very top with water. Set the glass onto a plate for collecting any spills that might occur.
- How many pennies do you think you could get into the cup without spilling any of the water? Make a guess!
- Very carefully, add a penny (or other coin) one at a time into the glass.
- Watch the glass from the side to see what is happening as you add more coins.
Depending on how careful you are, you’ll get quite a bit of coins into the glass before spilling the water. You should notice the shape of the surface of the water is a curved surface after about 10 coins. The ability for water to hold to itself like that is the surface tension I mentioned earlier. Instead of spilling or breaking apart, the surface of the water holds together, and instead rises above the surface of the cup!
Procedure for the Drops of Water on a Penny Experiment:
- Put a penny on a plate or other flat surface.
- Using a full medicine dropper, you will be adding one drop of water at a time onto the surface of the penny to see how many drops it can hold before spilling over.
- Take a guess on how many drops!
- Carefully add a drop of water at a time to the surface of the penny, have someone help keep track of how many you do.
Again, you will notice the shape of the water drop as it grows on the penny. It will have that curved shape that forms when water is by itself. Put a single drop on the counter and see what it looks like from the side. Water has strong cohesive forces, which accounts for ability to hold its shape, and for things to float on it.
You used water for these experiments, what else could you use? (Of course, before you start filling cups with expensive liquids, ask your parents, and maybe use a smaller cup?) How wide of a cup did you use? Will it matter if the cup is really narrow? What side of the penny did you put the drops of water on? What does soapy water do to the experiment? Lots of questions! I hope you try to test them out.
I hope you enjoyed this simple experiment. If you have more questions about this, or need tips about science fair ideas around this topic (or others), contact the author.
Steve Davala is a middle school science teacher who likes to write and work with Photoshop. He’s got two kids of his own and subjects them to these science activities as guinea pigs.