Explore Science with a Penny
Discovery Place Science
Welcome to our Stay-at-Home Science series! While most of us are stuck at home and going stir-crazy, why not make science accessible for everyone with some fun experiments you can do as a family?
First up, we’re going to explore surface tension.
Did you know that surface tension allows some insects to glide or float on the surface of water? It is an effect where the surface of a liquid is strong. That strength varies by the type of liquid.
Today, we'll explore this phenomenon by carefully placing drops of water on top of a penny.
This experiment will take about 15 minutes, with three of those minutes used for prep time. This works best with an elementary-aged child.
- Pipette or eyedropper
- Paper towel
Material Substitution Note: Don’t have a pipette or eyedropper sitting around? Placing a piece of tape at the end of a straw and poking a pinhole in the tape will work just fine.
- Fill pipette/eyedropper with water.
- Fold a paper towel in half and set the penny on top.
- Carefully add one drop of water to the top of the penny.
- Look at the dome of water forming on the top of the penny.
- Keep going until it the water spills onto the paper towel.
- Dry off the penny and repeat two more times.
What we learn
Within the body of water, cohesive forces acting on a water molecule pull in all directions and by all molecules within proximity. On the surface, however, molecules feel attractive forces from inside the fluid, but none from outside. This causes the outer layer of water molecules to act like a stretched membrane and minimizes the surface area, creating surface tension. How many drops did your penny hold? Did the number of drops change or stay the same?
How to adjust for younger or older learners
Younger learners might not have the dexterity or muscle control to add one drop at a time. Someone older might have to add the drops while the younger child counts out the number of drops.
For older learners, try using different types of liquids, like olive oil or vegetable oil and compare the results. How do the liquids behave differently? Also try experimenting with the opposite side of the penny, or a different coin to see if the results vary.