Measure Surface Tension with a Penny

A soapy science activity from Science Buddies

By | |

Learn about the secrets of soap in this surprising surface tension-testing activity!

Key conceptsMolecules

Introduction

Background

Now, think about washing dirty dishes or clothing. There will be lots of tiny little holes and cracks that water needs to get into to wash away dirt and grime, such as the spaces between the fibers of a shirt or between a plate and bits of dried food. In order for water to flow more easily into these small spaces, you need to its surface tension. You can do this by adding soap, which is a surfactant (a material that decreases the surface tension of a liquid). In this activity you will see how soap decreases the surface tension of water by putting water droplets on top of a penny.

Materials

• Penny
• Medicine dropper or eyedropper
• Glass, cup or small bowl
• Tap water
• Dish soap
• Dish towel or paper towel
• Flat, level surface that can get wet, such as a kitchen counter
• Paper and pencil or pen (optional)

Preparation

• Place your penny on a flat, level surface that can get a little wet, such as a kitchen counter.
• Fill a glass, cup or small bowl with tap water.

Procedure

• Fill the medicine dropper with water.
• Now carefully add one drop of water at a time to the top of the penny. Hold the medicine dropper just above the top of the penny (not touching it) so each new drop has to fall a short distance before it merges with the drop on the penny. You can write down the number of drops you add if you like. Watch the drop on top of the penny carefully as it grows. It should keep getting bigger and bigger until it touches the edges of the penny.
• Keep adding drops (refill your medicine dropper as necessary) one at a time.
• Once the drop spills over the penny’s edge, use a towel to completely dry off the penny and surrounding surface.
• Mix a small amount of dish soap with your tap water.
• Now, repeat the experiment using soapy water. Again, slowly add one drop at a time.
• Extra: Try the experiment with different liquids or other things you can find in your kitchen. (Make sure you have an adult's approval to use any liquids before you handle them.)
• Extra: Try using something other than a penny to collect the droplets. What happens if you use different materials, such as the flat top of a small plastic bottle cap or a button?

Observations and results

More to exploreSticky Water, from ExploratoriumSoap, from ExploratoriumMeasuring Surface Tension of Water with a Penny, from Science BuddiesSurface Tension Science: Build a Raft Powered by Soap, from Science Activities for All Ages!, from Science Buddies

Science Buddies

see also: