Measure Surface Tension with a Penny

Summary

Key Concepts

Molecules, surface tension

Credits

Ben Finio, PhD, Science Buddies

Introduction

Have you ever noticed how, on a rainy day, water forms droplets on a window? Why does it do that instead of just spreading out evenly over the whole surface? And what could that possibly have to do with washing the dishes or doing laundry? It all has to do with something called surface tension. Try this activity to learn more!

This activity is not recommended for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.

Background

You have probably noticed that, if you look at a surface outside on a rainy day, or spill some water inside, it tends to form droplets that stick up from a surface, instead of spreading out into an even sheet. This occurs because water is made up of many tiny molecules that are all attracted to each other. Molecules in the middle of a drop of water are pulled evenly in all directions by all the nearby molecules. However, molecules near the surface of the droplet are pulled mostly inward by the water molecules below them. This creates something called surface tension – the surface of the water droplet is held together by the attraction between molecules.

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, like 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 decrease its surface tension. You can do this by adding soap, which is a surfactant, or material that decreases the surface tension of a liquid. In this experiment, you will prove that soap decreases the surface tension of water by putting water droplets on top of a penny.

Materials

Penny
Medicine dropper or eye dropper
Glass, cup, or small bowl
Tap water
Dish soap
Dish towel or paper towel
Flat, level surface that can get wet, like a kitchen counter

Instructions

Place your penny on a flat, level surface that can get a little wet, like a kitchen counter.
Fill a glass, cup, or small bowl with tap water.
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.
Watch the drop 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. How big does the drop on the penny get before it finally spills over the edges?
Once the drop spills over the edge of the penny, 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. How big does the drop of water on top of the penny get before it breaks and flows over the edges?

Extra: try the experiment with different liquids or other things you can find in your kitchen. How do different soaps and detergents like hand soap or laundry detergent compare to each other? What about other liquids like milk or juice? Which ones make the biggest (or smallest) drops?

Extra: try using something other than a penny to collect the drops. What happens if you use different materials, like a small plastic bottle cap or a button?

Observations and Results

You should find that plain tap water can produce a much larger, stable drop of water on top of the penny than the soapy water. This is because plain tap water has higher surface tension, so the surface is “stronger” and can hold together a larger drop. Adding soap lowers the surface tension of the water, so the drop becomes weaker and breaks apart sooner.