Overview:
In this activity, we will explore how insects like the water strider are able to “walk on water” with a few surface tension experiments. Water is a molecule (a group of two or more atoms) that consists of two atoms of hydrogen and one atom of oxygen. A water droplet that we can see with our eyes contains millions of molecules of water that are so small, we cannot see them with our eyes.
Each water molecule exerts a force upon its neighboring molecules, known as intermolecular forces. These forces are due to attractions between molecules. The hydrogen ends of the water molecule tend to be slightly positive, while the oxygen tends to be slightly negative. Positive and negative forces attract each other, i.e. opposites attract. Like forces would push against each other. You’ve likely seen a similar phenomenon with magnets. If you hold the north ends of two different magnets together you feel the forces repel against each other! We refer to these forces as hydrogen bonding and Van der Waals forces. It takes a certain amount of energy to break the forces, just like it would take a certain amount of force to pop a balloon.
The surface of the water is unique because it experiences forces below and to the sides. Whereas inside of the interior of a water droplet has uniform attractive forces that go all the way around each molecule, on the surface of the droplet only the sides and bottom of the molecule experience attractive forces.
Procedure
- Obtain your kit for the Surface Tension of Water experiments.
- Set up your station by arranging your cups and the penny on a table or other level surface. Beside one cup, place the paper clip. Beside the second cup, place the packet of pepper. Place the rest of the supplies to the side.
- Once you have all of your supplies on your table, fill both cups about one half to three quarters of the way full with the water.
In the next step, you’re going to be adding drops of water one and at time to the flat surface of the penny. How many drops do you predict you will get?
- Using one of the droppers, dip into one of the cups of water. Begin adding drops of water to the penny one at a time. Keep track of how many drops you get on the penny before the surface tension breaks and the water falls off the side of the penny. Record your count. Was your prediction close?
- Next, we will observe how the surface tension of water can be broken. We are going to place the paperclip carefully in one of the cups with water. Do you predict it will sink or float?
- Pick up the paperclip and carefully place it on top of the water in one of the cups. Was your prediction correct? The surface tension of the water is strong enough to hold the paperclip!
- If the paperclip does sink on your first try, try using a piece of tissue paper a little larger than the paperclip and placing it on the surface of the water first. The tissue will eventually sink, and the paperclip should be supported by the surface tension!
- Take the second unused dropper and dip it into the liquid soap. Add one drop of soap to the cup with the paperclip. What happened? Why do you think that is?
- Carefully open the pack of black pepper. We’re now going to add it to the second cup of water. Do you think it will sink or float? Once you have made your prediction, shake it carefully into the second cup of water.
- Observe what happens with the pepper. The surface tension, again, is strong enough to hold the pepper on the surface. Now, once again, see what happens when you add one drop of liquid soap to the cup with the pepper. What happened? Can you explain why that might happen?
Adapted from: Home Science Tools. https://learning-center.homesciencetools.com/article/experiments-with-water-science-project/ (Accessed January 20, 2021), Surface Tension Experiments.
