Facilitator/Educator Guide: Investigating Surface Tension with Paper Fish
What causes dew to form into drops on plants, water to bead on a waxed car, and bubbles to accumulate in soapy water? The answer is surface tension. Learn more about surface tension with a pan of water, some dishwashing liquid, and a paper fish.
A glass filled to the brim with water is a visual demonstration of the physical phenomenon of surface tension. Collections of molecules, like the molecules in a glass of water, experience intermolecular forces. Any and every molecule in the glass of water will be pushed and pulled by its neighboring molecules. Intermolecular forces are also known as Van der Waals forces. Since each molecule in the glass of water is being pushed and pulled by its neighbors, the net force on each molecule is zero.
The molecules at the top surface of the water also experience the effects of intermolecular forces, but these molecules aren't surrounded on all sides by other water molecules. Instead, there is an interface between the water molecules at the surface and adjoining air. As a result, the water molecules at the surface cohere more strongly to neighboring water molecules at the surface. The intermolecular forces between the water molecules and the air at the surface are weaker than the intermolecular forces between the water molecules at the surface. The enhanced intermolecular forces between neighboring water molecules at the surface form what is called surface tension and cause the water at the rim to curve upward slightly above the level of the rim.
Because the water molecules at the surface cohere strongly to each other, the amount of force required to push an object through and beneath the surface of the water is greater than that required to push the object through the water within the glass. This explains how water-strider insects seemingly "walk" on water and why a needle or paper clip floats on the surface.
In this physical science classroom activity, you will demonstrate surface tension by making a paper fish "swim" in a pan of water by placing the fish on the surface of the water and then adding dishwashing liquid. Adding the dishwashing liquid to the water reduces the surface tension, and the change in surface tension will cause the fish to move forward as if it were swimming.
This science activity can serve as a starting point for a variety of physics discussions. Here are a few questions that can be used to start a discussion:
- What are some examples of surface tension that you can see around you?
- How does surface tension play into creating soap bubbles?
- Will the surface tension of a liquid change if you mix another liquid into it? Why or why not?
Needed for preparing ahead:
- Fish template (1)
- Printer paper
- Scissors (1 pair)
- Medicine dropper (1 per demo or small group)
- Dishwashing liquid (enough to fill one medicine dropper for each small group)
- Aluminum roasting pan, 12 x 15 inches (1 per demo or small group)
- Cold tap water, enough to fill the roasting pan halfway
Needed for each demo or small group at the time of the science activity:
- Paper fish (1)
- Medicine dropper filled with dishwashing liquid (1)
- Aluminum roasting pan, 12 x 15 inches, filled halfway with cold tap water (1)
|Figure 1. You can gather the materials needed for this classroom activity from around the house and classroom.|
What to Do
Prepare Ahead (10-20 minutes)
- Print out the fish template shown on paper and then cut it out along the solid lines. Cut out one paper fish per demo or small group.
|Figure 2. Fish template|
- Fill each dropper (one per demo or small group) with dishwashing liquid.
- Fill each roasting pan (one per demo or small group) halfway with cold water.
Science Activity (< 10 minutes)
- Each group should have a roasting pan filled halfway with cold water, a dropper of dishwashing liquid, and a paper fish.
- Have a member from each group gently place their fish on the water's surface at one end of the roasting pan.
- Using the medicine dropper, a member from each group should carefully place a drop of dishwashing liquid in the hole in the center of the fish. What does the paper fish do?
Placing a drop of dishwashing liquid in the hole on the fish will reduce the value of the surface tension of the water in the hole. The surface of the water surrounding the drop of dishwashing liquid has a higher surface tension, so the surface pulls away from the drop of dishwashing liquid, which results in the paper fish moving forward. When the dishwashing liquid mixes evenly with the water, and the surface tension equalizes in the pan, the fish will eventually stop.
For Further Exploration
This science activity can be expanded or modified in a number of ways. Here are a few options:
- Try dropping other liquids, like vegetable oil, in the hole on the fish and see if the fish still swims.
- How long does it take for the fish to "swim" from one end of the pan to the other? Does the speed depend on the kind of liquid you drop into the hole on the fish?
- Try to make the fish from different kinds of paper, such as wax paper or construction paper, as well as aluminum foil. Do fish made from different materials swim just as fast as the original paper fish?
- Replace the cold water with hot tap water and repeat the experiment. How does the water temperature affect how the fish swims?
- Try these additional Science Buddies Project Ideas Measuring Surface Tension of Water with a Penny and Build a Raft Powered by Surface Tension that investigate surface tension.