• Gather materials and make copies of the Surface Tension Boat Template.
• Divide the class into teams of three students each.

Part 1: Surface Tension

1. (Optional) Conduct the quick pre-activity demonstration described in the Assessment section. The demo requires wax paper, pennies and water.

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1. Tell students that they are going to test all of the methods that were suggested in the activity introduction for how to get a paper clip to "float" on water.
2. Ask for student predictions. Who thinks that they will be able to float a paper clip on water?
3. Pass out materials to students.
4. Have student teams begin testing ways to get the paper clip to "float." Which ways work? Which ways do not work?
5. If students cannot get the paper clip to float at all, suggest they place a paper clip on a small piece of toilet paper, and then place it gently on top of the water. The toilet paper should come apart, leaving the paper clip floating on the water.
6. After all teams have a floating paper clip (see Figure 1), bring teams back together for a class discussion:

• Who was able to get the paper clip to float on water?
• Which methods worked? Which did not?
• What property does water have that let us do this? (Answer: Surface tension.)

Figure 1. A paper clip floats on the surface tension of water. click for copyright

Part 2: Breaking Surface Tension

1. Tell students that you are going to drop a few droplets of liquid soap into their water before trying to get the paper clip to float again.
2. Ask for predictions: With soap added to the water, who thinks it will be harder to float the paper clip? Who thinks it will be easier?
3. Drop a few drops of liquid soap into each group's water.
4. Have students test all of the ways that worked before to get the paper clip to float on water.
5. After all teams tried to float the paper clip, bring them back together for a class discussion:

• Who was able to get the paper clip to float on water? (Expect that fewer students were able to get a paper clip to float than in Part 1.)
• Which methods worked? Which did not?
• What happened? What was the difference? (Answer: Soap made it harder to float the paper clip.)

Part 3: Surface Tension/Surfactant-Powered Boats

1. Using the attached Surface Tension Boat Template, cut out small paper boats. They should look like and be shaped exactly like those illustrated in Figure 2.

Figure 2. Surface tension boat template. click for copyright

1. Place the paper boats into a small tub filled with water. Ask the students: How might we power this boat? How can we propel it from one end of the tub to the other?
2. Let's think about surface tension. What happens when we release any kind of tension? Do you think that happens with surface tension? How can we release surface tension? Let's try the soap.
3. Have students drop 1-2 droplets of liquid soap into the small triangle cutout at the back of the boat. The boat should propel at least a few inches. Will it work again? Try it? Did it work? (It only works a few times.)

Part 4: Post-Activity Discussion and Closure

Why were we able to get a paper clip to float on water? Well, water and other liquids have a property called surface tension. What do the words "surface" and "tension" make you think of? Surface may make you think of the top of the water, where the water meets the air. Tension might remind you of something tight, pulled or strained. Put those together and you have it; water on the surface has a tendency to be tight with the water around it, resulting in a property that pulls it together. This is called surface tension.

So, what happened to the surface tension when we added soap? Well, soap is what we call a surfactant. This word comes from two words: surface and active. (Write these on the board.) Surface, because soap acts on the surface, and active because it acts on the surface. Surfactants act on the surfaces of liquids, in this case water, to reduce surface tension.

What property kept the drops of water together on the wax paper and pennies in the first demo? (Answer: Surface tension. See the pre-activity demo in the Assessment section.) In what situations might a surfactant be useful? (Answer: When trying to remove something floating on the surface of water, like an oil spill.) In what sort of situations might a surfactant not be useful? (Answer: When you are trying to float something on water, like a paper clip.)

In what ways have engineers used the property of surface tension to design things to help people? Engineers understand how surface tension works and use it in many ways. They design printers and printing presses in which surface tension is used in the ink application process. Environmental engineers who clean water and air pollution take advantage of surface tension. They design systems that use the condensation of water, for example, to make drinking water from salty ocean water, or use water vapor to remove pollutants from dirty air. Surface tension determines whether things can float or not, which applies to many everyday situations. Surface tension is a consideration in space flight (zero gravity) — liquids cannot be stored in open containers because they will run up the vessel walls.

In what ways have engineers used surfactants to design things to help us? Engineers figure out ways to use surfactants to break up surface tension. For example, surfactants, like soaps and detergents, break up all kinds of oil, which helps in cleaning up messes. Environmental engineers use surfactants to clean hazardous waste areas or spilled airplane fuel on runways or spilled oil on the surface of the ocean. Chemical engineers design cleaning products using surfactants — everything from laundry soap to shampoo.

How did the surfactant help the little boats move? What happened? (Answer: The surfactant acted to break the surface tension.) Why does it only work once or twice? (Answer: You can only break the surface tension a few of times, and then it stays broken.)

Who knows what a water strider is? (At least one student might know that water striders are insects that "walk" on water using surface tension; see Figure 3.) Do you know what property water striders use to "walk" on water? (Answer: Surface tension.) Engineers at MIT and Carnegie Mellon have recently developed a mechanical robot water strider that can also "walk" on water (see photo at http://news.bbc.co.uk/2/hi/science/nature/3126299.stm). How might something like this be useful? (Encourage creative ideas.)

Figure 3. A water strider. click for copyright

Think About It Demo: Drop a few drops of water on a sheet of wax paper placed on a tabletop. Drop a drop of water on the face of pennies scattered on students' desktops. Ask students to brainstorm with their teammates to come up with ideas about why the drops of water stay together. Why don't they go all over the place? Why do they seem to form spheres? Have students keep these ideas in mind throughout the activity.

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Class Questions: Throughout the activity, ask students the questions embedded at each stage of the Procedure section.

Drawing: Have students draw or explain in their own words their answers to these questions: What is surface tension? What does a surfactant do?

Story time!: Have students write a story about a water strider that lives happily in a very clean pond, until — somehow — a surfactant is added to the pond water. Have them include in the story how the water strider lived before, what happened because of the surfactant, from where the surfactant may have come, and what an engineer might do to help the poor water strider. Learn more about water striders at MIT's Water-Walking website: http://www-math.mit.edu/%7Edhu/Striderweb/striderweb.html.

Re-Engineering: Have students redesign their paper boats from Part 3 (of the Procedure section) to travel even further. Test the new designs and keep making improvements.

Penny Challenge: Challenge students to see how many drops of water they can put on the face of a penny before the water spills off. What property holds the drops of water together on the penny? (Answer: Surface tension.) What happens if you add a drop of diluted liquid soap? (Answer: The water spills off the penny.)