All Washed Up, by John Cowens

Aluminum foil boats, dishwashing liquid, forks and paper clips make for great science adventures with water

I love any science activities involving water! It's a plentiful, inexpensive material, and my students' spills and splashes give me the perfect opportunity to clean my classroom.

Bonding time
Every molecule in water attracts its neighbor. The reason for this is because the two hydrogen atoms that are attached to one side of a water molecule attract the single oxygen atom of another molecule. This mutual attraction of molecules in water is called hydrogen bonding.

To strengthen the bond, hydrogen atoms are positively charged while oxygen atoms are negatively charged. This creates surface tension (see Figure 2).

When soap is added to water, the surface tension is weakened. This is due to sodium dodecyl sulfate, which is the chemical found in soap that reduces the surface tension of water.

Here are three short activities involving the weakening of surface tension.

Soap-powered boats (Grades K-6)
Duration: one 45-minute session (unless you include the extension activities).

Materials: two four-inch square pieces of heavy-duty aluminum foil, a 3 x 5 index card, pencil, scissors, dishwashing liquid, a large pan that is approximately 12 x 20 x 1, water, masking tape, ruler or meter stick.

Use clean, soap-free water for each experiment and make sure your pan is large enough so the boats can't sail to the other end of the pan. I have good luck using a large cookie pan from our school's cafeteria.

Draw boat shapes on two index cards and cut them out (see pattern, Figure 1). Trace the patterns on heavy-duty aluminum foil and cut them out carefully. Pour cold tap water into a large flat pan to a depth of approximately 1/2". Place the aluminum "boat" on the water at one end of the pan (Figure 2). Place a single drop of dishwashing soap into the round hole (Figure 3).

Next, use a measuring tape to measure how far the boat travels before it stops. Then, empty the soapy water and rinse the pan thoroughly. Repeat the above procedure starting with adding tap water to the pan. Use the other aluminum "boat." Which boat traveled father? Why? Which travels faster? Why?

This is the perfect time to make kids realize that good scienceing involves repeating experiments several times in order to collect accurate data.

What makes the boat move?
Soap breaks the attraction and lets water molecules spread farther apart. That lessens the "pull" that the water molecules exert on the foil boat. When set free, the boat can skim across the water, riding a "wave" of surface tension.

Extensions:

1. Is there a way to improve your boat's design? Try different shapes. Try different size holes and positions in the back of the aluminum boats.

2. Make a boat of different materials: playing cards, index cards, file folders, vinyl sheets or plastic-coated paper. Which material makes the best boat?

3. Try different brands of liquid dishwashing soaps, bubble bath crystals, hand soap and so on.

4. Try racing your boat across orange juice, punch or Kool-Aid.™ Do these liquids have more water tension, i.e. a thicker "skin?"

Surface tension is sticky water? (Grades 4-7)
Here's a lesson that proves metal paper clips can float on water.

Materials: a small paper clip, a medium-size bowl, fork, liquid soap detergent.

Fill the bowl almost to the top with water. Place a paper clip on the tines of the fork. Slowly and carefully lower the fork onto the surface of the water. As the paper clip clings to the surface, carefully pull the fork away from the clip. The paper clip should be floating on the water's "skin." (See Figures. 5, 5a and 5b.) Why is the clip floating?

Place one drop of liquid detergent toward the edge of the dish. What happens to the paper clip?

Try this activity several times, cleaning the dish thoroughly each time to remove any soap residue.

Assessment: Have students explain why the metal paper clip floats on the water, but sinks when a drop of liquid soap is added to the dish.

Extension: Have students research and find several organisms that benefit from surface tension. For example, water striders (order Hemiptera, family Gerridae) are insects that live on the surface of the water in slow moving streams as well as in oceans. Their speed across the "skin" of water is about one meter per second!

Cooking oil and water (Grades K-4)
A quick (less than 30 minutes), fun experiment packed with lots of learning.

Materials: You'll need cooking oil, water and a cup for each child.

Place a few drops of cooking oil on the palm of the hand. Fill a cup with water and slowly pour some onto the palm of the oily hand. Notice what shape the water retains. (Figure 6). Wash hands thoroughly with hand soap. Now, pour water onto the palm of your cleansed hand and notice what happens to the water.

When the hand was oily, the water molecules bonded together tightly and formed small balls or drops which rolled over the palm of the oily hand. The surface tension of water was easily observed.

When soap was used to wash the oiled hand, its molecules were attracted to both water and oil. Soap molecules bonded to oil and water causing the surface tension of water to be greatly reduced. As a result, the oil washed off the hand.

Wetter water
Under a microscope dirt, oil and grease (grime) can be seen locked in the fibers of material. When soap is added to water, surface tension is reduced making water "wetter." As the washing machine's agitator shakes, rattles and vibrates, the wetter water is able to penetrate the nooks and crannies of the material and flush out the grime.

The major reason for using hot water when washing clothes is that its surface tension is lower, making it a better wetting agent. If the detergent lowers the surface tension, the heating may be unnecessary. Which laundry soap makes water wetter to combat the grime? Discuss the various findings in group presentations.

John Cowens teaches science at Fleming Middle School, Grants Pass, OR.