You and Your Students!
Vicki Cobb, Education World Science Editor
Watch a battle as two liquids struggle for supremacy.
Physical science, Water
Setting the Scene (Background)
The surface of water, where it meets the air, acts like a skin. That's because water molecules are more attracted to each other than they are to air, so they pull together with a force called surface tension. Needless to say, surface tension is not a very strong force but some liquids, like alcohol, have an even lower surface tension than water does. If you drop some alcohol in the middle of a shallow pool of water, you can see a dramatic struggle between the two liquids!
This is a very dramatic demonstration that uses an overhead projector in an unusual way. Some kids will want to try this experiments themselves after you demonstrate it.
The "war zone" between the battling liquids is a glass baking dish placed on an overhead projector.
Students will observe how the water retreats from the alcohol, and the alcohol follows -- leaving a clear, dry spot in the center of the dish. Let students describe the struggle between the two liquids at the borderline where they meet. Eventually, the liquids merge and the troubled waters are peaceful again.
Challenge students to explain what's happening
Behind the Scenes
Both the water and the rubbing alcohol have surface tension, but the water's surface tension is stronger. When you drop in the rubbing alcohol, you're creating a surface of alcohol in the middle of the water, which creates the "tug of war" between the liquids. The water pulls away because, with its stronger surface tension, it is more attracted to itself than to the alcohol. With its weaker surface tension, the alcohol is more attracted to the water than to itself. So the alcohol follows the water, leaving a dry spot behind.
But that is just the beginning
How is the conflict of the warring liquids resolved? Solution is the solution. The activity at the borderline is created as the alcohol and water mix. Eventually, the alcohol spreads evenly throughout the water. Only then does the new alcohol-water solution move over the dry spot and re-form an unbroken surface.
Another way you can see the difference in surface tension between alcohol and water is to put equal-sized drops on some waxed paper. If you look at the drops from the side, you will see that the water forms a rounder drop. It pulls together more strongly than the alcohol.