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SOME THINGS SURFACE TENSION CAN DO

This project will let us in on some more secrets of surface tension. Since pure water has the strongest surface tension of all liquids, we will not limit this experiment to using water only.

For the first test, we need clean, well-rinsed plates (not paper), and a four-inch length of lightweight thread. Fill the plates with water, and make a loop with your thread, by tying the ends together with a knot. Make the loop really round. Lie the loop on the surface of the water; gently, now; don't just plunk it on there. What happened to our lovely, round loops? Surface tension changed that in a hurry, didn't it?

Now, we'll pass around a little bottle of water with dishwashing soap added to it, and each of you may get one drop of soapy water (using your medicine droppers), and put the drop right into the center of the loop.

Now look at that! Nice round loops again! How on earth did that happen? Apparently, the detergent reduced the surface tension of the water, so that it couldn't pull the sides of the loop toward the center.

Or could it be the other way around? Maybe the surface tension (before we added detergent) pushed the loop sides together, rather than having pulled them. That's a heavy thought. Let's rinse our plates and try it again, with fresh thread and plain water. This time, we'll put the detergent outside the loop, and see what it does.

Here's another way to test it: Since we have detergent in our water anyway let's add plain water to the plate, in the center of the loop and see if the loop changes.

Let's do another experiment to compare surface tensions of water and of soapy water. We'll start out again with plain water on a plate. Sprinkle some powder onto the surface and see what it does. We have talcum powder, because it is inexpensive, but other powders work the same (face powder, ground pepper, cinnamon, nutmeg, powdered tempera paint). Wait until the water is calm. When the surface of the water is smooth, put one drop of the soapy water that is passed around the class onto the surface of the water. It's best to put it in the center.

Now what's happening? The surface tension breaks up and draws together away from the drop of soapy water. You can see this easily, because the powder is floating on top due to the strength of the surface tension. The water molecules are staying together and trying not to let another liquid interfere. When the soapy water is stirred into the rest of the water, the powder will sink, because the soapy water broke the surface tension.

 

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