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WHAT DO DROPS OF LIQUID DO?

Today we are going to look at drops. We have water, honey, cooking oil, and alcohol. There is waxed paper, four medicine droppers, and a box of toothpicks.

Let's pass the waxed paper and tooth-picks so that each student has a square of paper and a couple of toothpicks. Our four liquids and our four medicine droppers on a tray will be passed around so that each student can put two drops of each liquid onto his or her waxed paper. Be sure to put the second drops next to the first drop of each liquid, so that water is next to water, and honey is next to honey.

Look at the drops now. Scrunch down and look at them from the side, so you can see their profiles. What shape the drops form after being placed on the waxed paper. Are they all the same, or are some of them fatter than others? Are the fatter ones smaller from side to side? We could make charts to show their shapes and sizes.

Now, with your toothpicks, push together two drops of the same liquid and see what happens. Do this with all four liquids. Do they all react the same?

It looks like the first drops of the liquids pull in the second drops, and become fatter, as well as wider. Just as the two drops touch, there is a jerking motion that combines
the two drops. It is almost as though the two drops don't want to be just next to each other and touching, but that they want to take up as little space as possible, and be one drop. (We know that liquids are not able to think or to want; the reaction they have is described that way so that we can understand what they do.)

The tendency of liquids to reduce their open surface is called "surface tension." Remember that.

When we get lucky, we'll learn more about surface tension. I'm wondering if that has anything to do with the reasons some things float? I don't know yet, but we'll all find out together on another day.

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