You know that saying, “You could cut the tension with a knife”? Well, this is like that, but also not. In this quick and easy science demo from Demo Science, you’ll be working with the surface tension of good ol’ tap water. You won’t be cutting anything, though—this one is closer to H2O acupuncture.
Don’t Drink the Water!
For this one, you’ll need a clear drinking glass, a mess of (preferably straight) pins, and enough water to fill the glass to the brim. Obviously, start by setting the glass on a desk or countertop and filling it with said water, up to the tippity-top brim. The water level should be even with the top edge of the glass.
Have your students gather ‘round, and ask them how many pins they think you can add to the glass before the water overflows. (At this point, I realize you should probably have a towel on hand, too, to clean up spilled water, so add that to the supply list.)
Once all the smelly little goobers have made their guesses, hold one pin over the glass so that its point just barely breaks the surface of the water. Let go gingerly so that the pin slides gently into the water, and wait as it settles to the bottom.
Then, keep adding pins, one at a time, in the same manner. As you do so, tell the students to watch the surface of the water; if they can stand or duck or crouch or whatever so that they’re pretty much eye-level with the top of the glass, they’ll have the best angle. Keep adding pins until the water finally overflows. If you set everything up right, it should take somewhere between six and four-thousand pins.
Chances are good that your students (or whoever you’re doing this experiment with) will have guessed way low on the number of pins needed to overflow the glass. As the number of pins increased, they should have observed the water level rising above the edge of the glass, long before it actually spilled over.
Surface tension is what kept the water from overflowing, even as you added more and more pins and the water kept rising. Water molecules are naturally “attracted” to each other via various intermolecular forces; this is what creates surface tension. It takes a good deal of force to break those bonds—the weight of the water that was “out” of the glass at the top had to be great enough to overpower the strength of the surface tension.