Liquid surface tension is a heck of a thing. The difference between something staying on top of the water, buoyed by that tension, or breaking the surface and falling in is infinitely minute. Even the smallest change can make something on top of the water suddenly sink. To help you explain to your students (or whoever) how the surface tension of water works, we present this fun and easy Demo Science science demo. Have fun!
All Aboard the SS Tin-tanic
You get to make—and sink—a tiny boat for this one. How weirdly fun is that? For this experiment, you’ll need three things: a fairly large container full of water (like a sink), a bottle of dishwashing soap (not dishwasher soap), and a little aluminum boat, which we’re going to say is tin, because Aluminum-tanic doesn’t work.
There are three options for crafting your watercraft. One, you can just use tinfoil: cut a piece about two inches wide and four inches long, fold it into a generally boat-like shape, and set sail. Two, if you care to be a little more, um, careful, you can cut up an empty beer soda can, press it flat by leaving it under something heavy for a while, then fashion that into boatesque form. Three: find a place that does metal stampings and have them crank out a custom, old-school Chris Craft-looking aluminum piece for you.
Gather your students ‘round, fill your sink with water, and float your boat. If you made it right, it should act like a real boat, more or less—by which I mean, “not sink.” Give it a few little pushes so it moves through the water, further demonstrating its boatness.
Ask the smelly little kids what they think will happen when you add a drop of the dishwashing soap to the water. Will there be any effect? Will the boat continue to float? Will it head straight to Davy Jones’ locker? Will the powerful outboard motor churn up a mighty lather?
Then, go Chekov’s gun and drop in a dollop of soap. What happens?
That’s Some Soapy Science, Son!
If all goes as it should, the boat will scoot forward once the soap hits the water. Why? Because the soap breaks the water’s surface tension behind the boat; the surface tension in front of the boat then pulls it forward.
Water molecules are held together by intermolecular forces called bonds, hydrogen bonds. Hydrogen bonds hold water molecules together tightly, creating surface tension. Soap molecules infiltrate the water molecules and push them apart. The increased molecular distance weakens the hydrogen bonds, making them less efficient in maintaining surface tension.