Water surface tension. We’ve talked about it here a time or two before, so you may well be an old pro. But it’s one of the most visually interesting demonstrable science concepts, and I know a butt-ton of fun experiments about it, so let’s take another look at it!
To demonstrate water’s surface tension, this Demo Science science demo requires a drinking glass, a cork (like the one from that bottle of shiraz you downed last night), and water. After all that wine, you’ll probably be in the market for a couple glasses of water anyway, so it’s a win-win.
After your whistle has been sufficiently wetted, refill the glass almost, but not quite, completely full. Plop the cork into the water and let it float of its own accord. Challenge your students (or whomever you’re performing this experiment with) to get the cork to float—and stay—in the center of the water without touching the glass.
Let as many of the smelly little buggers as want to give it a try take a crack at it. Maybe even make a few bets, if you can find any takers. The little punks will likely determine very quickly that such a thing is impossible. That’s your cue to prove those nerds wrong.
Leaving the cork in the glass, carefully—carefully!—add more water until it just slightly rises above the rim of the glass. This is often called “full to the brim,” which is about as perfect a description for anything as you’ll ever encounter.
What happens then is nothing short of magical! sciencetastic!
Centered As an Old Center Horse
As your students look at the glass from the side—without touching the table, ‘cause it’ll spill—they’ll see that the surface of the water is actually above the top of the glass. Thanks for keeping the water from spilling over the sides, surface tension!
Surface tension is created by water molecules’ natural attraction to other water molecules, as well as hydrogen bonding and the numerous other intermolecular forces at work. Though water seems pretty easy to move your hand through, for example, it’s actually pretty hard to break the water’s molecular bonds.
Anyhoozle, the cork will now be more or less perfectly centered in the glass, as it will naturally float to and remain where the water is highest—i.e., the center of the glass where surface tension allows the water to swell upward. As if by magic science.