Air: you know it, you love it, you can’t live without it. But did you know that it behaves differently, all the way down to the molecular level, at different temperatures? (I mean, I’m sure you knew that, but do your students?) Did you know it’s also fairly easy to manipulate air’s behavior with just your two hands? It’s true! Read on for science.
Soda Bottle Science, That Is!
For this Demo Science science demo, all you’ll need is a large, empty soda bottle, the cap of said bottle, and a soupçon of water. If you want to be the cool teacher for a day, you’ll bring in multiple bottles of soda and give each student an appropriately kid-sized glass of tasty beverage in order to drain the bottle “for the experiment’s sake.” (If you’re going that route, you’ll also need Solo cups, and possibly Solo ice.)
With your bottle emptied, wet the top of the bottle cap and place it upside-down on the open top of the bottle. Rub your palms together to work up a little friction heat, then place your mitts lightly around the bottle. Hold on to it, but don’t squeeze it.
Now, with almost all of the experiments you’ll find on this site, I automatically say “bigger is better” when it comes to supplies. Therefore, you may be inclined to go with the largest bottle of soda you can find for this one—for once, I recommend restraint. A two-liter bottle will be plenty big. Any bigger, and the demo might not work.
If all goes according to plan, the cap will eventually pop off the bottle of its own accord. But how?
Full of Hot Air
Remember at the beginning of this post when I was like “air behaves differently at different temperatures”? Well, whaddaya know, that’s precisely what’s happening here. Your hands, with their natural approximately-98.6° temperature amplified by friction, gradually warm the air inside the soda bottle.
As the air warms, its molecules expand and begin vibrating against each other in an attempt to create more space around them. The only way to create more space is by leaving the bottle. The wet cap creates a seal at first, holding in the warming air, but as the air gets warmer and warmer it will vibrate against the cap hard enough to blast it off so the air can escape.