Even though we can’t see it or feel it or touch it or really interact with it in anyway, apart from breathing it, air does exert pressure, even in the most natural of settings. One factor that influences air pressure is the temperature of the air—this is, in part, what causes wind (which is actually a whole thing for which we as yet have no reliable demonstration).
We DO have a handy dandy Demo Science science demo to help students understand how the difference in temperature can affect air pressure, however. It’s fast, easy, and pretty derned fun. Read on!
Splish Splash Swish Swoosh Swoop Bloop Scrunch
All you’ll need for this one is an empty, two liter, plastic soda bottle (and its cap), some very hot tap water, and the sink that goes with said tap. First, fill the bottle roughly half full with the hot water, leaving the cap off. The water needs to be as hot as you can possibly make it right out of the tap, so be careful not to scald yourself. Gloves might be a good idea.
Have your students (or whoever) gather around as you swirl the water around in the bottle for about a minute or so, heating the air inside the bottle above the water. Keep swirling it around, then quickly flip the bottle and pour out the water. If you’ve got a good swirl game, the water should keep sloshing circularly and pour out right quick. Faster than just upending the bottle, anyway.
Quickly put the cap on the bottle and twist it tight to create a solid seal. Watch the kids’ reactions as the bottle collapses in on itself as if by magic. See if any of them can suss why that happened.
Invisible Science In Action
The bottle collapsed because the hot water made the air inside the bottle hot, too—as mentioned in the water swirling step. With the water gone and the bottle capped, the air inside rapidly cools back to room temperature.
Because its molecules are closer together, cool air actually has less volume than warm air. Ergo, the air inside the bottle “shrinks” as it cools, leaving a lot of extra space. As the bottle is not in a vacuum, the air pressure outside the bottle is now, suddenly, greater than that in the bottle. The difference in air pressure causes the bottle to collapse, and little kids’ minds to be blown.