It's Okay To Be Smart

A gas more dense than a solid? Solids floating on thin air? Surely you’re joking! That can’t exist, right?

Wrong.

We can create real-life magic of levitation if we simply apply a few simple pieces of chemistry and physics. Let’s review a few basic principles first. Density, I’m sure you all remember, is how much mass something has per unit volume (which can vary at different temperatures, etc.). Liquid water has a density about 1 kilogram per liter, and air is nearly a thousand times less dense at ~1.3 grams per liter.

So why do things float? Besides weighing the same as a duck? Remember that gravity is always pulling objects toward the center of the Earth, at a force of 9.8 Newtons for every kilogram of mass. Mass is different than weight, of course, because weight is really the result of gravity’s force acting upon the mass. For something to float, no matter if it’s on water or air or what, it must have a force acting against and overcoming the pull of gravity. It’s why planes fly and rockets go up to space and why anyone knows who Archimedes is. Where does that force come from when things float?

Let’s hit the beach and hop on a stand-up paddleboard. Although they look thin, an average SUP board can “displace” up to 200 liters of water. That means that it will support a force equal to that of the 200 kg of water it displaced, or 1,960 Newtons (or 200 kg worth of surfer dudes/dudettes). It works the same way with a gas. Whatever mass of gas is moved out of the way, that amount of “mass force” will float.

Pretend you have a swimming pool full of sulfur hexafluoride, one of the densest gases around (6.17 grams per liter, or about five times denser than air). If I place a foil boat inside that moves a liter of gas out of the way, as long as it weighs less than 6.17 grams it will float! Don’t believe me? Watch this:

Those of you with a keen attention to detail may be saying “Yeah, Joe … but that’s not really a solid.” Ok, I’ll give you that. It’s more like a foil-boat bag of air. There are “real” solids that could float on that, though. Like this space-age aerogel, a silicon dioxide solid that has a density of 3 grams per liter (the least dense solid ever created)!

Before you rush out and try to get your hands on some sulfur hexafluoride, keep in mind that it is one of the most potent greenhouse gases known to man, about 23,000 times worse than carbon dioxide. Filling that aquarium is the same as burning about 600 gallons of gasoline.

Finally … in the name of science, and since you’ve sat through this lesson so patiently, here is what happens to the human voice when a person breathes in sulfur hexafluoride (it’s basically the opposite of helium, being so much denser than air and changing the speed of sound in the vocal chords). Hit it, Kelly Ripa and Neil Patrick Harris: