How can you make water invisible?

So why does sulfur hexafluoride behave like a liquid? It all comes down to weight and density, or how close the molecules inside a substance are to each other.

Sulfur hexafluoride is a particularly dense gas — almost five times as dense as air. This just means that more of its molecules inhabit a given space than with most other gases. On top of this, each molecule of SF₆ also weighs more than an air molecule. If you released a balloon full of sulfur hexafluoride (molecular weight 146) and a balloon full of helium (molecular weight 4) into the air (molecular weight 29), the helium balloon would rise to the ceiling and the one filled with SF₆ would sink to the floor.

This is why sulfur hexafluoride just sits at the bottom of the tank during the invisible water experiment. As to how a small tinfoil boat can float on its surface, you have to understand how a similar object would float in water — by displacement. While you can drive a tank across a frozen lake, you have to cheat to float that kind of weight on water in its liquid form. An object in a fluid experiences an upward force equal to the weight of the fluid displaced by the object. For instance, a 500-pound (227-kg) boat will sink in the water until it has displaced 500 pounds of water.

But even the heaviest ocean vessels don’t have all their weight at the bottom. There’s a substantial amount of air inside, so a well-designed 500-pound ship will displace the 500 pounds of water to cancel out its weight well before it sinks below a desired point. This is called buoyancy. The same relationship holds true with a vessel floating on a sea of sulfur hexafluoride. However, since SF₆ is still a gas and is less dense than liquid water, the floating object has to be much lighter to stay afloat. This is why invisible water experiments typically involve light boats made from a sheet of tinfoil.

Many scientists also like to show off what happens when you inhale sulfur hexafluoride. Fill your lungs with the gas, and your voice will deepen temporarily. This is because the speed of sound depends on the molecular weight of the gas involved. A few mumbled words will come out as squeaks in a light gas such as helium, but will slow to Darth Vader proportions with a heavy gas like SF₆. Sound waves travel through colliding molecules. Generally, the denser the substance, the faster the sound — but heavier molecules don’t move as far. 

As fun as this voice experiment sounds, experts warn against inhaling sulfur hexafluoride without proper instruction. The heavy gas will continue to sit at the bottom of your lungs if not properly expelled — and that’s room you need for life-giving oxygen.

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Explore the links on the next page to learn more about buoyancy and view videos of the experiment in action.