How can water be made to disappear?

With his silk-gloved hands gracefully moving through the air, the magician carefully reveals his next illusion: an empty glass fish tank. Yet, he insists that the tank holds something extraordinary: invisible water. To prove it, he places a tinfoil boat inside, and to everyone's surprise, it floats, bobbing on an unseen surface.

The magician then produces a plastic cup and begins to 'remove' the invisible water from the tank. Though it seems like nothing more than miming, the water level appears to decrease with every cupful, and the little boat bobs more vigorously, as if disturbed by each invisible ripple.

You may have witnessed this impressive trick on a talk show or seen it circulating online. Ever wondered how it's done? No sleight of hand, wires, or digital effects are involved. The secret lies in an invisible gas called sulfur hexafluoride (SF₆). This intriguing gas is heavier and denser than air, which allows it to behave like a phantom liquid. When poured into a tank, it settles at the bottom and can support even the lightest objects on its surface, creating the illusion of floating.

Sulfur hexafluoride is non-toxic and non-flammable, making it perfect for a variety of scientific applications. It is used by medical professionals as a test gas in respiratory studies, and the electric power industry relies on it for insulation purposes.

This substance has made its way into everything from Nike sneakers to NASA's spacecraft, but the floating boat trick isn't as complicated as rocket science. Continue reading to discover the simple physics behind this illusion.

Floating on Sulfur Hexafluoride

So why does sulfur hexafluoride behave like a liquid? The answer lies in its weight and density, or how tightly packed the molecules within the substance are.

Sulfur hexafluoride is an unusually dense gas—almost five times denser than air. This means that a greater number of its molecules occupy the same space compared to other gases. Additionally, each SF₆ molecule is heavier than an air molecule. If you released two balloons—one filled with sulfur hexafluoride (molecular weight 146) and one with helium (molecular weight 4)—into the air (molecular weight 29), the helium balloon would rise, while the SF₆ balloon would sink.

This explains why sulfur hexafluoride remains at the bottom of the tank during the invisible water experiment. As for how a small tinfoil boat can float on it, the key concept to understand is displacement. While you can drive a tank across a frozen lake, floating such heavy weight on liquid water requires a bit of trickery. An object in a fluid experiences an upward force equal to the weight of the fluid it displaces. For example, a 500-pound (227-kg) boat will sink until it has displaced 500 pounds of water.

Even the largest ocean vessels don’t have all their weight at the bottom. There’s a considerable amount of air inside, allowing a well-engineered 500-pound ship to displace the necessary 500 pounds of water to balance out its weight before sinking too far. This principle is known as buoyancy. The same principle applies to an object floating on sulfur hexafluoride. However, because SF₆ is still a gas and less dense than liquid water, the floating object must be much lighter to stay afloat. This is why invisible water experiments often use lightweight boats made from tinfoil sheets.

Many scientists enjoy demonstrating the effect of inhaling sulfur hexafluoride. When you breathe in this gas, your voice temporarily becomes deeper. This occurs because the speed of sound is influenced by the molecular weight of the gas. In a lighter gas like helium, your words will come out as high-pitched squeaks, while in a heavier gas like SF₆, the sound slows and deepens to Darth Vader-like proportions. Sound travels by colliding molecules, and generally, the denser the substance, the faster the sound – though heavier molecules don’t travel as far.

While this voice-changing experiment can be entertaining, experts caution against inhaling sulfur hexafluoride without guidance. If the heavy gas is not properly exhaled, it will remain in the bottom of your lungs, taking up space that should be reserved for life-sustaining oxygen.

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