Floating on Sulfur Hexafluoride
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.
Explore the links below to learn more about buoyancy and view videos of the experiment in action.
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More Great Links
- "Active Pharmaceutical Ingredients Sulfur Hexafluoride (SF6)." Scott Medical Products. 2002. (July 16, 2008)http://www.scottecatalog.com/images.nsf/Images/API_SF6/$FILE/API_SF6.pdf
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- Cotton, Simon. "Sulfur hexafluoride." University of Bristol School of Chemistry. (July 16, 2008)http://www.chm.bris.ac.uk/motm/SF6/SF6h.htm
- "High Global Warming Potential (GWP)" U.S. Environmental Protection Agency. Oct. 19, 2006. (July 16, 2008)http://www.epa.gov/highgwp/scientific.html
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- "SF6 Emission Reduction Partnership for the Magnesium Industry." U.S. Environmental Protection Agency. March 25, 2008. (July 16, 2008)http://www.epa.gov/magnesium-sf6
- Shakhashiri, Bassam Z. "Floating Soap Bubbles." Science is Fun in the Lab of Shakhashiri. (July 16, 2008)http://scifun.chem.wisc.edu/HomeExpts/SOAPBUBL.html
- Spangler, Steve. "Anti-Helium - Sulfur Hexafluoride." Steve Spangler Science. (July 16, 2008)http://www.stevespanglerscience.com/experiment/from-donald-duck-to-barry-white-how-gases-change-your-voice
- "Sulfur Hexafluoride." Medical Dictionary. (July 16, 2008)http://www.medicaldictionaryweb.com/Sulfur+Hexafluoride-definition/
- "Sulfur Hexafluoride (SF6) Gas Introduction." Liming Research Institute of Chemical Industry. 2006. (July 16, 2008)http://www.lrici.com/Eng/Sulfur%20Hexafluoride%20(SF6).htm
- "Voice with Helium and SF6." UCLA Physics: Lecture Demonstration Manual. (July 16, 2008)http://www.physics.ucla.edu/demoweb/demomanual/acoustics/effects_of_sound/voice_with_helium_and_sf6.html
- "Why can boats made of steel float on water when a bar of steel sinks?" HowStuffWorks.com. April 1, 2000. (July 16, 2008)https://science.howstuffworks.com/question254.htm