Heavy Gas – Sulfur Hexafluoride

This is a gas that can lower your voice to freaky-weird levels.

Chemistry and physics teachers often use a classic science demonstration to show what happens to your voice when sound travels through a gas that’s six times lighter than air. Steve Spangler got to thinking about what happens to that same sound when it travels through a gas that’s six times heavier than air. It’s fun but the demonstration must be conducted by a trained professional. See the Safety Information section below.

Experiment Videos



Sulfur hexafluoride (SF6) is inert (won’t mix with anything) and non-toxic. It can do some surprising things even in small quantities. Except, don’t try these at home for two reasons: (1) you have to be trained in its correct use and handling and, (2) SF6 is quite expensive.


SF6 is a gas that’s heavier than air. Like carbon dioxide (CO2), SF6 sinks and fills any low spots first. It can fill a sealed container to the point where you can actually feel it when you put your hands into a container filled with it.


Like CO2 , sulfur hexafluoride is dense enough to allow things to float on it, too. You can fold an aluminum foil boat that sits right on the top of a layer of SF6 as though it were floating on water. It’s pretty cool.


There is a commercial use for SF6 as a non-conductor of electricity. You can see from its formula that SF6 is comprised of one sulphur atom and six fluorine atoms. In this symmetrical form, even the very reactive fluorine is locked up tightly. That makes SF6 a perfect electrical insulator which can effectively extinguish arcs, even arcs from a Taser.


Of course, the fun side of SF6 is that you can safely inhale small quantities and hear the temporary, but dramatic, change it makes in your voice. Inhale a little, talk normally, and your voice has dropped to a very low pitch. Again, there are safety considerations based on how heavy the gas is and what it takes to clear it out of your lungs completely.

How Does It Work

You may know that when you inhale a little helium (He), which is also an inert gas, your voice sounds like a high-pitched cartoon character. Why does this happen? Sound waves travel much faster through helium than they do through regular air. Helium is much less dense than regular air, about six times less dense, in fact.

So, you’re wondering, “What inert gas is about six times heavier than the air we breathe and won’t result in death if I breathe it – as with CO or CO2?” Sulfur hexafluoride (SF6) is inert, not poisonous, and can affect the sound of your voice when inhaled in small quantities (Again, don’t try this at home!). When SF6 is inhaled, the pitch of a person’s voice drops dramatically. The speed of sound in SF6 is way slower than it is in air. Sound travels through air at about 1100 ft/sec (335 m/s) but through SF6 it’s only about 394 ft/sec (120 m/s). On the other hand, a helium voice is heard at about 2953 ft/sec (900 m/s). It’s much faster!

As with helium, a person trained by an expert can breathe a small quantity of sulfur hexafluoride without harm to demonstrate how sound travels through a very dense gas. Of course, do not use any quantity of SF6 without proper training and precautions.

Safety Information

Even though helium is an inert gas which can be inhaled briefly without risk of death, we do not recommend that you breathe helium or any other gas besides “regular air.” The science teachers and demonstrators in the video were trained by professionals. Do not try this experiment at home (if for no other reason than sulfur hexafluoride (SF6) is extremely expensive).

Dr. Steve Harris has a fair amount of experience using the gas in his practice. He has also written about SF6 extensively and understands its properties. “It should be safe to inhale SF6 using the way science teachers do in their demos so long as you do not fill your lungs repeatedly over a short period of time. You must beware of hypoxia (oxygen deficiency) and fainting, except you will not get much warning because human hypoxia sensors are not very good.” He continues, “Breathe pure oxygen for ten breaths or so before you inhale the SF6 gas. That will have your lungs full of nearly 100% oxygen. That means you’ll have a lot of reserve against blackout if you go too high on the SF6 content.”

When you’ve finished your tests,  breathe slowly and deeply in and out several times to help remove the gas from your lungs. Don’t hyperventilate, just inhale and exhale deeply. SF6 needs to be pushed out of your lungs a few times since it’s so heavy.

Real World Applications

SF6 is used extensively in electrical power equipment. It’s colorless, odorless, non-flammable, and chemically stable. At room temperature it does not react with anything else. Its stability is what makes it useful in electrical equipment. SF6 is a very good electrical insulator and can effectively extinguish or prevent arcing because it doesn’t conduct electricity. It’s an insulating and arc-quenching material in switchgear for high- and medium-voltage applications. In electrical power systems, high- and medium-voltage switchgear is used to cut off the power in case of a fault to protect people and equipment. Electric arc strikes can occur between circuit-breaker contacts in this equipment. Breakers filled with SF6 are electrically insulating and effectively control arcing.

SF6 is mixed with argon (Ar), and used to insulate multi-pane windows. SF6 is used in the metal industry when casting magnesium. Eye surgeons use it as a cooling agent in operations. SF6 can be used as a fire extinguishing material because it is non-flammable and cooling.