Chemical Mystery #12: Baffling Balloons


In this chemical mystery, the floating and sinking behavior of balloons is observed:


Can you figure out how this experiment was conducted? If so, leave your answer in the comments! The solution will appear in a few days.

Check out the solution to this chemical mystery.


General Safety

For Laboratory Work: Please refer to the ACS Guidelines for Chemical Laboratory Safety in Secondary Schools (2016).  

For Demonstrations: Please refer to the ACS Division of Chemical Education Safety Guidelines for Chemical Demonstrations.

Other Safety resources

RAMP: Recognize hazards; Assess the risks of hazards; Minimize the risks of hazards; Prepare for emergencies


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Comments 9

Michael Farabaugh's picture
Michael Farabaugh | Thu, 09/20/2018 - 18:23

The lavender balloon is filled with hydrogen (or helium.) The lavender balloon floats in air because the density of the hydrogen (or helium) in the lavender balloon is less than the density of the air.

The yellow balloon is filled with air, and the aquarium/tank is filled with sulfur hexafluoride (or carbon dioxide). The yellow balloon floats in the tank because the density of the air inside the yellow balloon is less than the density of the sulfur hexafluoride (or carbon dioxide) in the tank.

The green balloon is filled with sulfur hexafluoride (or carbon dioxide). The gas inside the green balloon is equal to the density of the gas in the tank. When we consider the combined mass of the gas and the green balloon, the density of the green balloon is greater than the density of the gas in the tank. So it sinks.

Tom Kuntzleman's picture
Tom Kuntzleman | Thu, 09/20/2018 - 21:15

And we have a winner! One balloon is filled with helium, one with air, and one with sulfur hexafluoride. The tank is filled with sulfur hexafluoride. Well done, Michael!

Amiee Modic's picture
Amiee Modic | Sat, 09/22/2018 - 22:10

Hi Tom, Would that also work with Carbon dioxide, or is there not enough density difference between that and air? Just wondering because I don't have access to SF6 that I know of...

Thanks, Amiee

Tom Kuntzleman's picture
Tom Kuntzleman | Sun, 09/23/2018 - 07:07

Hi Aimee:

Are you familiar with the trick where you can float soap bubbles on carbon dioxide? This is one way you can demonstrate that air floats on CO2. Given that the density of CO2 (about 2 g / L) is not quite twice the density of air (about 1.2 g /L), getting air-filled ballloons to float on carbon dioxide is pretty tough to pull off. Sounds like you might have a project for your students to work on. Let me know if they figure out how to do this!

Bob Worley's picture
Bob Worley | Sun, 09/23/2018 - 17:24

First of all, in the UK sulfur hexafluoride, is very expensive. Teachers in the UK have seen University demonstrators float aluminium boats on this gas in special demo lectures. The teachers then ask us at CLEAPSS about it because SF6 is not in their usual list of chemicals. The expense soon puts them off.

I have seen people breathe in this gas and they talk with a very deep voice (often with someone who has breathed in helium with a very high voice). Breathing SF6 is even more dangerous than breathing to helium because the gas, being so dense, is not quickly expelled from the lungs. (You are reducing O2 in th4e body when breathing in these gases.) In any case  inhaling gases sets a bad example.

You can bounce air filled soap bubbles in a glass aquarium filled with CO2, H2  and CH4 soap bubbles rise in air . I wonder if a LPG or butane soap bubble would sink?

Tom Kuntzleman's picture
Tom Kuntzleman | Mon, 09/24/2018 - 08:30

Hi Bob. Thanks for chiming in on the safety and cost related to this experiment. You are right that SF6 isn't cheap. I'm wondering if just the right mix of helium and exhaled air in a balloon would make a balloon float on CO2 but sink in air. I'm guessing that the size to which a balloon is inflated would play a role in its ability to float on CO2 also. I might be tinkering around with this over the next few months. If anyone out there figures out how to get balloons to float on CO2 be sure to let us know!

Tom Kuntzleman's picture
Tom Kuntzleman | Mon, 09/24/2018 - 09:01

I filled a balloon mostly with helium, and then added a little bit of exhaled breath. The balloon did not float in air, but when I added it to a tank full of CO2 the following happened:

Balloon floating on carbon dioxide

A balloon filled with a mixture of helium and exhaled breath floats on CO2.


Bob Worley's picture
Bob Worley | Thu, 09/27/2018 - 01:21


I wish all my bright ideas worked the first time.It is great , it worked first time and possibly more science in that than in the original demo.. If you cover the tank of CO2 I suppose the balloon will rise and fall with warnth and cooling  but most likely sink as the He diffuses through the balloon rubber membrane. Talking of gases, I mananged to make liquid ammonia by pasing ammonia through a narrow tube immersed in propanone/solid CO2 in the the fume cupboard (no naked flames). I then dropped a small piece of lithium down the tube and into the colouless liquid ammonia and got the fantastic blue colour of the solvated electron.
Not in the syllabus I know but you do  not see that everyday. In fact to me, 50 years!! Worked first time!



Tom Kuntzleman's picture
Tom Kuntzleman | Fri, 09/28/2018 - 08:46

The solvated electron sounds very interesting; I am not familiar with this experiment. You should consider posting a video of the experimental process!