In Chemical Mystery #12, curious floating and sinking behavior is exhibited by three different balloons. To pull off this experimental trick, a tank is filled with sulfur hexafluoride (SF6), and three balloons are filled with three different gases: helium, exhaled breath, and SF6. The balloon filled with SF6 sinks when placed in the tank. The balloon filled with exhaled breath floats in the tank, and the helium filled balloon floats up out of the tank and through the air.
Bob Worley and Amiee Modic commented on this experiment, noting that many teachers do not have access to SF6 and therefore would likely be unable to conduct this experiment in their classrooms. This gave me the idea of trying to figure out how to pull off the trick using easily obtained items. I have experimented a little bit and found that it is easy to pull off this experimental effect using baking soda, vinegar, a large deep plastic bin, balloons, and a tank of helium (which can be purchased in the party section of stores such as Wal Mart). It is important to note that this experiment works best when using balloons with very thin plastic skins when inflated. Water balloons work well.
First, baking soda and vinegar are mixed in the plastic bin to fill the container with CO2 gas:
NaHCO3 + HC2H3O2 --> NaC2H3O2 + H2O + CO2
One balloon is filled with helium, a second balloon with exhaled breath, and a third balloon with half exhaled breath and half helium. Some experimenting may be required to get the right density of the balloon filled with helium/exhaled breath. The balloons display different floating and sinking behavior when placed into the tub filled with CO2 - quite similar to the experiment done with SF6:
Amiee and Bob, thank you for commenting on Chemical Mystery #12 and noting that many teachers do not have access to SF6. Your comments provided me with inspiration to get into the lab to try some further experimentation, which I always enjoy having the chance to do!