Well, that didn't take long. Less than 24 hours after posting Chemical Mystery #12: Baffling Balloons, Michael Farabaugh correctly explained how the experiment was conducted.
Three different balloons - one filled with helium, one with air, and one with sulfur hexafluoride (SF6) - were placed into a tank filled with sulfur hexafluoride gas. The helium-filled balloon floated out of the tank and up through the air. The air-filled balloon floated on the sulfur hexafluoride contained in the tank. Finally, the balloon filled with SF6sunk in the tank filled with SF6. You can watch the solution in the video below.
Well done, Michael. I think I’m going to have to come up with some chemical mysteries that are a bit more difficult for you all to solve…