To demonstrate buoyancy and diffusion in an interesting way.
Cylindrical plastic tank, dry ice, soap bubble blowing device.
It may be necessary to remove static electricity from the plastic container.
A block of dry ice is placed in the bottom of a clear plastic cylinder sealed on the bottom, trapping carbon dioxide in the bottom of the container as the dry ice sublimates (evaporates). Bubbles produced with a standard soap bubble blowing gizmo will float on top of the more dense carbon dioxide. Some carbon dioxide diffuses into the bubbles, so they get larger and sink! Some of the bubbles freeze when they sink to the bottom of the container near the dry ice.
John Satterly, Rate of Rise of Small Air Bubbles in Water, AJP 23, 387 (1955). ◙Gerald F. Hinderholtz, Buoyant Bubble, TPT 16, 490, (1978). ◙Frederick W. Kantor, Floating Balloons and Soap Bubbles on an Air-Freon 12 Gas Interface, TPT 1, 82-83, (1963). Charles Waiveris, Soap Bubbles on a Cold Day, TPT 32, 404-405 (1994).