This winning photo of children's faces seen through an aquarium filled with floating soap bubbles was sent in by Ken Lyle from Duke University, Durham NC. Surprisingly, these bubbles do not float away. Trapped in the open aquarium, these ordinary soap bubbles are fascinating to watch as they float about, bob up and down, and slowly sink downward.
This effect can be easily replicated. A few pieces of dry ice placed in the aquarium are allowed to sublime. The gaseous carbon dioxide, being denser than the surrounding air, displaces the air inside the aquarium filling it with carbon dioxide. Using ordinary soap bubble solution, air-filled soap bubbles are created and allowed to "fall" into the aquarium where, being less dense, they float on the invisible layer of carbon dioxide. Over time, the bubbles get larger and sink down into the carbon dioxide layer. This is the result of carbon dioxide passing through the soap film into the interior of the bubble and increasing the overall density of the bubble. The transport of CO2 through the film is not a simple diffusion process but rather one of equilibrium between dissolved and gaseous carbon dioxide. The high concentration of CO2 gas outside the bubble drives the equilibrium in the direction favoring the dissolved CO2 while the relatively low concentration of CO2 gas in the interior favors the process of CO2 returning to gaseous state. In simpler terms, the CO2 outside the soap film dissolves in the soap film that, in turn, comes out of solution entering the interior of the bubble.
This simple demonstration can begin many chemistry conversations, such as sublimation, density, fluid properties of gases, and equilibrium. And, as you can see on the faces of the children, the motion of the bubbles captures their attention, fully engaging their minds in the chemistry at hand.
Ken is the lecture-demonstrator and the chemistry outreach coordinator for Duke's Department of Chemistry. He is one of the winners of the 2012 Chemistry in Pictures contest. As in previous years, each of the coming issues of Chem 13 News will feature winning photos. Ken has won the book The Elements written and donated by Theodore Gray.