Regular and Diet Coke Cans in Water

Due to the difference in density, the can with the sugar in it sinks while the diet can floats. For a further refinement, mix in a bunch of salt--the density of the salt water increases enough that the sugared coke now floats.

More Detailed Explanation

The density of a can of regular soda is just over 1 gm/cc, while that of a can of diet soda is just under. This makes for a dramatic display of density differences. A 12-oz can of regular soda will sink in the same container of water in which the same brand of diet soda floats. Brand is not a factor, and only in the case of 12-oz aluminum cans does the small density difference straddle the density of water. All plastic containers sink. The reason for the behavior is the presence of 39 grams (nearly 8 teaspoons!) of sugar dissolved in the water of the regular soda. It takes a much smaller mass of aspertaine (Nutrasweet/Equal) to flavor the diet soda. The effects of the air space in the can, and the mass of the can, combine with the liquid densities to end up just over and under the density of water. This is best displayed in a transparent container of at least 4-liter capacity. To show that buoyancy depends on both the object and the liquid, the soda can demonstration can be taken further by adding salt to the water in which the cans have been placed. The dissolved salt raises the density of the water (ref. swimming in the Great Salt Lake). When enough salt has dissolved, the sunken regular soda can will rise to the top of the water. At the same time, the floating can of diet soda will rise high enough to tip over and float on its side. It takes about 13-oz of salt (half a standard cylindrical cardboard container) in a little more than 3 liters of water to bring about the change. The water can be pre-warmed to hasten solution of the salt, but even with regular tap water the change happens in less that twenty seconds with a little stirring. (To quiet skeptics it might be worth stirring the water before adding salt to prove that stirring alone does not raise the can.)