Answer: Sweetener Type

When it comes to floating on water, low density is the name of the game. Rocks have a high density and sink, the foam in life jackets has a low density and it floats. While that wouldn’t make for much of a showy experiment (because nobody would expect the life jacket to sink and the rock to float, after all), you can perform a pretty nifty density experiment with two cans of soda.

To perform our little experiment, you’ll need a can of regular soda of any brand and an equivalent can of the diet version, say Coca-Cola and Diet Coke, for example. Place both cans into a bucket of tap water. The Diet Coke will float at the top and the Coca-Cola will sink to the bottom. But why?

Soda cans are filled with water, carbonation, sweetener, and very minute amounts of other things like dye and flavoring. In the case of regular soda, the sweetener is sugar and they put a lot of it in a can. Your average can of soda has approximately 40 grams of sugar dissolved in it. This huge amount of sugar tips the density of the can’s contents to just over 1 gram per cubic centimeter (the density of plain water) and the can sinks to the bottom. Diet sodas, however, use artificial sweeteners that are hundreds of times sweeter than sugar. Because it takes hardly any aspartame (or the equivalent) to make a can of Coke taste sweet, the addition of the sweetener does next to nothing to change the density of the liquid in the can. The diet cola plus the carbonation, then, has a lower density than water and it floats.