Editing ExplodingSoda (section)

== Why It Is ==

Carbonated beverages are composed of carbon dioxide (CO2 gas) dissolved in water, as well as sweeteners, dyes, and flavorings. The dissolved CO2 gas is kept within the liquid water by the pressure of the container (the bottle or can). If this pressure is released by opening the container, bubbles of CO2 will escape from the soda and produce effervescence, a “fizzing” effect. The effervescence can be increased if the volatile soda solution is disturbed by shaking the container or inserting an object into it. This is why soda bubbles up if you pour it into a new container, but not if it is just sitting around.

Two properties of the Mentos and Diet Coke make them ideal for this experiment:

The Mentos has a rough surface, covered with many microscopic pits. These pits increase the surface area of the mint by a very large amount – just like a golf ball has a larger surface area than perfect sphere would. Each pit serves as a great nucleation site – a place for a bubble of CO2 to form.

The Mentos contains gum arabic and the Diet Coke contains aspartame and potassium benzoate. Gum arabic is a surfactant that reduces the surface tension of water (see Science Theatre's Tie-Dye Milk writeup for more information). Reducing the surface tension of the water makes it easier for bubbles to form. The aspartame and potassium benzoate in the Diet Coke also reduce the energy necessary for bubble formation. 

Experiments suggest that caffeine is not a significant ingredient in this reaction, primarily because there is only a very small amount of caffeine in soda (see reference 2).

When the Mentos is dropped into the soda, the disturbance of the solution, the appearance of a rough surface rich with nucleation sites, and the chemicals in the Mentos and Diet Coke all contribute to the rapid development of CO2 bubbles. The appearance of gas bubbles on the candy is visible during the experiment. This sudden and rapid formation and escape of CO2 bubbles sends a powerful flow of CO2 gas spewing from the container, taking the liquid soda along with it.