Nucleation in a Soda Geyser

By Tracy V. Wilson, HowStuffWorks.com

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What else can aspartame do besides make bubble-making easier? Find out in How Artificial Sweeteners Work at HowStuffWorks. You can also learn more about the peculiar properties of water in How Water Works.

By now, it's common knowledge that you can drop a few Mentos candies into a bottle of Diet Coke to produce an impressive geyser of soft-drink foam. The key concept at work is nucleation. Think about blowing an ordinary soap bubble with a bubble wand. Particles are moving from your mouth into the bubble, which eventually seals itself off and floats away. To make the bubble, you had to do some work, and you also needed a gathering point -- or a nucleation site -- for your breath to collect as the bubble formed.

In a carbonated drink, the process of bubble formation is a little different, but the basic idea is the same. It takes work to make a bubble, and the carbon dioxide molecules dissolved in the liquid need a jumping-off point to do it. The molecules gradually collect at the nucleation site until there are enough of them to overcome the water molecules' natural tendency to stick together. When this happens, the resulting bubble can stick to an interior surface of the container, or it can break free and rise to the top. The rough surface of a Mentos candy is covered with pits and peaks on a microscopic level, and these all act as nucleation sites.

In the reaction between Mentos and Diet Coke, several factors cut down on the work required to make a bubble, which is why the display is so impressive. Diet soft drinks contain a couple of ingredients that lower surface tension, making bubble formation easier. These include potassium benzoate -- a preservative -- and aspartame -- a sweetener. Gum arabic, an ingredient in Mentos, lowers the surface tension even more. Caffeine can play a role, too, but perhaps not in the relatively small amounts found in soft drinks.

So when you drop your Mentos into your Diet Coke, here's what happens:

1. Since the candy is much denser than the drink, it quickly sinks to the bottom, giving the rest of the reaction plenty of room to move.

2. Bubbles quickly form on the nucleation sites on the candy's surface.

3. In a cascading reaction, each bubble becomes its own nucleation site, as do all the new bubbles.

4. All these bubbles rise to the surface in an explosive eruption that's made even stronger by squeezing out through the narrow opening at the top of the bottle.