How Things Work: Mentos in Diet Coke

Scientists have performed an experiment involving Mentos and Diet Coke that takes the legend out of “urban legend.” The experiment was first performed for David Letterman by school teacher Lee Marek in 1999.

The experiment involves something that you can do with soda that will not rot your teeth, unless you decide to drink Diet Coke combined with Mentos candy. Unlike Pop Rocks and soda — which don’t actually make your head explode if consumed together — combining Mentos with Diet Coke really does cause a chemical reaction.

The result of combining Mentos with Diet Coke, which has been showcased around the Web thanks to YouTube, is not the typical “fizz” that arises from opening a new bottle of soda. Drop a Mentos candy into a two-liter bottle of any diet soda, and the soda will erupt like a geyser that puts all sixth-grade vinegar and baking soda projects to shame.

More recently, the experiment has been made popular by the creators of the website EepyBird.com, which contains numerous videos of the Coke-Mentos explosions. In particular, EepyBird shows videos of synchronized Diet Coke geysers, domino fountains, and even bloopers.

The Mentos and Diet Coke experiment has also received national attention. It was featured on an episode of Discovery Channel’s Mythbusters and on National Public Radio’s nightly news show All Things Considered.

The science behind this experiment goes deeper than the surface.

Water molecules are strongly attracted to one another and will collect around a bubble of carbon dioxide that is found in the soda, forming a sort of coating.

For more carbon dioxide bubbles to form, or for existing ones to grow larger, they must break the surface tension of the water molecules.

The theory with Mentos is that the gum arabic in the candy breaks the surface tension of these molecules, making the carbon dioxide bubbles form faster.

While breaking the water molecules’ surface tension contributes to the reaction, the experiment really requires the use of another property found in Mentos.

The second property is the wealth of tiny, irregular cracks and crevices that can be found all over a Mentos candy at a microscopic level, which are nucleation sites, or locations that are ripe for carbon dioxide bubbles to form. Drop a Mentos candy into the soda, and a large number of bubbles will form all over the candy at the nucleation sites. As the candy sinks, it passes through more soda and creates more bubbles. Collectively, these bubbles create foam and the pressure necessary to cause an eruption.

One might wonder why this experiment does not work with regular soda. According to Eepybird’s scientists, dropping almost any object into any soda will create some fizzing, but usually less so than Mentos and Diet Coke.

In addition, Jamie Hyneman and Adam Savage, the hosts of Mythbusters, theorized that the sugar molecules in the soda, due to their size, prevent the fast formation of bubbles, and also that the sticky sugar can better contain carbon dioxide bubbles.

Other sites have suggested that diet soda is favored because the cleanup is far less sticky than regular soda.

Mentos candies are convenient to use in the experiment because they are sizeable, and they sink. But anything with gum acacia should do the trick. Hyneman also said that table salt causes a similar eruption and actually works a little better than Mentos.

The ideal condition for achieving high-flying geysers is to get as many Mentos as possible into the diet soda at the same or nearly the same time. The scientists of EepyBird, for instance, drill a hole in each Mentos candy to make a string of Mentos that can be dropped into the soda. Of course, increasing the number of Mentos that are dropped will increase the spray.

Another way to increase the spray is by drilling a hole in the cap, holding the Mentos on a string, and then dropping them. The smaller hole creates a higher, more dramatic geyser.

Keep in mind, though, that it is probably wise to do this experiment outside if you want to stay dry.