Here's what happens...

There's been shit for content lately. Yeah, I know that people are starting to be busy with families and school, and whatever else it is that Cohort does with his free time (I really don't care). And that's fine, but when my lillte brother was over the other day to help move some furnature around he told me about Mentos and diet Coke. My first reaction was along the lines of "Yeah right, whatever. This is like the poprocks thing." I couldn't have been more wrong...


Here's what happens when you slam a 2L of diet coke down your gullet, then eat a tube of mentos whole...


And a little bit O' science...

At first glance, it might seem as if a chemical reaction is taking place. Perhaps the Mentos are alkaline and the soda (despite its name) is acidic. That would make the whole thing a variation on my all-time favorite science bit: the baking soda and vinegar volcano. But a quick read of Mentos ingredients reveals precious little alkali: sugar, glucose syrup, hydrogenated coconut oil, gelatin, dextrin, "natural flavor," corn starch and gum arabic. While hydrogenating the coconut oil most likely involves some caustic soda or other alkaline agent, this would not be enough to drive the spectacular fountains.

So the process must be physical rather than chemical. And that leaves several possibilities that singly or in combination may be creating the effect, according to the chemists I interviewed. But underlying them all is the basic principle of surface tension in liquids.

Surface tension is the energy required to separate liquid molecules as a bubble forms. Put another way, it is the strong attraction between two molecules in a liquid, in this case water. The water molecules in the soda attract each other strongly but equally throughout the Diet Coke when undisturbed by frisky Mentos. This attraction enables the soda makers to pump extra carbon dioxide into the liquid and keeps all the gas from bubbling out immediately because it takes even more extra energy to push the water molecules apart.

Dropping a Mentos (or, as in the demonstrations above, a slew of Mentos) into the soda breaks the surface tension in two ways. First, the candy is uneven on a microscopic level. These tiny bumps and pits make it easier for bubbles to form because the surface tension is suddenly spread out over a wider area. Anywhere the surface tension is lowered by the Mentos, bubbles will appear at what are known as nucleation sites. "If there's a way for that gas molecule to get more freedom, it will find it," notes Joel Finegold, a detergent chemist. "It will actually break through that weakness in the surface tension." This is also the reason that almost any substance--salt, ice cream, or even quarters in a glass of beer--will quickly be covered in a coating of tiny bubbles when dropped into a carbonated beverage.

But the makeup of the Mentos is also helping here. Gum arabic--an oily gum derived from the acacia tree--and the coconut oil contribute to the effect by paradoxically making the water molecules even more strongly attracted to one another than usual. Remember, oil and water don't mix. This frees up space in surrounding areas for the carbon dioxide to bubble free again. "A simple experiment mixing soda and and gum arabic in a beaker results in fairly rapid evolution of gas and large bubble formation," writes Michael Montague-Smith, a chemist at the University of Maryland. "In a container with a narrow neck, sufficient gas could cause quite the geyser." Finegold speculates that balls of candlewax would work just as well, thanks to the oiliness of the wax.

Whichever mechanism is responsible for breaking the surface tension, the geyser forms as follows. The dense column of Mentos sink to the bottom of the bottle of Diet Coke, releasing more and more of the carbon dioxide gas as it settles and dissolves. The soda fizzes violently and, voila, explodes out the narrow top of the container.

So much for the Mentos end, but the question remains why Diet Coke? Steve Spangler has a simple explanation: it's less sticky when it comes time to clean the geyser up. But the truth is no one is entirely sure why it seems to work best. It is likely that either Diet Coke is pumped full of more carbon dioxide than its soda peers or its artificial sweeteners permit more carbon dioxide to dissolve in the liquid (once again by strengthening the surface tension). Or it could be that Diet Coke is kept colder than other sodas; cold water can hold more carbon dioxide.

Ultimately, this is a fun way to coat oneself, one's friends and one's surroundings in soda without resorting to the simple expedient of pouring it on them. It joins a long list of entertaining tricks with everyday substances that illustrate basic principles of science, like using Taco Bell hot sauce to de-grime a penny or making a volcano out of baking soda and vinegar. It also joins the long list of urban legends--anyone else remember Pop Rocks and Coke?--surrounding things you might not want to combine in your mouth or your stomach, though it would be hard to get enough of both into you to make it truly dangerous. Mentos and Diet Coke: the stickymaker!