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Why does that candy and soda volcano work?

In a recent animated movie about a video game character having a mid-life crisis, a volcano erupts when a certain kind of candy falls into a certain kind of carbonated beverage. You've probably seen this demonstration or tried it yourself. Some kids did it on Letterman in 1999 and some have turned it into performance art. I wanted to know what was the science behind this explosive meme.

The Gas

The explosive fountain of foam is a physical reaction not a chemical one involving the rapid release of the carbon dioxide in the liquid. So it is more like shaking a can of pop before opening it, than mixing baking soda and vinegar. 

The Liquid

The diet cola in question has more carbon dioxide in it than its regular counterpart. So it has more gas to release. As well, the aspartame used as the low calorie sweetener acts as a surfactant, lowering the surface tension of the water molecules and making it easier to form bubbles. And it releases more gas when it is warmer.

The Solid

The microscopic roughness in the surface of the candy serves as nucleation sites, where bubbles formed. Mythbusters showed that another form of the candy with a waxy covering did not produce much of an effect. And they got an even higher result with rock salt. Gum arabic in the coating of the candy further reduces the surface tension. And because the candy is relatively dense, it sinks quickly, creating a bunch of bubbles that seed more bubbles as they rise. When you find a means to drop in at least five of the candies at once, so much gas get released at the same time that has nowhere to go but up. And if the opening is smaller, then the effect is exaggerated further. My own attempt was a miserable overflowing of foam rather than a catastrophic explosion. I only used one candy and the bottle was still cold from the fridge. Afterward, the soda tasted a little flat and a little minty (my stomach did not explode). I should refine my technique but I don't like the taste of aspartame and my daughter is a little too fond of the candies.

Do you have any tips for best results?

Learn more

Build a Coke and Mentos Rocket Car on Science World Resources