The Coke and Mentos Experiment


This 3 minutes movie was released about two weeks ago. It displays The Extreme Diet Coke & Mentos Experiment #137. Incredible and funny, is it not? I read somewhere that Mentos company hired both guys for their next advertising campaign.

Since we are in a serious place here, it was but underhand lead-in for a lesson of physics and chemistry: how can little Mentos have such a tremendous effect on a peaceful bottles of Diet Coke? Everyone will agree on the premises: the carbon dioxide (CO2) compressed into the soda escapes so rapidly that the pressure pushes it out of the bottle. It's like shaking a bottle before you open it, but even more dramatic.

Several people theorized that a substance called gum arabic in the Mentos breaks the surface tension of the soda, allowing the CO2 bubbles to escape rapidly. This explanation doesn't completely work though, since several items that contain no gum arabic also cause soda to foam violently.

The primary cause appears to be physical, not chemical: when a liquid is supersaturated with gas, like soda is with CO2, gas is able to form bubbles on nucleation sites (that are places with high surface area in a very small volume, such as scratches on a surface or specks of dust).

Mentos seem to be loaded with nucleation sites: there are so many microscopic nooks and crannies on their surface that an incredible number of bubbles will form when you drop one in a bottle of soda. Since the Mentos are also heavy enough to sink, they react with the soda all the way to the bottom. The escaping bubbles quickly turn into a raging foam, and the pressure builds dramatically.

This mechanism is somewhat similar to the way people whose urine is supersaturated with uric acid or calcium crystals often develop kidney stones. Fortunately enough, crystals are not gas, which probably explains why people with kidney stones usually don't explode.

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