ChemCafé

The Internet is seething with videos of “chemical geysers“, the most famous -and spectacular- being the one one obtains by adding mint Mentos candies to diet coke. A host of moreless reliable explanations can be found, but they are seldom based on good old science. The main explanation to the phenomenon -which by the way tells why the ‘experiment’ works with any sparkling beverage- relies on a very chemical concept known as heterogenous catalysis. Usually chemists use this phenomenon to promote reactions like hydrogenations by adding a porous catalyst such as palladium on carbon to their reaction medium. What happens (in the case of a ‘chemical fountain’) is that the pores in the mentos candy facilitate nucleation of carbon dioxide molecules which are dissolved in the beverage, which results in a fast formation of bubbles, resulting in the ‘geyser effect’ – nucleation is also what you observe when you see bubbles forming in a cup of Champagne. Of course, the faster the bubbles form, the faster they will escape from the bottle, ultimately creating this foaming mixture that can be projected meters away!

A detailed study conducted at Appalachian State University in Boone, North Carolina, by Dr. Tonya S. Coffey, showed that, in addition to the candy roughness and porosity (they even made AFM study on Mentos candies !!), a second factor deeply influences the success or failure of a coke fountain: the surface tension of the liquid directly influences the bubbles formation rate, and a liquid containing an artificial sweetener (such as aspartam in diet coke) has a lower surface tension than a liquid containing a natural sugar. Furthermore, the gum arabic contained in mentos candies also brings a contribution to lowering the surface tension of the mixture.

It was also observed than, contrary to what is frequently postulated, no acid-base reaction is involved (actually, there is no base in a mentos candy, and the pH of the beverage does not change during the course of the ‘reaction’), the caffeine present in coke has hardly any influence on the result, contrary to the speed at which mentos candies sink into the liquid, which turns out to be of great importance!

In summary, the chemical geyser is due to the presence of a rough, porous ‘material’, in addition to chemicals that decrease the surface tension of the liquid, which contains dissolved carbon dioxide: these are the three basic ingredients of this funny recipe.

For more details:
T. S. Coffey, Am. J. Phys 2008, 76, 6, 551-557. DOI: 10.1119/1.2888546

Thanks Sophie for having suggested the topic of this post – btw, ideas always welcome!