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Name: Gary
Status: Other
Grade: Other
Location: KY
Country: N/A
Date: October 2005


Question:
I just read an article at a tv news program's web site where a person dropped Mentos candy into soda, and the soda fizzed out all over. [Before you try this at home, use diet soda! It is sticky. And go outside!]

The article described what was happening as the arabic acid causing the surface tension between the carbon dioxide and the water to weaken, allowing the CO2 to escape. My question is, is it really surface tension at work here? What force(s) is at play when carbon dioxide is dissolved in water?



Replies:
The Mentos candy experiment is in the web literature:

http://antoine.frostburg.edu/chem/senese/101/consumer/faq/mentos.shtml

The ingredients in that candy are (from the citation above): All variations of Mentos contain sugar, glucose syrup, hydrogenated coconut oil, gelatin, dextrin, "natural flavor", corn starch, and gum arabic. They are basically just a big pellet of flavored sugar with gummy stuff added to give them structural integrity and to keep them from sticking together in the package.

This is a complex mixture of ingredients. In general, for a substance to reduce the surface tension of pure water (72 ergs/cm^2) the substance must be "surface active". This means that it has a hydrophobic end (water hating) and a hydrophilic end (water loving). Compounds like sugars (sucrose, glucose, etc.) with lots of hydroxyl groups do not generally meet this requirement. Arabic acid (one of the components of gum arabic) is a high molecular weight poly-sugar. So I would not think that it is the cause of the fizz. But there are certainly other components in the candy recipe that could do this. In my opinion, a more likely mechanism is the introduction of efficient "seeding" sites for the formation of gas bubbles, and there are many candidates either in the recipe explicitly or implicitly to do so. For example, starch and/or fine particle silica is often added to sugar to keep it free flowing, so either of these could be in the mint without having been added explicitly as one of the ingredients. Seed sites are more likely to induce a rapid fizz than is a nominal reduction in surface tension. The surface tension of the soda may already be significantly lower than that of pure water because of the presence of components like hydrogenated coconut oil which certainly would be a candidate surface active agent. It is difficult to pinpoint the exact cause of the effect in such a complex mixture.

Vince Calder



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