Ask A Scientist Materials Science Archive

name         Gary
status       other
grade        other
location     KY

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?
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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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