A Diet Coke and Mentos eruption (or Diet Coke and Mentos
geyer) is a reaction between a carbonated beverage and Mentos candies that
causes the beverage to spray out of its container.
The numerous small pores on the candy's surface catalyze the
release of carbon dioxide (CO2) gas from the soda, resulting in the rapid
expulsion of copious quantities of foam. Although any carbonated beverage will
produce a similar effect, the reaction was popularized using Diet Coke for
seemingly producing the best results.
When the Mentos come into contact with the Diet Coke, a
reaction causes the rapid formation of foam.
MythBusters concluded that the potassium benzoate, aspartame,
and CO2 gas contained in the Diet Coke, in combination with the gelatin and gum
arabic ingredients of the Mentos, all contribute to the formation of the
foam.
The structure of the Mentos is the most significant cause of
the eruption due to nucleation. MythBusters reported that when fruit-flavored
Mentos with a smooth waxy coating were tested in carbonated drink there was
hardly a reaction, whereas mint-flavored Mentos (with no such coating) added to
carbonated drink formed an energetic eruption, affirming the nucleation-site
theory. According to MythBusters, the surface of the mint Mentos is covered
with many small holes that increase the surface area available for reaction
(and thus the quantity of reagents exposed to each other at any given time),
thereby allowing CO2 bubbles to form with the rapidity and quantity necessary
for the "jet"- or "geyser"-like nature of the effusion.
This hypothesis gained further support when rock salt was used
as a "jump start" to the reaction. A paper by Tonya Coffey, a
physicist confirmed
that the rough surface of the Mentos candy helps speed the reaction. Coffey
also found that the aspartame in diet soda lowers the surface tension and
causes a bigger reaction, but that caffeine does not accelerate the
reaction.
