A Soda and Candy eruption (also known as a Diet Coke and Mentos eruption or a Coke geyser) is a reaction of carbonated beverage and Mentos candies. When the candies come into contact with the liquid, a reaction causes the formation of foam at a rapid rate. A 2006 episode of the television show Mythbusters concluded that the caffeine, 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 formation of the foam.
The structure of the Mentos is the most significant cause of the eruption due to nucleation. Mythbusters reported that when flavored Mentos with a smooth waxy coating were tested in carbonated drink there was hardly a reaction, whereas standard Mentos added to carbonated drink formed an energetic eruption, affirming the nucleation-site theory. The surface of the mint Mentos is littered 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 a rapidity and in a quantity that are responsible for the “jet”- or “geyser”-like nature of the effusion.