we know well about the giant eruption of carbon dioxide in diet cola-mentos mix and also it is a physical reaction. But we must know what causes the diet cola to liberate carbon dioxide in short period.
Proof for the physical reaction1 (not acid-base reaction):
There was a myth that an explosive acid-base reaction taking place when we mixup diet cola and mentos, but it has been disproved as following; Usually we can observe the changes in pH value if it’s a acid-base reaction. Diet cola has the pH of 3.0 and this value didn’t change after the addition of Mentos, which means clearly it’s a not acid-base reaction. But if you add baking soda into the diet cola then the pH values increases to 6.1.
Potassium benzoate, Aspartame and Gum Arabic:
Mythbusters1 and Tonya Shea Coffey2 described that the presence of gum arabic (a surfactant) in Mentos, and potassium benzoate (a preservative), aspartame (a sweetener of food and most common in beverages) in diet cola causes the eruption of carbon dioxide. The detailed investigation gives an idea that when we drop a Mentos into the diet cola container it reaches the bottom and the surfactant “gum arabic” which reduces the surface tension of water (water resists the expansion tiny carbon dioxide bubbles by means of forming strong attraction between water molecule and the tiny bubbles require more energy to push water molecule away from each other for the purpose of expansion; this phenomenon called “surface tension”; ultimately the dissolved tiny carbon dioxide bubbles around the Mentos escapes, these liberated bubbles from the bottom of the container will act as an growth site, which will help the remaining dissolved carbon dioxide to over the surface tension of water and causes the giant eruption of gas.
There’s an further explanation for the origin of fizz:
Cola already has carbonic acid (potassium benzoate will be the form of benzoic acid at lower pH) and carbon dioxide in the a certain equilibrium which gets disturbed upon the addition of Mentos.
1. Adam Savage and Jamie Hyneman. “Episode 57: Mentos and Soda.” Mythbusters, Discovery Channel, first aired August 9, 2006.
2. Tonya Shea Coffey, “Diet Coke and Mentos: What is really behind this physical reaction?,” American Journal of Physics 76, no. 6 (June 0, 2008): 551-557, doi:10.1119/1.2888546.