"Many people speculated that the geyser was the result of an
acid base reaction, given the low pH of soda. However, none of the ingredients in mentos are basic, and the experiment works to some degree with any type of soda and any type of candy.
Mentos candies are not as smooth as they appear to the naked eye. They are covered in bumpy craters, which increases the total surface area. A Mentos dropped into a bottle of soda acts as a
surfactant, meaning it reduces the surface tension of the soda. Water molecules are polar and attracted to each other. Anything that breaks them apart allows for bubbles of carbon dioxide gas to form in the solution. A rougher candy surface translates to more places for bubbles to grow, or more
nucleation sites. Surfactants are compounds that lower the surface tension (or interfacial tension) between two liquids or between a liquid and a solid. Surfactants may act as detergents, wetting agents, emulsifiers, foaming agents, and dispersants.
This is essentially speeding up the process that makes sodas fizzy. Bottled sodas are kept under pressure so that more carbon dioxide can be forced into solution. When the pressure is released, the carbon dioxide is forced out of solution and makes little gas bubbles. So, if you open a bottle of soda gently, you get a pleasant beverage. If you shake the can first, you disrupt the solution and get a face full of soda. And if you add a big enough surfactant, you get a geyser. It's the same chemistry, but a different magnitude.
The two biggest factors affecting the geyser are the roughness of the candy used and the rate at which it sinks to the bottom of the soda bottle.
[1] Other factors that affect the growth rate or total number of carbon dioxide bubbles also changed the geyser's height, such as temperature and the original surface tension of the soda. Diet Coke makes a better spectacle than regular Coca-Cola because both aspartame and benzonatate (a preservative used in artificially sweetened drinks) lower surface tension more than sugar does."
SOURCE:
Why Do Mentos Explode in Coke? | Brilliant Math & Science Wiki - https://brilliant.org/wiki/why-do-mentos-explode-in-coke/
EDIT: The watermelon played no role in this reaction. It was merely a way to produce a focal point for the stream to expel out of into a more straight geyser.
