We have seen this demonstration of a chemical reaction in many places, but have never done it ourselves. We decided to hold our own experiment by comparing the reactions of Diet Coke with that of Diet Pepsi. We used five Mentos with a 2-liter bottle. I taped a 3-foot measuring stick to the bottle to give us an idea of height comparisons.
I actually could only get 4 Mentos in the Pepsi before it exploded on me.
As you can see, Diet Pepsi won out on our test by a landslide.
Why does this reaction happen with diet soda and Menthos?
It is caused by something called nucleation. Bubbles of carbon dioxide "nucleate" shortly after the pressure is released from a container of carbonated liquid. Nucleation often occurs more easily at a pre-existing "interface", or a surface forming a common boundary among two different phases such as using string to make crystals form rock candy from a sugar-water solution. The surface of the mint Mentos is such an interface, as it is covered with many small holes that increase the surface area available for reaction, thereby allowing CO2 bubbles to form with such a rapidity and in such a quantity that a geyser is formed. A 2006 episode of the television series 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.