Frederick Sparber wrote:
http://www.stevespanglerscience.com/experiment/00000109 " For example, adding salt to soda causes it to foam up because thousands of little bubbles form on the surface of each grain of salt. Many scientists, including Lee Marek, claim that the Mentos phenomenon is a physical reaction, not a chemical one. Water molecules strongly attract each other, linking together to form a tight mesh around each bubble of carbon dioxide gas in the soda.
What's more, the ions in the water form complex ions with the carbonate ions, consisting of many small units loosely bonded together. Physical agitation, among other things, can break up the complexes, allowing the soda to suddenly gallop in the direction of equilibrium, which is why soda foams when you shake it.
Or, at any rate, that's what my high school chemistry teacher said when I asked him why it does that, many many years ago. I've never read anything since about that particular phenomenon, but I've also never run across another explanation that seemed so plausible for why carbonated beverages behave the way they do.
If the Mentos tabs also disrupt the complex ions, then the effect might still be "physical" but at a lower level than anything involving actual gas bubbles.
In order to form a new bubble, or even to expand a bubble that has already formed, water molecules must push away from each other. It takes extra energy to break this "surface tension." In other words, water "resists" the expansion of bubbles in the soda. When you drop the Mentos into the soda, the gelatin and gum arabic from the dissolving candy break the surface tension. This disrupts the water mesh, so that it takes less work to expand and form new bubbles. Each Mentos candy has thousands of tiny pits all over the surface. These tiny pits are called nucleation sites - ! perfect places for carbon dioxide bubbles to form. As soon as the Mentos hit the soda, bubbles form all over the surface of the candy. Couple this with the fact that the Mentos candies are heavy and sink to the bottom of the bottle and you've got a double-whammy. When all this gas is released, it literally pushes all of the liquid up and out of the bottle in an incredible soda blast. You can see a similar effect when cooking potatoes or pasta are lowered into a pot of boiling water. The water will sometimes boil over because organic materials that leach out of the cooking potatoes or pasta disrupt the tight mesh of water molecules at the surface of the water, making it easier for bubbles and foam to form. "
