Mentos candies are disc-shaped tablets made with sugar, wheat glucose syrup, hydrogenated coconut oil, rice starch, gum arabic, sucrose, esters of fatty acids, gellan gum and natural flavors. To the eye, the candy tablet looks smooth, but if you look up close with a microscope, the surface has many tiny holes and cracks. This plays a part in how the candy reacts to the Pepsi.
Pepsi is made using carbonated water, high fructose corn syrup, caramel color, sugar, phosphoric acid, caffeine, citric acid and natural flavors. The key ingredient that aids in the reaction to the Mentos is carbonated water.
When you drop Mentos candy pieces into Pepsi, a chemical reaction makes the liquid in the Pepsi expand. It expands by producing tons of tiny bubbles that shoot out of the mouth of the bottle at high force -- like a geyser. If you try this experiment, put on safety glasses, and don't stand in front of the bottle. Perform this experiment with a freshly opened bottle of Pepsi. Do the experiment outside, so any mess soaks into the ground.
Carbonation, chemically known as carbon dioxide, in the Pepsi fills the tiny holes and cracks in the Mentos. This breaks apart the water bond in the surface of the Mentos. The carbon dioxide acts as an acid. The base chemicals in Mentos like starch and sucrose create the bubbling, foaming action when they come in contact with the acid. Mixing baking soda and vinegar creates the same reaction. Baking soda acts as the base, and vinegar acts as the acid. Combining the two creates a big, foaming mess.