Mentos makes soda fizz due to the Mentos breaking the bonds in water molecules and encouraging the growth of more carbon dioxide gas, which is used to carbonate sodas. This is due to a combination of a rough surface area, the density of Mentos candies and their coating.
Sodas contains water molecules and dissolved carbon dioxide gas. The carbon dioxide gives sodas their characteristic bubbles. Water molecules prefer to bond together, creating a stable network of molecules in the soda. This network of molecules also prevents carbon dioxide from escaping due to surface tension. Any item dropped into soda can disrupt this network of water molecules, allowing more areas for carbon dioxide growth.
Mentos candies are particularly effective for breaking water molecule bonds for several reasons. Due to their rough surface and large surface area to volume ratio, Mentos candies allow more sites for carbon dioxide to grow. Mentos candies are also coated with gum arabic, a surfactant that is effective in reducing the surface tension of liquids. As Mentos are also dense, they sink quickly and create more bubbles in the process. The growth of bubbles increases the pressure in the soda, eventually to the point where the bubbles can escape.
Mentos are most effective in liquids with lower surface tension. The lower the surface tension, the easier it is for carbon dioxide to escape the liquid. Mentos are particularly effective when placed in Diet Coke. Diet Coke contains aspartame, which has low surface tension and allows for more dramatic reactions.