How does Mentos and coke cause a chemical reaction?

The reaction between Mentos and Coca-Cola is primarily a physical reaction rather than a chemical one. When Mentos are dropped into Coke, the rough surface of the candies facilitates rapid nucleation, allowing carbon dioxide gas dissolved in the soda to form bubbles quickly. This sudden release of gas creates an explosive fountain of foam, as the pressure from the carbonation forces the liquid out of the bottle. The combination of the candy's ingredients and the soda's carbonation enhances this effect.