Final answer:
Mentos candy rapidly catalyzes nucleation in the Coke and Mentos experiment due to its rough surface, leading to the swift release of carbon dioxide gas and causing an explosive foam jet.
Step-by-step explanation:
Mentos is needed for the Coke and Mentos experiment because it causes rapid nucleation, the formation of gas bubbles, in the carbonated beverage. The surface of Mentos candies is rough and contains many tiny pits where carbon dioxide bubbles can form rapidly. Once a Mentos is dropped into a bottle of Coke, the carbon dioxide gas that is dissolved in the soda quickly forms bubbles on the surface of the candy. This rapid formation of gas bubbles causes a rapid release of gas, leading to the famous jet of foam that erupts from the bottle.
This phenomenon illustrates principles of nucleation in physical chemistry and demonstrates the effect of surface area on reaction rates. The rough surface of the Mentos accelerates the CO2 bubble production, resulting in a dramatic reaction.