SurfinOnDryLand replied: "Many scientists...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. 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. When a scoop of ice cream is added to root beer, the "float" foams over for essentially the same reason. The surface tension of the root beer is lowered by gums and proteins from the melting ice cream, and the CO2 bubbles expand and release easily, creating a beautiful foam on top ----------------- There has been a lot of discussion online about why Diet Coke and Mentos make such an interesting combination. What everyone can agree on is that the carbon dioxide that has been compressed into the soda escapes so rapidly that the pressure pushes the soda out of the bottle. Its like shaking a bottle before you open it, but even more dramatic. Several people theorized that a substance called gum arabic in the Mentos breaks the surface tension of the soda, allowing the carbon dioxide bubbles to escape rapidly. This explanation doesnt completely work. One thing that casts doubt on the gum arabic theory is that several items (like pennies!) that contain no gum arabic whatsoever also cause soda to foam violently. Several chemists have since stepped forward to say that while gum arabic may contribute to the reaction, that cant be the only cause. These chemists are saying that the primary cause is physical, not chemical. Their explanation: nucleation sites. If you have a liquid that is supersaturated with gas (like soda, which is pumped full of carbon dioxide), a nucleation site is a place where the gas is able to form bubbles. Nucleation sites can be scratches on a surface or specks of dust anywhere that you have a high surface area in a very small volume. That's where bubbles can form. Mentos seem to be loaded with nucleation sites. In other words, there are so many microscopic nooks and crannies on the surface of a Mento that an incredible number of bubbles will form when you drop it in a bottle of soda. Since the Mentos are also heavy enough to sink, they react with the soda all the way to the bottom. The escaping bubbles quickly turn into a raging foam, and the pressure builds dramatically. Before you know it, you've got a big geyser happening!"