<P> The eruption is caused by a physical reaction, rather than any chemical reaction . The addition of the Mentos leads to the rapid nucleation of carbon dioxide gas bubbles precipitating out of solution: </P> <P> CO 2 (aq) ⟶ CO 2 (g) (\ displaystyle (\ ce ((CO2 (aq)) -> (CO2 (g))))) </P> <P> The conversion of dissolved carbon dioxide to gaseous carbon dioxide forms rapidly expanding gas bubbles in the soda, which pushes the beverage contents out of the container . Gasses, in general, are more soluble in liquids at elevated pressures . Carbonated sodas contain elevated levels of carbon dioxide under pressure . The solution becomes supersaturated with carbon dioxide when the bottle is opened, and the pressure is released . Under these conditions, carbon dioxide begins to precipitate from solution, forming gas bubbles . Normally, this process is relatively slow, because the activation energy for this process is high . The activation energy for a process like bubble nucleation depends on where the bubble forms . It is highest for bubbles that form in the liquid itself (homogeneous nucleation), and lower if the bubble forms on some other surface (heterogeneous nucleation). When the pressure is released from a soda bottle, the bubbles tend to form on the sides of the bottle . But because they are smooth and clean, the activation energy is still relatively high, and the process is slow . The addition of other nucleation sites provides an alternative pathway for the reaction to occur with lower activation energy, much like a catalyst . For instance dropping grains of salt or sand into the solution lowers the activation energy, and increases the rate of carbon dioxide precipitation . The physical characteristics of Mentos (surface roughness, easy dissolution into the liquid, etc .) have the effect of drastically reducing the activation energy for carbon dioxide bubble formation, so that the nucleation rate becomes exceedingly high . The activation energy for the release of carbon dioxide from Diet Coke by addition of Mentos has been found to be 25 kJ mol . This formation of gaseous carbon dioxide within the water, which due to hydrogen bonding and its high surface tension wants to maintain a connected matrix of molecules, causes the water to foam, ultimately generating the "jet"--or "geyser"--or eruption - like nature of the effusion . The foaming is aided by the presence of chemicals like potassium benzoate and aspartame in Diet Coke, and gelatin and gum arabic in the Mentos candy, all which influence the nature of water's surface tension and degree to which it can foam . </P> <P> The nucleation reaction can start with any heterogeneous surface, such as rock salt, but Mentos have been found to work better than most . Tonya Coffey, a physicist at Appalachian State University, found that the aspartame in diet drinks lowers the surface tension in the water and causes a bigger reaction, but that caffeine does not accelerate the process . It has also been shown that a wide variety of beverage additives such as sugars, citric acid, and natural flavors can also enhance fountain heights . In some cases, dissolved solids that increase the surface tension of water (such as sugars) also increase fountain heights . These results suggest that additives serve to enhance geyser heights not by decreasing surface tension, but rather by decreasing bubble coalescence . Decreased bubble coalescence leads to smaller bubble sizes and greater foaming ability in water . Thus, the geyser reaction will still work even using sugared drinks, but diet is commonly used both for the sake of a larger geyser as well as to avoid having to clean up a sugary soda mess . </P>

What is the science behind mentos and diet coke
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