Diet Coke and Mentos eruption
A soda geyser (alternatively Diet Coke and Mentos geyser or Mentos eruption) is a reaction between the carbonated beverage Diet Coke and Mentos mints that causes the beverage to spray out of its container. The gas released by the candies creates an eruption that pushes most of the liquid up and out of the bottle. Lee Marek and "Marek's Kid Scientists" were the first to demonstrate the experiment on television in 1999. Steve Spangler's televised demonstration of the eruption in 2005 went viral on YouTube, launching a chain of several other Diet Coke and Mentos experiment viral videos.
In the 1980s, Wint-O-Green Life Savers were used to create soda geysers. The tubes of candies were threaded onto a pipe cleaner and dropped into the soft drink to create a geyser. At the end of the 1990s, the manufacturer of Wintergreen Lifesavers increased the size of the mints and they no longer fit in the mouth of soda bottles. Science teachers found that Mint Mentos candies had the same effect when dropped into a bottle of any carbonated soft drink.
Lee Marek and "Marek's Kid Scientists" performed the Diet Coke and Mentos experiment on the Late Show with David Letterman in 1999. In March 2002, Steve Spangler, a science educator, did the demonstration on KUSA-TV, an NBC affiliate, in Denver, Colorado. The Diet Coke and Mentos geyser experiment became an internet sensation in September 2005. The experiment became a subject of the television show MythBusters in 2006. Spangler signed a licensing agreement with Perfetti Van Melle, the maker of Mentos, after inventing an apparatus aimed to make it easier to drop the Mentos into the bottle and produce a large soda geyser. Amazing Toys, Spangler's toy company, released the Geyser Tube toys in February 2007. In October 2010, a Guinness World Record of 2,865 simultaneous geysers was set at an event organized by Perfetti Van Melle at the SM Mall of Asia Complex, in Manila, Philippines. This record was afterwards beaten in November 2014 by another event organized by Perfetti Van Melle and Chupa Chups in León, Guanajuato, Mexico, where 4,334 Mentos and soda fountains were set off simultaneously.
In May 2010, EepyBird, Perfetti Van Melle and The Coca-Cola Content Factory created a Coke Zero and Mentos Rocket Car that drove 221 feet (70 m) propelled only by Coke Zero and Mentos. The car was driven by Fritz Grobe with assistance of white-lab-coat partner Stephen Voltz; the commercial film was directed by Rob Cohen along with producer and Coca-Cola Content Factory founder Stafford Green. The car was later driven down the street in New York by David Letterman. 
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:
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. Gases, 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−1. 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 food additives such as potassium benzoate, aspartame, sugars, and flavorings in Diet Coke, and gelatin and gum arabic in the Mentos candy, all which influence the degree to which water can foam.
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.
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