page_title: Diet Coke and Mentos eruption
section_title: Cause
hierarchical_section_title: Diet Coke and Mentos eruption / Cause
caption_attribution_description: English: SEM image of the surface of a Mentos candy
context_page_description: A Diet Coke and 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 candies catalyze the release of gas from the beverage, which 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 publicly demonstrate the experiment on the Late Show with David Letterman in 1999. Steve Spangler's televised demonstration of the eruption in 2005 became popular on YouTube, launching a chain of several other Diet Coke and Mentos experiment viral videos. Experiments carried out at altitudes ranging from below sea level in Death Valley to the summit of Pikes Peak have demonstrated that the reaction works better at higher elevations.
context_section_description: 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. 
  The activation energy for bubble nucleation (formation of bubbles) depends on where the bubble forms. It is very high for bubbles that form in the liquid itself (homogeneous nucleation), and much lower if bubble growth occurs within tiny bubbles trapped in some other surface (heterogeneous nucleation). Bubble nucleation and growth in carbonated beverages almost always occurs by heterogeneous nucleation: diffusion of carbon dioxide into pre-existing bubbles within the beverage. When dissolved gas diffuses into bubbles that already exist in a liquid, it is called Type IV bubble nucleation. When the pressure is released from a soda bottle upon opening it, dissolved carbon dioxide can escape into any tiny bubble located within the beverage. These ready-made bubbles (which are nucleation sites) exist in things such as tiny fibers or non-wettable crevices on the sides of the bottle. Because there usually are very few of such pre-existing bubbles, the degassing process is slow. Mentos candies contain millions of cavities, roughly 2-7 μm in size, that remain unwetted when added to a soda. Because of this, addition of Mentos candies to a carbonated beverage provides enormous numbers of pre-existing bubbles into which dissolved carbon dioxide can escape. Thus, adding Mentos candies to a carbonated beverage introduces millions of nucleation sites into the drink, which allows for degassing that is rapid enough to support a jet of foam out of a bottle.  
  Pre-existing bubbles provide a way for the reaction to occur without requiring bubbles to form within the liquid itself (homogeneous nucleation). Because Type IV nucleation sites (such as found on Mentos) allow the reaction to proceed with a substantially lower activation energy, Mentos candies can appropriately be considered a catalyst of the process. As another example, dropping grains of salt or sand into the solution provides Type IV nucleation sites, lowers the activation energy compared to that of homogeneous nucleation, and increases the rate of carbon dioxide precipitation.  
  The physical reaction that drives the eruption also causes a chemical reaction that slightly increases the pH of the soda. This is because when carbon dioxide is dissolved in water, carbonic acid is formed: 
   
  Loss of carbon dioxide shifts the above equilibrium to the left, removing carbonic acid and increasing pH. It is the physical reaction (carbon dioxide leaving solution) and not the loss of carbonic acid that causes the eruption. 
  The physical characteristics of Mentos (surface roughness) 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. The foaming is aided by the presence of food additives such as potassium benzoate, aspartame, sugars, and flavorings in Diet Coke,  all which influence the degree to which water can foam. It has been claimed that gelatin and gum arabic in the Mentos candy enhance the fountain, but experiments have shown that these candy additives have no effect on the fountain.
  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 
caption: SEM image of the surface of a Mentos candy.
exif: {}