This is another very interesting reaction, which can be very messy and is best performed outdoors. Many people mistake what is happening here is a chemical reaction but in fact it is a physical reaction where the "trapped" carbon dioxide is suddenly released from solution, quite violently and causes the liquid (Coca-Cola) to spurt in a considerably high jet out of the top of the bottle.




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 (which is why the bottles are rock hard when you buy them, if yours isn't, then it's been opened, pick another one).

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 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 must be stressed that it is the physical reaction (carbon dioxide leaving solution) and not the loss of carbonic acid that causes the eruption.