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The Science Behind Viral Soda Fountains

This article first appeared in the 5/31/20 edition of the Davis Enterprise

The Science Behind Viral Soda Fountains

Menots geyser with 5 drops of plain Mentos. From left: carbonated water (Perrier), Classic Coke, Sprite, Diet Coke. The green marks are with 0.5m separation. Photo by K. Shimada

The science behind viral soda fountains


By Sara Thompson

Special to the Enterprise


The popular at home experiment of dropping Mentos into Diet Coke became viral on the internet in 2005.  Prior to this it had been featured on evening programs and news affiliates in the late 90’s and early 2000s.  But before the famous pairing of Mentos with Diet Coke, there was another popular variation in the 1980’s.  Life Savers had a flavor called “Wint-O-Green” that could fit through the tops of soda bottles and create the same soda geyser we love now.  In the mid-late 90’s, the candy size increased and could no longer fit through the bottle top, and thus Mentos was found to produce the same effect and became the go-to candy for soda geysers.


Many assume the reaction to produce the tall soda geysers is a chemical one, similar to baking soda and vinegar, but it is in fact a physical reaction!  The soda has carbon dioxide dissolved in the solution that is held under pressure, when you open a soda you hear a hiss, that is the pressure releasing and the bubbles that form are some of the carbon dioxide expanding and being released in its gaseous state.  Not all of the carbon dioxide escapes just from opening the soda, which is why it stay bubbly for a while after opening, but it will begin to precipitate, or leave, the solution, resulting in an eventual flat soda.  When you add a Mentos to the solution, it introduces a new means for the carbon dioxide to escape. Mentos are not entirely smooth like they appear, they are actually quite rough when looked at under a microscope.  When dropped into the soda, the rough texture creates more surface area for the carbon dioxide to become gaseous and expand.  Because the Mentos is denser than the liquid, it sinks, helping to release more and more carbon dioxide as it goes down, seeding more bubbles above.  The rapid expansion of gasses causes the liquid solution to erupt out the top because it has nowhere else to go.


One of the reasons that Diet Coke works better than regular is because the artificial sweetener creates a weaker surface tension than the sweeter Coke.  When trying this at home, remember to do it outside as you will make a huge mess!  Try the experiment with different flavors of soda and with different flavors of Mentos, do they react the same?  Does cutting up the Mentos or crushing it up also change the experiment or share the same results?



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