I first saw the Diet Coke and Mentos experiment during a science fair at an elementary school in 2005, and I was instantly hooked! To perform this experiment, Mentos candies are dropped into a bottle of carbonated beverage; Diet Coke tends to be the beverage of choice. In the video below you can see this experiment play out in slow motion. My son, John, captured the video from his second-story bedroom window:
The fountain results from the rapid formation and expansion of carbon dioxide gas bubbles in the beverage as a result of the addition of the candy. You have probably noticed bubbles of carbon dioxide form in the liquid any time a bottle of carbonated beverage is opened. However, the formation of these bubbles occurs very slowly because the activation energy for bubble formation in water is relatively high. The addition of Mentos candy to a carbonated beverage tremendously lowers this activation energy. That’s because pits and pockets on the surface of the Mentos candy, called nucleation sites, provide already-formed gas bubbles into which dissolved carbon dioxide can easily escape. Thus, adding Mentos candy to a carbonated beverage allows for rapid expansion of gas bubbles, which results in a fountain. You can watch this process in slow motion around a Mentos candy placed in a carbonated beverage in the video below:
In my opinion, Diet Coke and Mentos has all the hallmarks of a great science experiment for teachers and students: It is easy to set up and conduct, it can be accomplished using simple and familiar materials, it produces a dramatic and unexpected result, and it relates to a large number of physical and chemical concepts. It will come as no surprise to you that I have performed this experiment hundreds of times during class lectures, laboratory sessions, and demonstration shows (and also while just goofing around at home!)
Even better, I think this experiment provides a fantastic vehicle to involve students of all ages in small, hands-on and exploratory research projects. Like many others, my students and I have investigated various aspects of this interesting fountain. It’s fun, for example, to try this experiment with carbonated beverages that have been incubated at different temperatures:
Most recently, we looked into a curious phenomenon that we discovered: Fountains produced using flavored seltzer water (which contains water, dissolved carbon dioxide, and natural flavorings) go much, much higher than fountains produced using unflavored seltzer water (which contains water and dissolved carbon dioxide alone). Check it out:
Wow! That’s a big difference. The presence of natural flavorings causes an enormous effect on fountain height! It has been known for some time that the beverage additives aspartame and benzoate contribute to higher fountains, but we were quite surprised to learn that natural flavorings have the same effect. This led to several questions such as “how might natural flavorings lead to higher fountains?”, and, “what other substances might cause higher fountains?” So we began adding carefully measured amounts of all sorts of stuff to seltzer water: citric acid, sugars, alcohols, etc. By doing so we learned that we could dissolve just about anything in seltzer water to produce higher fountains, so long as enough of the material was added.
We also carefully looked at the bubble sizes formed during the experiment, and noticed that smaller bubbles formed when adding Mentos to carbonated water that contained dissolved materials. You can see this effect in the video below:
We worked on this for a while and were able to show a strong correlation between decreased bubble size and increased fountain heights in the Diet Coke and Mentos experiment. You can learn a lot more about our findings by checking out our article published in the Journal of Chemical Education or this infographic on the Compound Interest site. Both of these sources describe in more detail how smaller bubble size leads to higher fountains. The article also provides some suggestions for new and simple demonstrations that connect to the Diet Coke and Mentos experiment.
Speaking of new demonstrations, if you and your students have any suggestions for experiments to try, please let me know. I’m always looking for new aspects of the Diet Coke and Mentos reaction to investigate, especially ones that can be explored in slow motion. Better yet, have your students get out there to try some experiments on their own and explain the results using chemistry!