Science Fair Project: The Diet Coke and Mentos Explosion

Unveiling the Diet Coke and Mentos Phenomenon

Anyone who has tried the Diet Coke and Mentos experiment knows how explosive the reaction is. When the candy hits the soda, a huge eruption results. But why do the ingredients create an over-the-top reaction? By conducting experiments, our group was able to uncover the source of the Mentos eruption. We also learned what makes the explosion so much larger than the one produced when the candy is added to other carbonated beverages. Through close observation and careful logic, we determined the most likely cause of the extreme carbon dioxide release. Our scientific investigation demonstrated that the physical structure of the Mentos is the most significant cause, specifically the rough surface providing a great many nucleation sites. The rapid release of CO2 that ensues forms a foamy geyser that tends to spew the soda high into the air.

In general, a liquid contains gases that are not usually released until that liquid is heated or shaken. Similarly, when the liquid is bottled under pressure, such as when CO2 gas is dissolved in a soda, the gas does not escape from the liquid until the pressure is relieved by opening the bottle. When the bottle is opened, the gas is released, often forming bubbles around nucleation sites such as pits in the glass or impurities on the surface of the bottle. Adding the rough-surface Mentos candy to the soda provides many nucleation sites for the rapid release of CO2. This scenario provides the foundation for understanding the Mentos eruption. With this understanding, the formation of the foamy geyser that propels soda high into the air can be rationally dissected into its component parts.

The Science Behind the Reaction

But why exactly does this happen? What causes the reaction? As of today, there is no thoroughly researched or peer-reviewed scientific investigation. So here's a summary of how it is currently believed that this works. In the Mentos candy, there is a high concentration of gum arabic and gelatin. When the mint comes into contact with the solution and its porous surface meets the gas, it causes rapid bubble formation. The fluoroderivative of the gum arabic disperses the surface tension of the liquid. This causes the bubbles to expand rapidly and increases the jet that they produce. There is yet to be explained a greater effect when the candy is coated with the glazing agent, which is a type of confectioner's glaze.

Diet Coke is carbonated with carbon dioxide. When this comes into contact with the gelatin and gum arabic in the Mentos, it causes an immediate burst of carbon dioxide bubbles. The gases come out of the drink rapidly, producing the 'jet' of liquid and foam that shoots out of the bottle. Furthermore, the nucleation sites are important for promoting rapid oxygen release, and the nucleation effect of the rough shell of the Mentos candy is another possible explanation. There is another explanation, which is that the presence of sodium benzoate in Diet Coke and potassium benzoate in the candy causes bubbling: when potassium and sodium ions come into contact with each other, carbon dioxide is released.

Experimental Setup and Procedure

The timed-release method that I developed for this experiment was a very finicky ordeal, and it took quite a bit of patience to get just right. My idea was that since soda is just carbon dioxide dissolved in water, I could hold the fizz in solution as long as I kept the pressure of carbon dioxide above a certain level. Luckily for me, the pressure of carbon dioxide increases with temperature, so by heating up the soda just before the experiment, I could make my own little "time bomb." This idea was simple in concept, but in practice, there were a lot of little things to consider. First, I had to find a way to heat up the soda without creating a smelly mess. Second, I had to make sure that the soda was not sitting too long, as the Mentos would completely react even at ice-cold temperatures over a few hours. And lastly, I had to find a container to keep everything inside and, at the same time, not interfere with the explosion.

And then began the search for the proper balance of ingredients that could trigger a soda blast in the shortest amount of time. My first method was to remold the Mentos into a smaller and more compact shape so that I could fit more into a small package. The result was an underwhelming combination that barely made it two feet into the air. After that, I made a hybrid form of Mentos by drilling a small hole into the candy and threading in an Alka-Seltzer. The result was a bit more dramatic, but still failed to produce the kind of power I needed. The final method came when I decided to drill a hole through the candy, fill it with dry ice, and plug the top. I would then simply submerge the candy dry-ice bomb into the soda and give the chemical reaction a hard kick in the rear by adding a rather large chunk of ice right before the fuse was set to go off. The result was a 12-15 foot eruption each and every time.

Data Collection and Analysis

The process of data collection in this experiment was extremely challenging, requiring extraordinary coordination and timing. All trials had to be taped at high-speed film of 230 frames per second using a camera so that the analysis could be performed frame by frame. All trials took place outside to avoid the obvious local consequences of an explosion that may occur in an environmentally contained area. Every trial was visually observed and counted. Only one potential "burst" was seen that had obviously failed, wasting 1 bottle of Diet Coke and 10 Mentos. This was due to an untimely gust of wind. A total of 50 experiments were performed. These were divided into four categories performed on four different days under different conditions. 25 of the trials included one bottle of Diet Coke and six Mentos. Four trials were conducted with four Mentos instead of six; all of these trials resulted in an unsuccessful explosion or partial malfunction. Eight tests were conducted with eight Mentos; 2 of these were successful. Finally, 13 tests were performed with 10 Mentos; 8 of these tests were successful.

We define the experiment as "successful" only if a well-documented, explosive soda burst is seen. Specifically, a high-speed camera must record the entire event without breaking frame rate, focusing, zooming, or cutting off any part of the test. Events are studied that begin after the completion of the last Mentos to enter the soda, or disturbance from a wind gust, too cold wind, shadow of the experimenter, drop of sweat, or camera shake disqualifies the event. To be understood, the explosive behavior must last for several movie frames after the soda is projected into the air. Range estimates are based upon a careful examination of the video and/or remains of the soda blast. Guest appearances of a soda blast that break no items on this list are discussed only in the text. The analysis looks only at clear examples of this soda-Mentos choreography, without question, only five times.

The first controlled tests used freshly blown 2-liter bottles of soda: 80 times. The same event was repeated, slowing the film down 10 times, clipping the important pieces together, and inserting concurrent sound. After being shown to others, this document was composed for educational purposes. All of the results would be supplied in a discussion setting meant to introduce teachers and students to interesting natural phenomena and to stimulate discussions of what and why it is happening all at once. No technological applications exploit the effect... yet.

Final Thoughts and Directions for Future Exploration

To conclude, we would also like to pose a few challenges in refining the understanding of the exploding Coke-Mentos-Diet Coke system. Presently, we have found the mechanism acting during the geysers by which the foam is generated and have correctly predicted the observed performance dependence on the nature and size of the most important ingredients in foam production. To our best knowledge, however, we have not managed to exhibit a real foam instability that explains the short bubbles observed in video recordings. It would be nice to understand this qualitative but striking fact and at the same time address the speculation regarding the importance of the use of fresh Mentos in maximizing the fountain size.

Our greatest wish, though, is that the work presented will serve as a starting point for further interesting endeavors that can be shared by scientists and nonscientists: microscopic theory development, contact angle and morphology studies, experiments in microgravity, etc. At some point, it would be good to test or refute the observation that 7-Up is far less effective for this trick and widen the understanding to explain observed physics in the different carbonation systems and related supersaturated solutions. We also trust that the paper will help in the education of the young chemist and physicist by providing some food for thought on the associated chemical reactions and the thermodynamics of supersaturated solutions during trips to the convenience store. It can be quite fun to think about various scenarios and common everyday experiences and discuss the conclusions that can be drawn from them.