Enlarge /. A well-coordinated mentos-and-diet-coke explosion that was filmed in slow motion in 2013 for The Slow Down Show.
As early as 2006, Fritz Grobe and Stephen Voltz – the self-described crazy scientists behind Eepybird – sparked an internet sensation with their viral video of an elaborate version of the well experiment Diet Coke and Mentos and simulated the choreography of the world-famous fountain display of the Bellagio in Las Vegas. The underlying physics and chemistry of the well effect are known.
However, an intrepid pair of scientists at Spring Arbor University in Michigan wondered if the height and associated changes in air pressure would have a measurable impact on the intensity of the foaming fountain, and did a number of experiments to find out. They reported their results in a recent article in the Journal of Chemical Education. The conclusion: If you really want to achieve the most foaming action for your money, carry out the Diet Coke and Mentos experiment at great heights.
Grobe and Voltz did not invent the basic demo. This has been around since at least the 1980s, although originally creative science teachers used Wint-O-Green Lifesavers that were threaded on a pipe cleaner to induce the foam fountain in soda bottles. In 1990 the size of the lifesaver changed and was too big to fit in the bottle mouth. Therefore, science teachers switched to Mint Mentos sweets to achieve the same effect.
More physics than chemistry
The effect is often misunderstood as a chemical reaction, similar to mixing baking soda with vinegar, but the underlying mechanism is actually a physical reaction: the nucleation of bubbles. A standard bottle of two liters of Diet Coke at room temperature usually contains about 12 grams of dissolved CO2. Release this seemingly insignificant 12 grams of dissolved CO2 at room temperature into the gas phase and it will expand to fill three times the total volume of the original soda bottle, or about six liters. This satisfying growl when you open a fresh soda bottle is the result of CO2 escaping. Then bubbles begin to form in the liquid and float upwards.
This usually happens slowly, which will make your soda flat over time, as the surface tension of the water in the soda helps trap the gas in place. Drop several Mentos candies in the freshly opened bottle, and that happens very quickly. This is because the surface of the candy has many small bumps. Dissolved CO2 diffuses into the bubbles, which inevitably form on the rough surface of the candy, and releases the bonds to the water much faster. The result is a towering foam geyser.
<img alt = "Fritz Grobe and Stephen Voltz conduct the" Diet Coke and Mentos Experiment "on 53rd Street in front of the Ed Sullivan Theater on Late show with David Letterman in 2006. "src =" https://cdn.arstechnica.net/wp-content/uploads/2020/04/mentos3-640×426.jpg "width =" 640 "height =" 426 "srcset =" https: / / cdn.arstechnica.net/wp-content/uploads/2020/04/mentos3.jpg 2x”/>Enlarge /. Fritz Grobe and Stephen Voltz conduct the "Diet Coke and Mentos Experiment" on 53rd Street in front of the Ed Sullivan Theater on David Showman's Late Show in 2006.
Jeffrey R. Staab / CBS via Getty Images
Co-author Thomas S. Kuntzleman first encountered the experiment in 2005 at a science fair in primary school and has been interested in the underlying physics and chemistry ever since. For example, he found that various beverage additives such as sugar, citric acid and natural flavors can improve the height of the well. Most recently, he decided to test the hypothesis that the intensity of the well effect would be greater at higher altitudes, since atmospheric pressure would be expected to play a significant role in the nucleation of bubbles.
Kuntzleman and his student co-author Ryan Johnson bought bottles of Diet Coke in the same store and made sure that all bottles had the same expiration date. For each experiment, they threw a single Mentos candy into a bottle through a 1-inch PVC pipe, and then watched the reaction take effect. They used a measuring bottle / tornado tube to measure the mass loss of the liquid over time. The measurements were all done within a week and the team made sure that all the bottles were at approximately the same temperature.
A number of experiments were carried out at different altitudes during an ascent to Pikes Peak; Another sentence took place while driving through North Carolina. and a third set of experiments were performed during Kuntzleman's family vacation as he drove through several national parks in California, Nevada and Utah. Overall, they conducted the experiment at altitudes from below sea level (Death Valley, CA) to over 14,000 feet (Pikes Peak). As expected, they found that more foam was produced at higher altitudes.
According to the authors, however, the influence of the altitude is not solely due to the simple application of known gas laws, namely that gases expand at lower atmospheric pressures. It was easy enough to test this with data collected on Kuntzleman's family vacation, as the soda bottle temperature had been carefully controlled and measured in this series of experiments. The gas laws state that the product of gas pressure and volume should be constant at constant temperatures, but the data showed an increase in the product of volume pressure with increasing altitude.
SEM image of the surface of a Mentos candy.
Bubbles begin to form on the porous surface of the Mentos candy.
When researchers dropped a candy into soda below sea level (left), the soda foamed less than 10,000 feet above sea level (right) than when performing the same experiment.
Kuntzleman & Johnson / Journal of Chem. Educ.
"While the gas laws are very likely to contribute to the observed effect, the data presented suggest that this is not the only explanation for the increased foaming," the authors wrote. "Therefore, additional kinetic effects are likely to play a role, which increase foaming at higher altitudes, such as increased bubble frequency and increased ulceration sites (due to a smaller critical radius) on the candy."
It is a useful addition to Kuntzleman's teaching repertoire. For example, the basic demo can be combined with central concepts such as chemical kinetics, surface research, gas solubility and chemical equilibria. He has led students in both non-large science courses and general chemistry courses to recreate this latest series of experiments as a small research-based project and to achieve the same result. "That the popular Diet Coke and Mentos experiment works better at higher altitudes is a memorable way to remind non-Majokr and general chemistry students of the important idea that atmospheric pressure decreases with altitude," the authors wrote.
For those scrubbing up extended on-site orders, it also offers a possible fun DIY group experiment. Identify people in your social circles who live at different heights. Anyone can do the Mentos and Diet Coke demo – highly recommended outdoors – anywhere, noting the relative levels of the foam effect so everyone can compare their results. (Scientific American has helpful instructions, as does Eepybird.)
If you feel really ambitious, anyone can do the experiment live simultaneously using Zoom or a similar conference tool. Simply protect your electronic devices. And make sure you use Diet Coke or Coke Zero, not only because they produce the greatest response – the aspartame lowers the surface tension of the water – but also because they are less sugary and sticky. Trust us, you will be drenched.
A spectacular, new version of the Bellagio Fountain in Las Vegas brought to you by the crazy scientists at EepyBird.com.
DOI: Journal of Chemical Education, 2020.10.1021 / acs.jchemed.9b01177 (About DOIs).