Wed. Sep 28th, 2022
A well-coordinated explosion of Mentos-and-Diet-Coke filmed in slow motion for The Slow Down Show in 2013.
enlarge A well-coordinated explosion of Mentos-and-Diet-Coke filmed in slow motion for The Slow Down Show in 2013.

In 2006, Fritz Grobe and Stephen Voltz — the self-proclaimed mad scientists behind Eepybird — caused an internet sensation with their viral video of an expanded version of the Diet Coke and Mentos fountain experiment, mimicking the choreography of the Bellagio’s world-famous fountain featured in Las Vegas. The underlying physics and chemistry of the fountain effect is well known.

But a few intrepid scientists at Spring Arbor University in Michigan wondered if altitude and the associated changes in atmospheric pressure would have a measurable impact on the foaming fountain’s intensity and conducted a series of experiments to find out. They reported their results in a recent article in the Journal of Chemical Education. The result: If you really want to get the foamiest action for your money, run the Diet Coke and Mentos experiment at high altitude.

Grobe and Voltz didn’t invent the basic demo. That’s been around since the 1980s, though originally creative science teachers used Wint-O-Green Lifesavers strung on a pipe cleaner to cause the foam fountains in soda bottles. In 1990 the Lifesavers were resized and they were too big to fit in the bottle mouths. So beta teachers switched to Mint Mentos candy 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: bubble nucleation. A standard two liter bottle of Diet Coke at room temperature usually contains about 12 grams of dissolved CO2† Leave that seemingly insignificant 12 grams of dissolved CO. loose2 however, in the gas phase at room temperature, and it will expand to fill three times the total volume of the original soda bottle, or about six liters. That satisfying whooshing sound when you open a fresh soda bottle is the result of CO2 escape. Then bubbles start to form in the liquid and float to the top.

This usually happens slowly, depleting your soda over time, as the surface tension of the water in the soda helps to hold the gas in place. Drop a few Mentos candies into the freshly opened bottle and it will happen very quickly. That’s because the surface of the candy has many small bumps. dissolved CO2 will diffuse into the bubbles that inevitably form on the rough surface of the candy, breaking the bonds with the water much faster. The result is a towering foam geyser.

Fritz Grobe and Stephen Voltz conduct the
enlarge Fritz Grobe and Stephen Voltz perform the “Diet Coke and Mentos Experiment” on 53rd Street outside the Ed Sullivan Theater on the Late Show with David Letterman in 2006.

Jeffrey R. Staab/CBS via Getty Images

Co-author Thomas S. Kuntzleman first encountered the experiment at an elementary school science fair in 2005, and he’s been interested in the underlying physics and chemistry ever since. For example, he has found that various drink additives such as sugars, citric acid and natural flavors can increase the height of fountains. Most recently, he decided to test the hypothesis that the intensity of the fountain effect would be greater at higher elevations, as atmospheric pressure would be expected to play a significant role in bubble formation.

Kuntzleman and his co-author, Ryan Johnson, bought bottles of Diet Coke from the same store and made sure the bottles all had the same expiration date. For each experiment, they dropped a single Mentos candy into a bottle through a one-inch PVC pipe and watched the reaction work its magic. They used a graduated bottle/tornado tube to measure the mass lost from the liquid over time. The readings were all taken within a week of each other, and the team made sure all bottles were kept at roughly the same temperature.

A series of experiments were conducted at different elevations during an ascent of Pikes Peak; another set took place while driving through North Carolina; and a third set of experiments was conducted during Kuntzleman’s family vacation, driving through multiple national parks in California, Nevada, and Utah. Altogether, they conducted the experiment at elevations ranging from below sea level (Death Valley, California) to over 14,000 feet (Pikes Peak). As expected, they found that more foam was produced at higher elevations.

However, according to the authors, the impact of altitude is not entirely due to a simple application of known gas laws, namely that gases expand at lower atmospheric pressures. It was easy enough to test this with data collected during Kuntzleman’s family vacation, because the soda bottle temperature had been carefully monitored and measured in that series of experiments. The gas laws state that the product of gas pressure and volume must be constant at constant temperatures, but the data showed an increase in the product of pressure and volume with increasing altitude.

“While it is highly likely that the gas laws 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 that enhance foaming at higher elevations are likely at play, 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 linked to central concepts such as chemical kinetics, surface science, gas solubility and chemical equilibria. He’s had undergraduate students in both non-major science courses and general chemistry courses recreate this latest set of experiments as a small research-based project and come to the same conclusion. “That the popular Diet Coke and Mentos experiment works better at higher altitudes is a memorable way to remind non-majors and general chemistry students that the important idea is that atmospheric pressure decreases with altitude,” the authors wrote.

For those annoyed by extensive on-site orders, it also offers a potentially fun DIY group experiment. Identify people in your social circles who live at different heights. Any person can do the Mentos and Diet Coke demo – outdoors is highly recommended – wherever they are, taking into account the relative height of the foam effect so that everyone can compare their respective results. †Scientific American has helpful instructions, just like Eepybird.)

If you’re really ambitious, everyone can do the experiment live simultaneously via Zoom or a similar meeting tool. Just protect your electronic equipment. And be sure to use Diet Coke or Coke Zero not only because they cause the greatest reaction — aspartame lowers the surface tension of the water — but because they’re less sugary and sticky. Trust us, you will get soaked.

A spectacular mint-powered version of the Bellagio Fountains in Las Vegas, brought to you by the mad scientists at

DOI: Chemical Education Magazine2020. 10.1021/acs.jchemed.9b01177 (About DOIs).

By akfire1

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