The geyser of sweet foam released when mint candies are dropped in carbonated beverages has become an Internet favorite, and a great way to introduce children to chemistry. Dr Thomas Kuntzleman of Spring Arbor University, Michigan has made a mini-career out of exploring this phenomenon. His latest work reveals how altitude – and therefore air pressure – affects the scale of the spectacle, and produces results beyond those predicted by theory.
We may never know who first dropped a Mentos into a soda bottle, but the dramatic consequence has been known at least since the 1980s. Performances on TV around the year 2000 brought the effect to wider attention.
For those not satisfied with following in other's footsteps there have been two ways to go; either make the whole thing bigger, such as by diving into a vat of diet soda in a suit made of Mentos, or tweaking the experiment slightly to learn more.
Kuntzleman has taken the second path. Three years ago he published a paper revealing why diet drinks work better than sugary sodas (short version, the viscosity is lower). He's recently used his favorite demonstration as a genius way to teach people about the importance of physical distancing to flatten the curve of the spread of the COVID-19 pandemic.
In between, Kuntzleman has explored the effect of ambient air pressure on the soda geyser. Fizzy drinks are made by forcing carbon dioxide at 4-5 atmospheric pressures into water. As soon as the top comes off the bottle, the gas begins to escape, since the pressure is no longer there to maintain such high solubility, but usually can only do this at the surface or specific sites. If release happens fast enough, the gas will take some of the liquid with it as foam. So it makes sense that lower air pressure, such as on a mountaintop, would lead to swifter gas release, and more impressive fountains.
To test the idea, Kuntzleman first added candies to cola in the lab at different pressures to measure the mass lost from the liquid. Then he went on location, carrying out the experiment everywhere from Death Valley in California, 13 meters (43 feet) below sea level to Pikes Peak, a 4,200-meter-high (14,100 feet) summit in Colorado.
Kuntzleman did indeed find more foam was produced where the air was thin, but in the Journal of Chemical Education he describes setting high school chemistry students the question of whether gas laws alone could account for the increased volume produced. The students were able to show the increase was more than gas laws would predict, and there must be an as yet unknown secondary effect operating at higher altitudes.
In the course of the experiment, Kuntzleman also explored why Mentos work so well for this experiment. He found the mint flavor is irrelevant. Instead, Mentos have just the right roughness, producing the perfect sized bubble (2−7μm, 0.00008-0.00028 inches) while also being rich in nucleation sites that induce bubble formation.