How the trick with cola and mentos works at different altitudes

US, WASHINGTON (ORDO NEWS) — The addition of Mentos peppermint candies to a bottle of cola, provoking a violent reaction and a fountain of foam, is already a classic entertainment used by teachers and popularizers around the world to arouse children’s interest in physics and chemistry. However, some of the micro processes that occur during this process have so far been little studied.

The authors of an article published in the Journal of Chemical Education found out how the pressure and size of micropores in the candy influenced the famous trick. The team conducted the experimental part of their research in such impressive corners of the United States as Death Valley and Pikes Peak peak in the Rocky Mountains, filming some of the results in the video.

At a basic level, the explanation for the reaction is quite simple: CO2 was initially dissolved in a liquid under pressure. Depressurization of the bottle leads to a change in pressure, as a result of which the concentration of gas in the liquid decreases, and part of it enters the atmosphere.

The more actively the solution interacts with the surrounding air, the more actively the gas comes out: for example, it will be if you shake the bottle. Mentos simply effectively accelerate this process. Previous studies have shown that the porous structure of sweets provides ideal “traps” that capture tiny air bubbles. When one such “tablet” gets into the drink, its surface provides excellent contact with air for dissolved CO2 deep inside the bottle. The resulting gas immediately rushes out.

Until now, the exact size of these tiny bubbles could only be estimated based on micrographic images of the textured candy shell. Moreover, from the point of view of the effect on the reaction, this is an important question: for carbon dioxide to leave the solution, each bubble must provide the necessary surface area for a sufficient gas flow.

Theoretically, they should be more than one micrometer across, but larger bubbles also take up more space, reducing the number of areas in which the reaction begins to start, which also can potentially affect the process.

Since it is impossible to record the moment of gas release at such a micro level under ordinary conditions, the scientists came up with a special solution. This required the use of key physical relationships of the reaction, namely, variables such as pressure and volume.

Thomas Kunzelman, a chemistry professor at Spring Arbor University, accidentally discovered that this reaction is even more dramatic if it happened at high altitudes. Having discovered this, he, having contacted his colleague from Colorado Ryan Johnson, decided to test his hypothesis. Kunzelman and Johnson conducted a series of experiments (and at the same time had fun, as you can see in Kunzelman incredible exploration video blog) under a variety of conditions: from Death Valley, where the height of the earth’s surface is below sea level, to the peak in the Rockies at 4300 meters.

They found that air pressure alone cannot explain the observations, leaving room for the derivation of more accurate variables that contribute to foaming. Combining data on air pressure fluctuations with measurements of mass lost during degassing, as well as comparisons between different sweets, Kunzelman and Johnson soon understood quite well why Mentos is the best choice for this type of activity.

Their equations suggest that these gas release centers have a diameter of two to seven micrometers, which provides a balance and compromise between the size of the bubbles and the desired number of centers on the surface of the candy. The conclusion is well combined with existing models explaining the reaction, as well as with micrographic images of pores based on these models.

The data obtained will help teachers and popularizers of science working with children to better understand the mechanics of the process, which means it is better to explain it to their viewers and show them the world at the same time more complex and understandable, attracting new generations to unravel the secrets of physics and chemistry.


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