Rapid degassing of carbon dioxide from carbonated beverages can produce fountains that reach several meters high: CO2(aq) → CO2(g) These impressive fountains have been used by chemical educators as a motivating way to instruct students about various topics such as gas solubility, kinetics, and chemical equilibria. Such fountains result when carbon dioxide gas bubbles form on nucleation sites that are introduced into carbonated beverages, which are supersaturated solutions of carbon dioxide. A very popular method of carrying out this experiment involves dropping Mentos candies into bottles of freshly opened Coca-Cola. In addition to Mentos candies, Chemical educators have used a variety of other nucleators such as chalk, salt, Lifesavers candy, iron filings, and ultrasound to cause the rapid degassing of carbonated beverages. We have used a variety of means to investigate the nucleation process in this experiment. First, we have used a kinetic approach to indirectly measure adsorption of CO2 on the surface of Mentos candies dropped into carbonated water. The results obtained suggest that CO2 interacts with the candy surface via physisorption, as expected. Adding to the list of nucleators, we have also discovered that dropping smooth spheres comprised of metal, plastic, or ceramic into carbonated beverages creates fountains. Interestingly, spheres with greater density produce higher fountains. The smoothness of these spheres suggests their surfaces contain few nucleation sites, making it unlikely that these spheres cause fountains via surface nucleation. On the other hand, it is known that bubbles can be produced when solid surfaces contact each other within a liquid in a process known as tribonucleation. Based on our observations, we propose the smooth spheres produce fountains when dropped into carbonated beverages in a process that involves tribonucleation.

Investigation of nucleation mechanisms in the candy-cola soda geyser

255th National Meeting (2018)

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