I know making geysers by adding Mentos to Diet Coke is soooo 2006, but bear with me for this one. A couple of months ago, I gave a small talk that was sidetracked for 15 minutes while the room discussed the subject. The second-most embarrassing aspect of the story is that I was responsible for leading us into the tangent, and the most embarrassing aspect is that, at one point, I minimized PowerPoint and opened YouTube to show a video of the phenomenon.
I was surprised to learn that many people in the room hadn’t even heard of the experiment. The set up is simple: take a 2 L bottle of your favorite carbonated beverage and several Mentos breath mints. As soon as the mints fall into the liquid, you get an instantaneous violent eruption. The more mints you add, the more violent the explosion. You can check out videos here and here.
From what I understand, there is no chemical reaction taking place—just the physical process of carbon dioxide being outgassed from solution. For some reason, the Mentos brand of mints provide especially nice nucleation sites for the CO2 bubbles to form. I’ve always heard that diet sodas are best for this demonstration because they aren’t sticky (making the clean up easier), but the other solutes (sweeteners, caffeine, etc.) appear to be important, too. These compounds alter the surface tension of the liquid, affecting the solubility of the gas and how quickly it can be released. I understand Mythbusters ran a number of “scientific experiments” on these geysers, but I’d like to see a hardcore experimental physicist get in on the action. It seems like there’s plenty of science left to be done.
In another example of YouTube science, people supercooled bottles of water (or beer) in a freezer and then shock froze the liquids. This involves cooling the liquid below its freezing point, which can be done if you handle the bottle gently enough. When the supercooled liquid is shaken or tapped, it quickly and spectacularly freezes over. I’m told by someone who has studied this phenomenon that it’s unclear what exactly is going on. Maybe the bubbles that are produced from the tapping provide the initial nucleation sites for the crystals to form? It might be similar to the process of selective flocculation used to purify potash salts. In this technique, a surfactant is added to a concentrated solution of the raw mixture of salts extracted from evaporite mines and bubbles are passed into the solution. I’m not sure whether pure crystals form at the air-water interface or if suspended crystals are simply swept along by the bubbles, but the “floc” floats to the top where it is skimmed off and sold. I’d like to see if the shock-freeze experiments work when there’s no air in the bottle, minimizing bubble formation.
Anyway, it seems like there’s a limitless supply of interesting experiments here that could engage high school students and get them to do some real science. It reminds me of the materials chemist who showed that M&Ms pack more efficiently than gumballs. This ground-breaking study merited a Science paper and extensive coverage in the news media (NY Times, CNN, etc.). While you may lament the publication of this paper in such a respected journal, we chemists need to do a better job of engaging the public and convincing them that “chemicals” ≠ “bad”. Undertaking studies involving food and other brand-name products might just help with that.