The experiment itself is quite easy to perform: procure some Mentos candies (the minty kind) and an unopened bottle of soda. Contrary to the experiment name, the soda in question doesn't need to be either diet or Coke (any opaque soda should theoretically work), but in practice it's good to use diet because it's going to go everywhere, and there's no sugar in diet soda so it doesn't leave a sticky, sugary mess when it's done.
Anyway, all you need to do at this point is set your bottle of soda (the 2-liter size works well) down somewhere it can stand up and a good ten feet or so away from anything you don't want to get showered in it. Then, with some Mentos in hand, unscrew the cap, quickly drop the Mentos in, and run! Shortly thereafter (we're talking no more than two seconds) a geyser of soda should erupt from the top of bottle in spectacular fashion for a few seconds.
Back in 2007 there were some videos on the fledgling website YouTube of some guys setting up chains of soda geysers in such a way that one's eruption would trigger the next, and so on down the line like a sweet, eruptive form of dominoes. They used some special equipment they'd made which screwed onto the tops of the bottles and let them drop the Mentos in with a quick-release system. I came across the video below, and decided to try it myself (with a single bottle of soda).
You can find out more about the making of this particular video here from the website of the guys who made it if you're interested.
I'm talking about this because last month I discovered that one of my co-workers had never tried the experiment before and was interested in doing so. Right after that, quite by chance I came across a variant of the quick-release devices used in the video meant to allow people to perform the experiment for themselves, so I snapped it up and together we set up the following demonstration just outside the office.
Pretty neat huh? We only used two Mentos in the quick-release magzine (which can hold up to five or six), so it wasn't the longest lasting eruption, but it's still pretty cool. You're probably wondering why exactly it does that. The short answer is that no-one is entirely sure, but we're pretty sure it's not a chemical reaction, as you might think; rather, it's a physical one. Soda, as you're no doubt aware, has a lot of carbon dioxide dissolved in it to give it its “fizz.” Mentos candies, for some reason, act as catalysts to bring that carbon dioxide out of solution by providing surface area where the dissolved gas can collect and precipitate out. It's similar to what happens when you shake or jolt a soda, which also causes the carbon dioxide to precipitate out of solution. As anyone who's tried to open a soda too soon after shaking it, all that carbon dioxide tries to forcefully exit the area as rapidly as possible, and often ends up carrying a significant amount of the soda along with it.
It's not only Mentos that can cause this reaction; dropping just about anything into a bottle of soda will cause at least a little fizzing. Mentos just happen to do it really, really, well on account of their surface structure having lots and lots of little tiny pits that make it very easy for dissolved carbon dioxide to collect and precipitate out.
Anyway, if you've got the time and inclination, give it a whirl! You can drink any leftover soda (it'll merely be a little flat), and eat the Mentos left at the bottom too if you want. It's a very fun experiment that's sure to get some remarks from onlookers.