As I mentioned the other day, some of the kids in the neighborhood and I decided to conduct some experiments into the Mentos + Coke = geyser phenomenon. I thought I’d have pictures and/or video to show you, but due to technical problems (technically, I was the problem) the experiments didn’t get captured for posterity. Still, we conducted some good experiments, got a little sticky, had a lot of fun, and learned a bit about doing science on the way.

Here’s what we did:

Hypotheses:
There were three different questions that the kids wanted to look at: if the kind of candy used makes a difference, if diet soda works better than regular, and if caffinated soda works better than decaf. After much discussion, the kids decided that their hypotheses were going to be that Mentos works better than other candies because of the mint used, that diet soda would work better than regular because it has “more chemicals” and that caffinated would work better than decaf for the same reason.

Materials:
5 2-Liter bottles of Coca-Cola classic
5 2-Liter bottles of Diet Coke
5 2-Liter bottles of Caffine-Free Diet Coke
5 2-Liter bottles of water (bottles from earlier trials that were rinsed and reused)
(the initial plan called for using Caffine-Free Coca-Cola, but that was unavailable)
Mentos mint candy – 6 mints per trial x 4 trials = 24 mints, or 2 rolls.
Mentos mixed fruit candy – 6 candies per trial x 4 trials = 24 mints (2 rolls)
Breath Savers candy – 7 per trial x 4 trials
Tic-Tacs – 15 per trial x 4 trials
Pennies – 25 per trial x 4 trials

The amount of candy (or money) used was selected to provide approximately the same volume of material.

Methods:
We placed a 2-liter bottle on a large sheet of foam (an old boogie board) to provide a stable surface, and put the whole setup against a convenient telephone pole. The pole was marked with masking tape at the level of the top of the bottle, and at one foot intervals after that (a meter ruler was unavailable). For each trial, the same technique was used. The cap was removed from the bottle, and we waited for the fizzing to die down. At that point, a paper tube containing the candy or coins was upended into the bottle, and the person conducting the test then ran like hell. The four types of candy listed above were tested, with the pennies used as a control, in all three varieties of soda, and with water.

Results:
As expected, dropping things into water produced no significant reactions of any type (much to the disappointment of the crowd). The two types of diet soda worked more effectively than the regular soda – the geysers of liquid were about one foot higher for all of the tested materials. The caffine free didn’t appear to work quite as well as the caffinated soda, but the heights were really too similar to tell for sure (particularly with the very small sample size used).

The material used did seem to make a difference, but the results here weren’t quite what was expected. The Tic-Tacs and the Breath Savers produced geysers that were in the 1-2 foot range. The two different varieties of Mentos produced 5-7 foot soda geysers. The pennies actually produced geysers that were in the 1.5-2.5 foot range – better than two of the candy types used.

Conclusions:
As the EepyBird folks point out, the violent explosion of soda from the bottle actually isn’t caused by a chemical reaction of any kind – it’s caused by nucleation sites. Those are imperfections on a surface that serve as the initial places for bubbles to form. Mentos (both fruit and mint) seem to have lots of these, which is why they work so well.

Next Time:
I’ve got the kids thinking about what to do for the next experiment. It’s probably going to be a variation on this one – they’re trying to come up with ideas for things to drop in that would have more nucleation sites than even the Mentos. So far, there have been some really creative ideas floated. In another couple of days, we should have a reasonable list worked out. We’re going to do another experiment on Tuesday, and I’ll be sure to report the results here when it’s done.

Final note:
I’m going to try to do one or two experiments a week with the kids, so please feel free to suggest experiments that we can try.