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:
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.
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.
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.
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.
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.
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.
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.
I heard some guys say that diet Sprite worked better than diet Coke.
One thing you could try- glue the Mentos to a little stick or rod and get it quickly deep into the liquid. Either the weight of the liquid will dampen the geyser or all that additional beverage foaming at the same time will give an interesting result.
One of the other teachers (one who works with higher functioning kids) did this experiment on Tuesday. I have a pic on my work computer. I'll try to send it along.
It is awesome.
Love the new digs. You *might* want to get a better picture of yourself, perhaps one with a paper bag over your head, Charlie Brown style.
I'd be willing to bet that the reason some sodas work better than others is all about liquid density. In other words, soda with more physical junk in it won't fly as high as a less concentrated solution. For instance, in my 3rd year chem lab I chose to do some bomb calorimetry on different sweeteners (it was a combined pchem biochem deal). A part of the experiment I abandoned (for reasons that will become clear) was this bit where I was going to compare the heats of combustion for diet coke and regular coke. So I figured I boil off the water (along with release a lot of CO2) so make some samples that I could dry and get an even weight. Ultimately, 20 oz of regular coke left me with a big fat layer of sugar goo and chemicals and the diet coke left me with a film that wouldn't wash off the beaker until I started trying various solvents. That means that the regular coke has lots of crapola in it and is probably a lot more viscous. (Judging from tongue-feel I would say this is a fair assessment.) I am not a physicist, so I couldn't tell you exactly what that has to do with bubbles, but I bet under the same propelling force as a lighter liquid it wouldn't fly as high. Also, I bet the candy sinks a little slower.
I did get to show the cub scouts Mercury, Saturn, Mars, Jupiter, and the Ring Nebula this past Saturday night. It's always fun. From a seat at the picnic table, Mercury was framed by two trees, and easy to describe, since it was the only thing visible there just after sunset. Then, i let them see it with small binoculars. Then through the 10 inch scope. So it wasn't just, here it is, they got to build observing skills. You have to start somewhere.
They had lots of questions. In a break with tradition, the leaders had some questions too. Here's the Ring Nebula - it looks like a smoke ring., i said. After a pause, Cool. Another pause. What is it? Too many adults seem to have forgotten how to ask them. I ask questions too, but often, no one knows the answers (yet).
FYI: The Mentos video guys are scheduled to appear on David Letterman tonight