Diet Coke & Mentos Reaction Part II

Friday's Science Club was our last session in our introduction to Chemistry before we transition into an exploration of the Timeline of Life and questions about earliest life on this planet (the answers to which, actually, involve a lot of Chemistry). We began with the First Great Lesson, which is the Big Bang and the Periodic Table. We've examined elements, compounds, mixtures, solutions, the states of matter, and physical changes vs. chemical reactions.

Friday, October 4

• discuss states of matter (solid, liquid, gas) and the effect of temperature on molecules; explain solutions and saturated solutions; do "Experiment 3: A Star is Boron" from pages 26-29 of The Book of Ingeniously Daring Chemistry by Sean Connolly
  
• make Lemon & Lime Volcanoes and consider the similarities between this reaction and our baking soda & vinegar reaction last week; consider the similarities between lemon juice, lime juice, and vinegar
• revisit Diet Coke & Mentos reactions; explain how this is a combination of both a chemical reaction (key concept: carbonic acid breaking down into water and carbon dioxide) and a physical reaction (key concept: nucleation); examine nucleation sites further by placing a piece of pumice into Diet Coke to see the result; allow students to design and test their own Mentos and Diet Coke experiments

Sometimes the bullet point lists make it look so deceptively simple!!!

First, I should explain that everything we did was really all around understanding the Diet Coke & Mentos reaction better.

We looked at borax going into solution because we were going to talk about carbon dioxide being dissolved in the soda. Water (H2O) and carbon dioxide (CO2) reversibly form carbonic acid (H2CO3). When the bonds are broken, the CO2 is released. The carbonic acid, by the way, is why soda is so bad for your teeth whether it has sugar in it or not.

We looked at baking soda reacting in a similar way with lemon juice and lime juice as it had to white vinegar because then we could consider how all these liquids are alike. They are all acidic. Seeing if pumice created a similar reaction to Mentos could help us discuss how pumice and Mentos are alike (ultra-craggy surface).

A lot of adults became interested in the Diet Coke & Mentos reaction from our Sep 27 session. Two Science Club dads -- both of whom work in electrical engineering -- got involved. Malcolm, who works in electronics manufacturing at EMAC, Inc., contributed a sturdy plastic graduated cylinder from his lab. This was the perfect diameter to hold our Mentos candies. Justin, who is campus plant and service operations engineer at SIU's Physical Plant, spent time looking into this reaction further and also came on Oct 4 to help explain nucleation sites to the kids. He explained to me that Henry's Law states that the equilibrium point (carbonic acid staying together as opposed to coming apart) is affected by BOTH temperature and pressure, so having the soda be warm creates a more profound reaction. He even found a peer reviewed paper written on this topic! I did some research of my own in this regard, and am happy to share here all of the links we collected.

Caution, Experiment! - Humboldt Kosmos

The Science of Coke and Mentos
(try the "Fun with Nucleation" follow up activity)

Erupting Diet Coke with Mentos - Scientific American
(also includes several follow up experiment ideas)

Solutions of Gases in Water: How Soda Pop Gets Its Fizz
(Henry's Law, equations 13.4.1 and 13.4.2)

Diet Coke and Mentos: What is really behind this physical reaction? by Tonya Shea Coffey
(this is the full paper published in the American Journal of Physics, close-up photographs of the surface of Mentos candy, figures and tables showing the results of many variations on this experiment)

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In addition to reading up on this matter, I will admit that I actually lost sleep thinking about how to create a better design for getting as many candies as possible into the bottle of Diet Coke.

The problem is that the reaction starts right away, so not all 12 of the candies are going to make it to the bottom to add to the high speed come-apart of the carbonic acid. The pushing up and out of the soda, as the CO2 bubbles form around the many nucleation sites on the candy (which is quite craggy when seen under an electron microscope... not to mention that each neighboring bubble is now itself a nucleation site) and the pressure builds and builds so the gas has to escape NOW, means that many of the candies are ejected before ever touching the Diet Coke. I wanted to find a way to get more Coke onto more candy before the thing erupted.

I also had a student who had predicted right at the beginning that the Diet Coke and Mentos would be a sodium-water reaction and then became completely confident in this fact when he read the ingredients in the candy and found the word sodium. He felt that ingredient list was proof that he was correct. So I wanted to do something to explore that idea (and debunk it).

And, before the donation of the graduated cyclinder, we were looking at ways to come up with a candy holder that could hold the candy correctly.

So when I couldn't sleep I made a list of all things that were on my mind as this fuzzed and fumed in my brain:

is it a chemical reaction or a release of pressure? isn't the CO2 already in the diet Coke?

after the reaction is over is all the CO2 out? if you shake the bottle up wildly will it make bubbles?

try rock salt and diet Coke - prove that it is not a sodium reaction

nucleation sites - try pumice and diet Coke

if the diet Coke has gone flat will it still work?

does temperature of the Coke affect the reaction?

cut top off 2 L, pour Mentos in, replace top

cut bottom off 2 L, place Mentos on plate, put plexiglas sheet under, pull plexi piece out so all candies get wet at the same time

put diet Coke in a pail with Mentos on a plate, flip pail over and put on top

place long PVC pipe over opening of bottle, slide candies down the pipe and run

get a larger test tube (25 mm)

use toilet paper roll but cut and retape to be 25 mm diameter

use a thinner piece of cardboard that slides out more cleanly

use same test tubes but cut Mentos in half

do some tests with smaller bottles to fine tune the design, then do it with a larger one

pour Coke into a bucket and stand around throwing Mentos into it

As much as all of the adults involved wanted to understand the science of it better, all of the kids involved just wanted to see it happen over and over and over again. We didn't have nearly as much testing of hypotheses on Friday afternoon as I had expected. It was more running around in the field, doing stuff with your friends, playing with candy, and soaking in the Diet Coke stickiness (as well as the pure enjoyment of something super cool).

Yes, I want them to take detailed lab notes, make thoughtful predictions, and test their ideas changing only one variable at a time... I want them to think critically and examine the results of their experiments and be open to the idea that sometimes the results will falsify their hypothesis.... I want them to realize that scientists are thoughtful and precise and they don't actually go racing full-tilt around the lab howling, "That was AWESOME!"

But, you know what? Maybe sometimes they do. I mean, who am I to judge? Sometimes Science is just plain FUN.

And, after seeing it happen over and over and over again, then your brain ticks into another gear and you start thinking "I wonder why" and "Maybe if..."

Isn't that how all of Science began in the first place?

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