Introduction

I. ResearchThe widely popular “Diet Coke and Mentos Geyser” occurs when Mentos are dropped into a

fresh bottle of Diet Coke and results in a jet of Diet Coke spray shooting out of the mouth of the bottle. Depending upon the number of Mentos dropped into the bottle, the height of the spray can vary between a few inches and tens of feet. The Diet Coke and Mentos reaction was featured in a 2006 Mythbusters episode and first shown in 1999 on the David Letterman Show. Since then, it has evolved to become a popular in-class physics and chemistry demonstration for kids of all ages. A Google query for “Diet Coke and Mentos” will return millions of hits, and YouTube has many home videos of this reaction.

The Mythbusters team did a great job of identifying the basic ingredients in this reaction. They cited the gum arabic and gelatin in the Mentos, and the caffeine, potassium benzoate, and aspartame in Diet Coke as the main contributors to the explosive result. It was also hypothesized by them that the rough surface of the Mentos can help break the strong polar attraction that water molecules have for each other by providing growth sites for the carbon dioxide.

This growth of carbon dioxide bubbles is referred to as nucleation. Each Mentos candy has millions of tiny pits all over the surface (see above pictures). These tiny pits are called nucleation sites - perfect places for carbon dioxide bubbles to form. As soon as the Mentos hit the soda, bubbles form all over the surface of the candy. Couple this with the fact that the Mentos candies are heavy and sink to the bottom of the bottle and you've got a double-whammy. When all this gas is released, it rises so rapidly that it takes the surrounding liquid with it, producing the light brown foam that we see in the geyser.

200 μm 20 μm

Parth Thakker5/6/2011Ms.Layman1st Period

So why do all of the chemicals identified in Mythbuster make any difference at all? Well, the answer has to do with the surface tension of the water. When you drop the Mentos into the soda, both of the surfactants (gelatin and gum Arabic) from the dissolving candy break the surface tension. This disrupts the water mesh, so that it takes less work to expand and form new bubbles. Using the contact angles of various solutions on a polycarbonate surface, Tonya Coffey of ASU also proved a similar effect of aspartame, potassium benzoate, and caffeine on surface tension. Hence, “Diet” and “Caffeine Free” sodas should hypothetically produce bigger reactions than standard sodas.

The basic science behind this explosive phenomenon can actually be put into simple terms via a chemical equation. Basically what happens is that Carbonic Acid (carbonated water) breaks up into Carbon Dioxide gas and Liquid Water. So, rather than being a reactant, the surface of the Mentos simply catalyzes this naturally occurring conversion at a very rapid rate (this is why soda gets flat over time).

H2CO3 (l) CO2 (g) + H2O (l)

Many scientists, however, claim that the Mentos phenomenon is purely a physical reaction, not a chemical one. Water molecules strongly attract each other, linking together to form a tight mesh around each bubble of carbon dioxide gas in the soda. In order to form a new bubble, or even to expand a bubble that has already formed, water molecules must push away from each other. It takes extra energy to break this “surface tension.” In other words, water “resists” the expansion of bubbles in the soda. So basically, this theory suggests that the Mentos form bubbles when they’re dropped into the bottle, and then act a place for these bubbles to expand along with the bubbles that were already in the soda. Nucleation still occurs, but rather than the Carbon Dioxide being converted from liquid to gas, all the little gaseous particles of CO2 converge to form a large bubble that erupts with dramatic speed. This would mean that rather than the Carbonic Acid being the essence of the reaction, it is the pressurized gas that remained in the soda, but failed to “dissolve” in it to form Carbon Dioxide.

Although the fundamental principle that drives this phenomenon is known to be nucleation, it is still disputed whether or not the Carbonic Acid is the reactant. The most widely accepted answer is no, because there is very little of the acid in the soda in relation to the CO2 gas itself. Also, decomposition of the acid would have to occur for it to be released, and no chemical catalyst is present for the reaction.

Mentos

II. HypothesisBased on research, we hypothesize that if a standard pack of Mentos Mints are dropped into

both the Diet and regular variations of a soda, then the resulting geyser will be higher from the Diet soda. This is because the only difference between regular and Diet sodas is the absence of High Fructose Corn Syrup, and the addition of Aspartame, Potassium Benzoate, and Citric Acid. Both Aspartame and Potassium Benzoate will contribute towards this change by catalyzing the reactions at a faster rate due to lowered surface tension. Citric Acid plays no role in the reaction.

Methods

I. Materials

2-liter Coca-Cola 2-liter Diet Coke 2-liter A&W Root Beer 2-liter AW Diet Root Beer 2 packs of Regular Mint Mentos (30 Mentos in total) Video Camera

For control purposes, the sodas were purchased with expiry dates ranging from April-May 2011. Also, in terms of storage, all of the bottles were stored in the together after purchase and were at equal temperatures (64° F) during experimentation.

II. ProcedureFor this experiment, the weather conditions were fair. The temperature was about 64 degrees

Fahrenheit. The area where the experiment took place was on a slab of concrete, outside the rear of “O building” at South Mecklenburg High School. To start off the lab, the regular coke and the diet coke were placed on the designated areas, with equal elevation and conditions. Then, the Mentos packet was opened. Once the work area and materials were prepared, the diet coke bottle was also opened.

Immediately, the five Mentos were placed inside, and the reaction instantly took place. The geyser shot upwards for about 2 seconds; however, the residual reaction kept going for about 45 to 60 seconds. This same process was repeated for each sample. All other variables were kept controlled.

Many control variables were implemented in this process to eliminate bias. For example, the Mentos were uniformly emptied from the original packaging, and were protected from contamination until experimentation. Also, the location used to test all of the bottles was identical in each test, and all four samples were tested within 30 minutes of each other. Each trial was conducted by the same person to eliminate procedural bias.

In this experiment, the independent variable was the type of soda, and whether or not it was diet. The dependent variable was the height of the resulting geyser. This height was measured after experimentation using the film of each trial. The door behind the bottles was used as a reference point. The door was measured to be 68” high from the ground. Still frames were taken from the video and then analyzed to produce the altitude attained by each individual geyser. We believed this method to be very simple yet accurate, and it provided us with results that could be verified and confirmed through our records.

Discussion

I. ResultsThe purpose of this experiment was to test the effect of Aspartame on the intensity of

nucleation in carbonated drinks. Aspartame is the artificial sweetener that is used in diet drinks, so this experiment was conducted by testing the reactions of Coke, Diet Coke, Root Beer, and Diet Root Beer with regular Mint Mentos. Our measured data was as follows (the numbers are the maximum altitudes that the nucleation “geysers” attained, in inches):

Type of Drink Coke A&W Root Beer

Plain (Control) 30” 38”

Diet (Experimental) 68” 78”

The mean of the Control Group was 34”, and the Experimental was 73”. This means that, on average, the addition of Aspartame boosted the resulting geyser by 39”. This is a substantial difference, and so it was assumed to be statistically significant without conducting complex analysis. Another observed difference between the Control and Experimental groups was the time it took for the nucleation to complete. The Diet drinks finished their geysers quicker than the Plain drinks did. This makes sense, though, because the increased height of the geysers implied that more gas was released, and so the drink ran out faster. This also means that the soda (or at least the CO2 in it) is the limiting reagent.

The research done prior to our experimentation strongly supported our hypothesis of Aspartame boosting the nucleation process. This conclusion was drawn from the fact that Aspartame is a surfactant and therefore reduces surface tension. This reduction was demonstrated by the contact angles of various solutions, which was also discussed in the introduction. Our experimental data strongly supported our hypothesis, and we believe this to have occurred due to the reasons cited above.

II. ConclusionsOur hypothesis for this study was that the addition of Aspartame in a carbonated drink would

intensify its rate of nucleation when reacted with Mentos. After analyzing our results, our hypothesis stood true, and was supported by our experimentation. Both of our correlated samples proved to have a substantial difference between the Diet and Plain versions of the drinks. Since all of the other ingredients for each drink were kept controlled between both versions, we can safely assume, in this

Diet

Plain

case, that correlation indicates causation. In other words, it can be concluded that Aspartame plays a big role in intensifying the nucleation process. Based on this finding, if aspartame was mixed in a bottle of carbonated water, then its nucleation reaction would theoretically outperform a standard, non-diluted bottle of carbonated water.

We learned a lot from this lab, and had a lot of fun as well. I don’t think any of us previously knew how this famed “reaction” worked. When I did find out how it occurred, I was very surprised by the fact that it wasn’t truly a chemical reaction. Rather it simply occurs due to all of the tiny nucleation sites on the surface of the Mentos. However, the sheer intensity and swift speed of the process still seems incredible to me, especially when you realize that it can be catalyzed by a single, tiny Mento. Other than learning this process and seeing it in action, I also got some practice in writing formal papers. Overall, this was an enriching lab experience and I hope to do more of its sort in the future.

III. Future ImprovementsAlthough this study was limited in budget, materials, space, and time, it was still able to provide

conclusive and reliable results. However, if I did it again, then I would definitely have at least 4 sets of samples to improve the reliability of the results. This is because “larger sample sizes generally lead to increased precision when estimating unknown parameters,” (Wikipedia, 2011). I would also refine the delivery system and method of measuring altitude to make the experiment more controlled and accurate. To analyze the results, I would use a multi-variable ANOVA which would analyze the differences between the experimental and control groups, and would tell me the statistical significance of my conclusions. It would also produce a “p-value” which represents the probability that the results of my experiment occurred by chance, and predicts whether or not my study could be replicated to produce the same results (Vassar Stats, 2011).

Another possible improvement would be the construction of a standardized nozzle for the soda bottle. Along with a standardized bottle shape, this would help un-bias comparisons between sodas of different brands (Spangler S., 2005). In fact, many such improvements could be made to make the results more reliable. These could include using a controlled environment, with standard 1 atm. pressure and purely oxygenated air. Although this may sound extreme, utmost precision is required to attain a sense of scientific validity and truly conclusive results. Hopefully someone in the science field will do this soon and gain some more insight into this enigmatic reaction (while maybe even assessing possible risks for nucleation inside the body!)

Works Cited

Coffey, Tonya: “Diet Coke and Mentos: What is really behind this physical reaction?” American Journal of Physics, June 2008. URL: http://www1.appstate.edu/dept/physics/coffey/nano/DietCokeandMentos.pdf

“Concepts & Applications of Inferential Statistics”: VassarStats. Retrieved May 6, 2011, from http://faculty.vassar.edu/lowry/webtext.html

Hyneman, Jamie et. al: “Diet Coke and Mentos.” MythBusters, Aug 9, 2006. URL: http://www.videobb.com/video/MLMFOT46GFXl

Kelly, Kieran: “What is the chemical equation that occurs when mixing soda and mentos?” MadSci Forum, January 4, 2006. URL: http://www.madsci.org/posts/archives/2006-01/1136425070.Ch.r.html

M., David: “What does potassium benzoate, and aspartame do in the mentos and diet coke reaction?” Yahoo! Answers, Feb 23, 2008. URL: http://answers.yahoo.com/question/index?qid=20080223132905AACEol9

Muir, Hazel: “Science of Mentos-Diet Coke explosions explained.” NewScientist, June 12, 2008. URL: http://www.newscientist.com/article/dn14114-science-of-mentosdiet-coke-explosions-explained.html

Norrie: “How is carbon dioxide dissolved into water, when it (Co2) is a gas and cannot ‘dissolve’?” Yahoo! Answers, May 6, 2008. URL: http://answers.yahoo.com/question/index?qid=20080506131643AAqwsLU

O’Hare, Kate: “The 'MythBusters' Take on the Mentos/Diet Coke Craze.” Zap2It, July 12, 2008. URL: http://www.zap2it.com/tv/news/zap-mythbustersmentos,0,4325641.story

“Sample Size”: In Wikipedia. Retrieved May 6, 2011, from http://en.wikipedia.org/wiki/Sample_size

Senese, Fred: “Why do Mentos mints foam when you drop them into soda pop?” General Chemistry Online: Frostburg State Univ., Oct 24, 1999. URL: http://antoine.frostburg.edu/chem/senese/101/consumer/faq/mentos.shtml

Spangler, Steve: “Mentos Diet Coke Geyser.” Steve Spangler Science, 2005. URL: http://www.stevespanglerscience.com/experiment/original-mentos-diet-coke-geyser