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New Demonstrations and New Insights on the Mechanism of the Candy-Cola Soda Geyser Thomas S. Kuntzleman,*,† Laura S. Davenport,‡ Victoria I. Cothran,† Jacob T. Kuntzleman,† and Dean J. Campbell§ †

Department of Chemistry, Spring Arbor University, Spring Arbor, Michigan 49283, United States Department of Chemistry, Hope College, Holland, Michigan 49423, United States § Department of Chemistry and Biochemistry, Bradley University, Peoria, Illinois 61625, United States ‡

S Supporting Information *

ABSTRACT: When carbonated beverages (which are supersaturated solutions of aqueous carbon dioxide) are confined within a narrow-necked container, events which rapidly release the gas from solution produce a fountain out of the beverage. One well-known variant of this experiment is the addition of Mentos candies to a bottle of Diet Coke. Previous reports have shown that the presence of aspartame and benzoate in carbonated beverages enhance the fountaining effect. These additives are thought to enhance fountaining by lowering the surface tension of the beverage, but the details of this process are not completely understood. This paper explores the relationship between geyser height and the type of carbonated beverage. It is shown herein that several other compounds commonly found in commercial carbonated drinks such as sucrose, glucose, citric acid, and components of natural flavors also enhance geyser heights. By examining how these additives affect bubbling and foaming behavior in seltzer water, it is postulated that solutes which inhibit bubble coalescence contribute to higher fountains. KEYWORDS: General Public, Elementary/Middle School Science, High School/Introductory Chemistry, First-Year Undergraduate/General, Demonstrations, Public Understanding/Outreach, Inquiry-Based/Discovery Learning, Consumer Chemistry, Phases/Phase Transitions/Diagrams

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INTRODUCTION If several Mentos candies are placed in a bottle of Diet Coke, an impressive fountain erupts out of the bottle. This phenomenon results in part from rapid escape of dissolved CO2 out of the beverage and into the gas phase: CO2 (aq) → CO2 (g)

and PCO2 is the pressure of CO2 under which the bottle is sealed. Thus, if PCO2 = 5 atm, it is estimated that 7.5 g of CO2 is dissolved per liter of carbonated beverage at 25 °C. Given this estimate, we can use the ideal gas law to calculate that all the CO2(aq) contained within a 2 L carbonated beverage released into the gas phase at 1 atm and 25 °C would take up about 8 L of space. When a carbonated beverage is opened, CO2 in the headspace of the bottle escapes into the atmosphere. As a result, the soda within is no longer subject to high PCO2 but rather to the atmospheric PCO2 of about 400 ppm.9 Under this lower PCO2, the amount of CO2 expected to remain dissolved at equilibrium is less than 1 mg. Thus, once unsealed, a soda is supersaturated in CO2(aq), essentially all of which will eventually escape as a gas from a carbonated drink. Fortunately, the release of CO2(aq) from an opened carbonated beverage does not happen all at once, but rather quite slowly. This is because even though the process in eq 1 is thermodynamically favored,4 it is associated with a high activation energy, Ea.10 This barrier is generally associated with the energy required for

(1)

The swift formation of several expanding CO2(g) bubbles pushes the beverage contents upward and out of the bottle, causing the fountain. This experiment can be done with just about any carbonated beverage, and several solids other than Mentos candies (e.g., Wint-o-Green Lifesavers, iron filings, salt, and chalk)1−3 can initiate the fountain. To gain an appreciation of how this experiment works, it is useful to explore facets of the solubility of CO2 in and degassing of CO2 from carbonated beverages. Carbonated beverages are sealed under pressures of approximately 4−6 atm of CO2.4−6 The amount of CO2 dissolved in a carbonated beverage can be estimated using Henry’s Law,7 which is a function of the temperature, T, and CO2 pressure under which the beverage is sealed: S = kH(T )PCO2

(2)

Received: November 8, 2016 Revised: February 7, 2017 Published: February 23, 2017

In eq 2, S is the amount of CO2(aq) dissolved, kH(T) (=0.0345 M atm−1)8 is Henry’s Law constant for CO2 in water at 25 °C, © 2017 American Chemical Society and Division of Chemical Education, Inc.

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(NISD) is a term that has been used to succinctly describe this experiment.11

bubbles to spontaneously form within a liquid, and is proportional to the cube of the liquid surface tension, γ:10

Ea =

16πγ 3 3(P0 − P)

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PREVIOUS EXPERIMENTS The popularity of this experiment has spawned several publications describing its use in classrooms, laboratories, and summer camps.11−13 In addition, several studies have elucidated aspects of the physicochemical processes involved in NISD.1,2,5,11,14,15 It has been shown that fountain height increases with temperature, and that this effect is due to kinetic rather than thermodynamic factors.2,5,11,15 The activation energy for the overall degassing process from Diet Coke has been found to be roughly 25 kJ mol−1.11 In a MythBusters episode,14 it was argued that additives contained within carbonated beverages increased the speed of NISD. In particular it was noted that addition of benzoate, aspartame, or (to a lesser extent) caffeine to sodas increased the rate of reaction. Following up on this work, Coffey1 also noted that carbonated beverages which contained these additives created more impressive fountains than seltzer water, which contains only CO2 dissolved in water. It was also found that diet sodas produced higher fountains than sugar-sweetened sodas.1 Coffey hypothesized that these differences could be explained due to the presence of aspartame and benzoate (but not caffeine) in the diet sodas. It was argued that because aspartame and benzoate lower the surface tension of aqueous solutions, less energy is required to form bubbles in diet sodas than in sugarsweetened sodas or seltzer water. We have also observed this trend in fountain heights (Figure 2): diet sodas > sugar-sweetened sodas > seltzer water (contains only CO2 and water). However, the results of Figure 2 call into question some current explanations on the effect of additives on fountain heights. Beverages that contain neither aspartame nor benzoate (such as Coke, Pepsi, and Caffeine Free Coke; see Supporting Information for ingredients in beverages tested) produce higher fountains than seltzer water. Thus, other additives likely increase fountain height. Because the addition of sucrose or glucose to water has been demonstrated to increase surface tension upon addition to water,16−18 this observation suggests that factors other than decreased surface tension contribute to higher fountains. In fact, it has been previously suggested that the presence of solutes in general might inhibit bubble coalescence, which in turn allows for smaller bubbles and higher fountain heights during NISD.5 Thus, the role of additives on fountain heights observed was explored. To do so, controlled amounts of several additives (sucrose, glucose, citric acid, aspartame, sodium benzoate, magnesium sulfate, and the flavor components linalool and citral19) were added to seltzer water to observe the effect of these compounds on fountain heights. Because increased foaming was associated with higher fountains, attempts were made to quantify the relationship between the presence of these additives and foaming behavior through measurement of bubble sizes. The results of these experiments shed light on the mechanism behind how fountains are produced in the NISD experiment.

(3)

Here, P0 is the gas pressure above the solution at equilibrium, and P is the gas pressure required to reach the supersaturated concentration. It should be noted that eq 3 arises from a simple and classical treatment and is typically inaccurate, and more sophisticated treatments have been attempted.10 Nevertheless, how do opened carbonated beverages degas at all, given this difficulty in forming bubbles? Tiny gas pockets, called nucleation sites, present within a carbonated beverage provide ready-made air bubbles into which CO2(aq) can easily escape into the gas phase.4,10 The presence of such gas pockets significantly lowers the activation energy required for bubble formation, in some cases lowering the activation energy to essentially zero.10 Nucleation sites can be found in a wide variety of sources such as cloth fibers and scratches on drinking glasses.4 The terms homogeneous and heterogeneous nucleation refer to whether bubble formation occurs in the presence (heterogeneous) or absence (homogeneous) of nucleation sites. It is in the case of heterogeneous nucleation that the activation energy is drastically lowered. Adding Mentos candy to a freshly opened carbonated beverage introduces countless heterogeneous nucleation sites into the supersaturated liquid. Indeed, scanning electron microscope (SEM) images have demonstrated that the surface of Mentos candies contains large numbers of nucleation sites.1 Upon addition of these innumerable nucleation sites to the beverage, the thermodynamically favored process depicted in eq 1 occurs with ease. Rapid degassing results, and several liters of CO2(g) are formed within the bottle, pushing the contents several meters high out of the opening (Figure 1 displays CO2 bubbles forming around a single Mentos candy within a carbonated beverage). Nucleation induced soda degassing

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MATERIALS AND METHODS

The majority of these experiments were designed to be simple to carry out, with the hopes that high school and undergraduate students can mimic these investigations in inquiry-based explorations of NISD. Details on modified versions of the

Figure 1. Image showing CO2 bubbles forming around a single Mentos candy within a carbonated beverage. 570

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Figure 2. Fountain heights achieved upon addition of mint-flavored Mentos to commercial beverages as described in the text. Diet beverages are to the right of the dotted line, and sugared beverages are in between the solid and dotted lines.

experiments presented here, which we have used as lecture demonstrations and laboratory explorations, can be found in the Supporting Information.

immediately after additions were made in a process that took less than 30 s. Samples were always compared to a control of seltzer water of identical brand, expiration date, and place of purchase. Control samples were prepared by opening a bottle, making additions of deionized water, and resealing in a similar time frame and fashion. In the very few cases where seltzer water was removed to prepare highly concentrated sucrose solutions, such samples were compared to similarly treated controls.

Fountain Heights of 2 L Commercial Beverages

There were 11 Mentos candies introduced into a 2 L bottle of commercial soda pop within 20 s of opening the beverage. A modified turkey baster was used to house the candies in a vertical column to aid in consistent placement of candy into the soda. The resulting fountain was filmed with a home video camera. The soda bottle was placed in front of a garage door that measured 2.24 m from ground level to the top of the door. The videos were analyzed using a laptop with a flat screen. The distance from the ground to the top of the door as it appeared in the video on the computer monitor was measured with a centimeter ruler. The video was played and paused at the maximum fountain height achieved. The height of the fountain as it appeared on the screen was measured with a centimeter ruler, and the actual fountain height was determined by ratio. Note that the top level of the liquid prior to addition of Mentos, and not the ground, was taken as the base of the fountain. Most brands required five trials. However, when significant variability was observed for a certain brand, as many as seven trials were run to ensure a representative height.

Measurement of Bubble Sizes

Samples were vigorously shaken for 2−3 s to form bubbles and then immediately placed in front of a Phantom Miro EX-2 camera fitted with a 10× magnifying lens filming at 1000 frames per second. It was necessary to use a high speed camera to capture images capable of resolving bubble sizes during the rapid bubble formation and coalescence events observed. The field of view was focused on the top 3−6 cm of the liquid in the bottle. To ensure consistent placement, bottles were placed in a block of polystyrene foam with a hole cut to snugly fit the 1 L bottles. The foam block was secured in place with duct tape. Upon video playback, the particular frame chosen for analysis was taken as the first video frame observed wherein the bottle was first locked in place and bubbles were observed. The bubble sizes observed often varied dramatically; thus, the average of at least 10 and as many as 25 bubbles in the field of view were randomly chosen for measurement. The bubble size for a particular bubble was taken as the longest horizontal distance across the bubble. It should be noted that by using this method bubbles down to about 0.2 mm in size (but not smaller) could be detected; smaller bubbles almost certainly existed that could not be reliably measured. Bubble sizes were measured in this manner in at least two but no more than three separately prepared bottles at similar concentrations.

Preparation of 1 L Seltzer Water Samples

One liter bottles of seltzer water purchased from grocery stores were used in these experiments. One liter rather than 2 L bottles were used due to the limited purchasing availability of the latter. After opening a bottle, addition of sucrose (Fisher Scientific), glucose (Fisher Scientific), citric acid (Fisher Scientific), aspartame (NuSci), sodium benzoate (Fisher Scientific), or MgSO4·7H2O (Fisher Scientific) was made from stock aqueous solutions. Additions of between 0.100 and 50.0 mL of stock solution were typically required. Substantially more volume (50−200 mL) of stock solution was occasionally required in the case of glucose and sucrose additions. In a very few cases to prepare samples at high sucrose concentrations, up to 100 mL of seltzer water was poured out prior to addition of stock. Addition of solid material was not made because doing so caused rapid degassing of the seltzer water. Additions of linalool (Acros Organics) and citral (Alfa Aesar) were made from pure liquid in microliter volumes. Bottles were resealed

Measurement of Fountain Height

After bubble size measurements were made, samples were left undisturbed until all visible bubbles that adhered to the walls of the bottle (which can act as nucleation sites) were dislodged. This typically took no longer than 5 min. A Geyser Tube20 was used to add 7 Mentos to each 1 L bottle within less than 30 s of first opening the beverage. (Seltzer water with added solute often fizzed much more readily than untreated seltzer water 571

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upon opening. Thus, slow opening of bottles was often required to avoid material spraying out of the bottle.) The resulting fountain was filmed with a home video camera. Samples were placed in front of a wall with an overhang that measured 2.60 m from ground level to the bottom of the overhang. Actual fountain heights were determined by video playback in a manner analogous to what was done with the commercial 2 L beverages. However, given the differences in brand of seltzer water and expiration date between the several samples tested, these measurements were always recorded (and are presented herein) relative to a control sample of the same brand and expiration date. Fountain heights of control samples averaged 110 ± 20 cm. In a few cases, bubbling vigorous enough to reach into the neck of the bottle was observed prior to addition of Mentos. The bubbling would often cause candy to become lodged in the Geyser Tube such that it would not fall into the liquid upon release. The vigorous bubbling could be stabilized by pouring out a small amount of seltzer water (20−50 mL) prior to attaching the Geyser Tube. Therefore, this practice was adopted for samples that displayed vigorous bubbling. Fountain heights were measured in this manner normally using three to four bottles prepared at similar concentrations. Most concentrations required three to four trials. However, when significant variability between trials was observed for certain concentrations, as many as eight trials were run to ensure representative heights.

Figure 3. Fountain heights observed upon addition of Mentos candy to seltzer water with added aspartame (●) or benzoate (○). Error bars in all figures represent one standard deviation.

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HAZARDS Citral and linalool are both flammable liquids. Citral, linalool, aspartame, sodium benzoate, and citric acid may cause skin or eye irritation. Sodium benzoate has been classified as a possible teratogen, suspected reproductive system toxin for males, and reproductive system toxin for females. Repeated or long-term exposure to sodium benzoate may be toxic to blood, the reproductive system, liver, and central nervous system. It is important to note that sodium benzoate is generally recognized as safe by the FDA as a food additive, and it is commonly found at levels around 0.015% by mass in beverages.21

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Figure 4. Fountain heights observed upon addition of Mentos candy to seltzer water with added citral (●) or linalool (○).

RESULTS

Fountain Heights

The addition of either aspartame or benzoate to seltzer water was observed to increase fountain height by as much as three times (Figure 3). Higher fountains resulted as more solute was dissolved up to roughly 0.03% by mass, after which the effect saturated. These two compounds have been previously shown to both decrease surface tension upon addition to water1,22 and increase fountain heights during NISD.1,14 Given that some commercial sodas that contain neither benzoate nor aspartame produce higher fountains than seltzer water alone (Figure 2), we attempted to examine the effect of other ingredients found in commercial carbonated beverages on fountain heights. Thus, we tested the effect of increasing concentrations of citral and linalool, both of which have been identified as components of natural flavorings in lemon-lime carbonated beverages,19 on fountain heights. Very small amounts of citral or linalool were observed to increase fountain heights by as much as four times; this effect saturated at about 0.002% by mass for each substance (Figure 4). Citric acid, another compound commonly found in commercial soft drinks, also increased fountain height by as much as six times (Figure 5). However, substantially more citric acid (about 2% by mass) was required to observe an effect.

Figure 5. Fountain heights observed upon addition of Mentos candy to seltzer water with added citric acid.

Citric acid, citral, and linalool are all commonly found in commercial carbonated drinks, yet none of these three substances has to our knowledge been previously reported to increase fountain heights during NISD. Of note, about 10 times 572

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than decreased surface tension contribute to increased fountain height during NISD. During the testing of fountain heights or when bottles were otherwise agitated, more foaming was observed in solutecontaining seltzer water (Figure 7B) versus seltzer water alone (Figure 7A). It is well-known that the presence of solutes inhibits bubble coalescence in water, which leads to smaller bubble sizes and greater foaming.25−31 It was therefore hypothesized that bubble size might be inversely related to fountain height. For a test of this idea, varying concentrations of different solutes were added to seltzer water. These bottles were agitated to form bubbles within each liquid; the resulting bubbles were filmed, and bubble sizes were measured as described in the Materials and Methods. Consistent with this hypothesis, the addition of any solutes tested herein to seltzer water correlated with decreased bubble size in vigorously shaken seltzer water (Figure 7C,D). For example, when citral or linalool was added to bottles of seltzer water and shaken, bubble size decreased with increasing concentration of either solute (Figure 8). Construction of an overlay plot of the fountain height (Figure 4) and bubble size (Figure 8) data for linalool showed good correlation between fountain height increase and bubble size decrease with added linalool (Figure 9). Similar results were observed when constructing overlay plots of concentration-dependent fountain heights and bubble sizes for all other solutes tested (see Supporting Information). These results strongly suggest that smaller bubble sizes, through inhibition of bubble coalescence, influence fountain heights during NISD experiments.

less (by mass) of either compound found in lemon-lime flavoring than either benzoate or aspartame was required to observe increased fountain heights. We further endeavored to test how the addition of sucrose, glucose, or MgSO4 (all of which have been shown to increase surface tension upon addition to water16−18,23,24) affects fountain height during NISD. Interestingly, all of these substances increased fountain height when added to seltzer water (Figure 6); this effect reached saturation with added

Figure 6. Fountain heights observed upon addition of Mentos candy to seltzer water with added sucrose (●), glucose (○), or MgSO4 (▲). Lines are drawn to guide the eye.

MgSO4 at about 2% by mass. Saturation was not observed at the concentrations of glucose and sucrose tested. Nevertheless, the results of these experiments demonstrate that factors other

Figure 7. Bubbling behavior observed in bottles of seltzer water. Bubbles formed from adding Mentos candy (A, B) or by vigorously shaking bottles (C, D). Seltzer water alone (A, C) or with added solute (B, D). Average bubble size is 3.1 ± 1.1 mm in part C and 0.8 ± 0.4 mm in part D. 573

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fountains than mint-flavored Mentos (which contain no citric acid). The surface tension of beverages can be lowered by the addition of certain solutes, such as aspartame and benzoate.1 Because the addition of aspartame and benzoate to seltzer water has been correlated with higher fountains, it has previously been suggested that lowering the surface tension of beverages increases fountain heights during NISD.1 However, in the experiments presented here, addition of solutes to seltzer water that has been demonstrated to increase surface tension also increases fountain height (Figure 6), so other factors must be involved. As stated earlier, it is well-known that the presence of solutes inhibits bubble coalescence in water, which leads to smaller bubble sizes and greater foaming.25−31 The correlation between solute addition, fountain height, and bubble sizes observed herein strongly suggests that inhibition of bubble coalescence due to solute addition is an additional factor related to fountain height during NISD. Notably, it has previously been suggested that inhibition of bubble coalescence might play a role in fountain heights during NISD.5 Inhibition of bubble coalescence could contribute to higher fountain heights by increasing the surface area through which CO2(aq) can enter bubbles. Coalescence of countless small bubbles into a single bubble drastically reduces the surface area between bubbles and the bulk solution. In the experiments on agitated seltzer water (Figure 7A,C), bubbles coalesce quickly into large bubbles. As a result, the surface through which CO2(aq) may diffuse from the bulk water into bubbles is reduced. If bubble coalescence is inhibited through addition of solute (Figures 7B,D and 8), bubbles formed during NISD remain small. In this state of affairs the surface area through which CO2(aq) may diffuse from bulk water into bubbles remains high. In this case the greater surface area would increase the kinetics of degassing, producing greater fountain heights. Inhibition of bubble coalescence due to solute addition is consistent with the trend in fountain heights observed (diet > sugar-sweetened > seltzer water) during NISD in beverages (Figure 2). In contrast to seltzer water, both diet and sugarsweetened sodas contain added solute in addition to dissolved CO2. Diet sodas generally contain aspartame, benzoate, and natural flavorings while sugar-sweetened sodas typically contain sugars and natural flavorings. Thus, both diet and sugarsweetened sodas would be expected to form higher fountains than seltzer water upon addition of Mentos candy. Given the ubiquity of natural flavorings in carbonated beverages and the ability of minute amounts of linalool and citral to increase fountain height, we suggest that natural flavorings play a major role in increasing fountain height during NISD experiments with carbonated beverages. The higher viscosity of sugarsweetened over diet sodas35 is a likely explanation for the somewhat lower fountains observed in the former over the latter. Both bubble nucleation5 and speed of bubbles moving through soda are likely to be inhibited in more viscous solutions, causing somewhat lower fountain height. It should be noted that, with respect to inhibition of bubble coalescence, different solutes do not necessarily act independently of one another. For example, it has been shown that electrolytes act cooperatively with sucrose to inhibit bubble coalescence.27 Therefore, the various components present in a typical soft drink might interact with one another to increase or decrease bubble coalescence, impacting the geyser effect as a result. Thus, while the studies presented here provide insight

Figure 8. Bubble sizes observed in vigorously shaken seltzer water with added citral (●) or linalool (○).

Figure 9. Replot of the data for linalool from Figures 4 and 8. Fountain height (●) and corresponding bubble sizes (○).

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DISCUSSION In the experiments presented herein, several components commonly found in soft drinks increase fountain heights during NISD in a concentration-dependent manner. Two of these (aspartame and benzoate, Figure 3) have been previously identified to cause increased heights; the remainder tested (citral, linalool, citric acid, glucose, and sucrose, Figures 4−6) have not. Interestingly, except for citric acid the amounts of each compound required to increase fountain height are roughly in accord with amounts typically found in soft drinks (Table 1). However, the observation that dissolved citric acid contributes to increased fountain height may shed light on the previously reported result15 that strawberry-flavored Mentos (which contain citric acid) tend to produce much higher Table 1. Amounts of Substances Typically Found in Soft Drinks19,21,32−34 Substance

% by Mass

Aspartame Benzoate Lemon-lime flavor (multiple components) Citric acid Various sugars

∼0.06% ∼0.015% >0.001% ∼0.18% ∼10% 574

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previously suggested that solutes which decrease surface tension are responsible for higher fountain heights during NISD, the results of the experiments presented here suggest that a wider variety of solutes are capable of enhancing this effect. Even so, we do not discount that surface tension likely plays a role in the fountain heights observed. In the experiments presented here, very small amounts or citral and linalool (both components of natural lemon-lime flavor) were observed to be quite effective at increasing fountain height. On the basis of these findings, it is likely that several other components of lemon-lime and other flavors (orange, cherry, vanilla) might behave similarly. Further, the effects of common cola additives such as caffeine and phosphoric acid were not explored herein, but are likely to affect fountain height. Discovering various components of flavorings and other additives in soft drinks that are effective at increasing fountain heights could provide a springboard for small research projects for high school and undergraduate chemistry students to explore.

into the role of various solutes on geyser heights in the Diet Coke and Mentos experiment, further studies could shed light on how different solutes might interact to enhance or diminish this effect. Probing the role of solutes in inhibition of bubble coalescence remains an active area of research. At present there is tentative agreement that bubble coalescence is inhibited by solutes which impart a high elasticity to bubbles. Addition of solutes to water tends to increase bubble elasticity, which in turn is affected by a solute’s ability to change surface tension with increased concentration (dγ/dc)2.25−28,31 While certainly interesting, these considerations are beyond the scope of this article. The experiments presented herein provide a wide range of chemical topics for students and teachers to explore. For example, students would likely be motivated to prepare bottles of seltzer water at varying solute concentration if they knew they could subsequently test the fountain heights of prepared samples. Such an activity could be used to give students handson experience calculating solution concentrations in various units (ppm, % by mass, molarity, and molality). In addition, these experiments naturally connect to the topic of chemical kinetics: Increased bubble surface area increases rates of degassing, and Mentos candy lowers the activation energy of the degassing process. Other possibilities for connecting this system to the chemistry curriculum have been previously reported in this Journal.2,5,11,12 Finally, we have found that allowing observers to compare fountain heights upon addition of Mentos to unflavored and flavored seltzer water (which contains only natural flavorings, CO2, and water) is a simple and engaging way to demonstrate the effect of natural flavorings in particular and solutes in general on fountain height. Flavored seltzer water achieves fountain heights 4−5 times higher than its unflavored counterpart (see Supporting Information). An advantage of this demonstration is that no preparation of solutions is required: Simply add Mentos under similar conditions to flavored and unflavored seltzer water, and then explain the differences observed. Citral or linalool may be added to seltzer water if it is desired to allow observers to see the addition of solute take place (see Supporting Information). Although obviously not contained in soft drinks, alcohols in general and isopropanol in particular provide conveniently obtained solutes that are easy to add, and these work quite well at enhancing fountain heights (see Supporting Information). Connecting the results of these experiments to the differential foaming observed in waves of freshwater and seawater tends to interest spectators. Seawater generally has a much higher concentration of solutes than freshwater, and this difference accounts for the foaming and formation of whitecaps observed in seawater but not freshwater. This differential behavior has been linked to inhibition of bubble coalescence due to dissolved solutes in seawater.29,30

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.6b00862. Fountain height/bubble size overlay plots for other solutes, ingredients list for beverages tested in Figure 2, and information on how to conduct experiments that demonstrate the effects discussed herein (PDF, DOCX)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Thomas S. Kuntzleman: 0000-0002-2691-288X Dean J. Campbell: 0000-0002-2216-4642 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We wish to thank Emily Brewer, Matthew Folkenroth, Mayuresh Gadgil, Keri Martinez, and Max Palmer for assistance; Wayne Bosma for assistance and helpful discussion; and Mark Ott for helpful discussion. The reviewers of this manuscript provided a wealth of helpful insights and suggestions that greatly improved this work. We thank one reviewer in particular for suggestions for future work. Spring Arbor University provided financial support of this project.

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REFERENCES

(1) Coffey, T. S. Diet Coke and Mentos: What is really behind this physical reaction? Am. J. Phys. 2008, 76, 551−557. (2) Huber, C. J.; Massari, A. M. Quantifying the Soda Geyser. J. Chem. Educ. 2014, 91, 428−431. (3) Liljeholm, A. Diet Soda and Iron Filings. Am. J. Phys. 2009, 77, 293. (4) Liger-Belair, G. The Physics and Chemistry Behind the Bubbling Properties of Champagne and Sparkling Wines: A State-of-the-Art Review. J. Agric. Food Chem. 2005, 53, 2788−2802. (5) Baur, J. E.; Baur, M. B.; Franz, D. A. The Ultrasonic Soda Fountain: A Dramatic Demonstration of Gas Solubility in Aqueous Solutions. J. Chem. Educ. 2006, 83, 577−580.

CONCLUSION The experiments presented here provide insight on role of solutes in fountain heights achieved during the famous Diet Coke and Mentos experiment. Addition of a surprising number of different solutes tends to increase fountain geyser heights during NISD in a manner that depends upon solute concentration. The presence of solutes appears to limit the size of bubbles produced during degassing, which contributes to faster degassing kinetics and higher fountains. While it has been 575

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Journal of Chemical Education

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DOI: 10.1021/acs.jchemed.6b00862 J. Chem. Educ. 2017, 94, 569−576