In the Classroom edited by
JCE DigiDemos: Tested Demonstrations
Ed Vitz Kutztown University Kutztown, PA 19530
The Ultrasonic Soda Fountain: A Dramatic Demonstration of Gas Solubility in Aqueous Solutions submitted by:
John E. Baur* Department of Chemistry, Illinois State University, Normal, IL 61790-4160; *
[email protected] Melinda B. Baur Department of Chemistry, Illinois Wesleyan University, Bloomington, IL 61702-2900
checked by:
David A. Franz Department of Chemistry, Lycoming College, Williamsport, PA 17701
The solubility of gases in liquids is a fundamental concept that is explored in classrooms from grade school through college. Common demonstrations of gas solubility range from comparisons of carbonation in warm versus cold beverages at the primary school level through measurements of Henry’s law constants in the undergraduate laboratory (1, 2). We describe a simple yet dramatic demonstration suitable for introducing the concept of gas solubility by exposing a carbonated beverage (uncapped) to ultrasonic energy in a common laboratory ultrasonic cleaner. As shown in Figure 1, the sudden degassing of the solution through the narrow bottle top produces a dramatic, foaming fountain that can reach a height of several meters. For advanced level classes, the demonstration might be used to introduce the dynamic aspect of gas solubility in liquids and nucleation in liquids supersaturated with gas. Henry’s law describes the relationship between the concentration of a gas dissolved in a solvent, [X], and the partial pressure of the gas, pX, [X] = KX pX where KX, the Henry’s law constant, is dependent on the chemical identity of the solute and solvent and on the temperature of the solution. After bottling, the headspace of a soda is comprised primarily of CO2 at a pressure of about 4 atm (i.e., pCO2 = 4 atm) (3). Earth’s atmosphere has a much lower CO2 partial pressure ( pCO2 = 3.7 × 10᎑4 atm) (4). Therefore, Henry’s law predicts that the concentration of aqueous CO2 should decrease dramatically after exposing the beverage to the atmosphere. Fortunately, this equilibrium is established slowly, and we can thus enjoy carbonated beverages for some time after opening the container. By exposing a carbonated beverage to ultrasonic energy, the rate of CO2 loss is greatly accelerated and this equilibrium with the atmosphere is achieved much more rapidly. A spectacular fountain of foam results from the large volume of CO2 ejected through the narrow opening at the top of the bottle. Although the height of this fountain depends upon several parameters, we routinely achieve heights of 1–2 meters with a 2-L bottle of soda and greater than 3 meters with a single-serving bottle having a reduced size opening. Under optimal conditions, more than half of the beverage volume is ejected within 5–10 seconds. www.JCE.DivCHED.org
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Figure 1. The ultrasonic soda fountain: (A) warm (left) versus cold (5 °C) diet 2-L sodas, (B) regular (left) versus diet sodas, (C) newly opened (left) versus flat diet sodas, and (D) 24-oz (710-mL) diet soda with reduced size opening (see text). Except for the cold soda in panel A, the temperature of all sodas was 24 °C.
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Procedure
Figure 2. Enclosed operation of the ultrasonic soda fountain for simpler cleanup using (A, B) 5-gallon (19-L) water cooler bottle or (C, D) vented 2-L soda bottle. The fountain is shown just after energizing the ultrasound (A, C) and during the maximum intensity (B, D).
The demonstration should be performed outdoors or in a room with a high ceiling and provisions for containing a large spill. Liquid can splatter several meters from the bottle, and therefore spectators should be kept at a safe distance. We prefer to use a colorless diet soda, as problems with staining and sticky residue are avoided. If single-serving bottles are used, they are fitted with a #3 rubber stopper into which a hole has been drilled so that a 4-cm piece of a plastic drinking straw can be held tightly. The straw is used to achieve a measure of directional control of the fountain. Before attempting the demonstration, a waterproof cover for the ultrasonic cleaner should be made. We cut a hole the size of the cleaner’s bath in an absorbent, plastic-lined laboratory shelf paper. Sufficient water is added to the cleaner so that when the beverage container is placed in the bath the water reaches the operating level. After the beverage cap is removed, the ultrasound can be energized. We have also used two versions of the demonstration that require less cleanup. While somewhat less dramatic, they are more appropriate for indoor demonstrations in smaller classrooms. In the first, an empty 5-gallon (19-L) water cooler bottle is used to contain the fountain (Figures 2A and B). To join the soda bottle to the larger container we used a Fountain Connection (Yeany Educational Products, Palmyra, PA), Tygon tubing, and a #10 rubber stopper configured as shown in Figure 3. Although we allowed soda exiting the vent to drip back into the bath, a drain tube could also be attached. The second version is an even simpler method of containment suitable for smaller audiences. For this demonstration, an empty 2-L bottle with small vent holes is joined to a single-serving soda (Figure 2C and D). A “Tornado Tube” (Burnham Associates, Salem, MA) commonly used to join two 2-L bottles for a vortex demonstration makes a simple coupler. Although the ejection of a small volume of foam from the vent holes is unavoidable, we found that placing the holes on the side near the top of the inverted 2-L receptacle minimizes splatter. The effect of a number of experimental parameters on the fountain can be investigated: • Temperature: The height of the fountain is greater for room temperature beverages than for chilled beverages (Figure 1A). • Presence of other solutes: Diet sodas generally produce higher fountains than sodas sweetened with sugars (Figure 1B). • Aqueous CO2 concentration: A beverage that has been opened for a period of time will produce a smaller fountain than a freshly opened beverage (Figure 1C).
Figure 3. A close-up view of the connector for the water bottle cooler.
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With the exception of seltzer water and club soda, all carbonated beverages we exposed to the ultrasonic energy produced a fountain. Table 1 shows the fountain height obtained with single-serving containers of different sodas using the restricted opening. Maximum effect was achieved with room temperature Diet Coke.
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In the Classroom
Hazards To avoid electrical shock hazards, the electronics and power cable of the ultrasonic cleaner must be covered with a waterproof material. Further, the experiment should only be attempted with the ultrasonic cleaner plugged into a groundfault interrupt (GFI) outlet. If no such outlet is available, an extension cord with an integrated GFI circuit should be obtained from a hardware store. Discussion When discussing the results of this demonstration, one should be careful to emphasize that the fountain results from kinetic effects. All carbonated beverages are supersaturated with CO2 upon opening, so loss of the gas is thermodynamically favored. At 25 ⬚C, a soda contains about 3 L of CO2 per liter of solution just after opening.1 If the container remains undisturbed, the carbon dioxide loss is slow and occurs via two processes: diffusion through the surface layer and bubble formation. Of these, the former is slow because of the relatively low area of soda exposed to the atmosphere. Bubble formation, therefore, is the predominant mechanism for CO2 loss, and the rate at which equilibrium is reestablished depends upon the rate of bubble formation. Because of the high degree of interfacial free energy associated with very small bubbles, spontaneous formation of bubbles is energetically unfavorable in carbonated beverages (5, 6). Instead, bubbles rise from a few discrete nucleation sites at which this interfacial energy barrier is overcome. In champagne, it has been shown that nucleation occurs at impurities on the container wall (6, 7). Shaking or dropping a carbonated beverage produces a spray of foam when the container is opened because a number of nucleation sites are formed by the mechanical agitation of the solution. Likewise, ultrasound produces a spray of foam because of increased nucleation. This was demonstrated by two simple experiments. First, when a closed bottle of soda was exposed to ultrasound there was no visible effect on the beverage. However, when the bottle was subsequently opened, a spray of foam qualitatively comparable in intensity to a shaken bottle of soda resulted. In the second experiment, a small volume of soda was placed in a beaker and closely observed during exposure to ultrasound. As shown in Figure 4, violent evolution of CO2 occurs primarily at the bottom and sides of the container. Before exposure to the ultrasound few bubbles are visible (Figure 4A), a large cascade of bubbles appears at the beaker surface shortly after initiating the ultrasonic energy (Figures 4B and C), and bubbles remaining at some nucleation sites are still visible just after the termination of the ultrasound (Figure 4D). This greatly increased nucleation probably results from both intense agitation (which forms nucleation sites in a manner similar to shaking) and from cavitation (the formation of very small bubbles during the low pressure portion of the ultrasonic wave) (8). Interestingly, the bubbles produced in the ultrasound are much smaller than the bubbles formed in the undisturbed solution. Table 1 shows that fountain height increases with temperature for a given soda and that diet sodas produce higher fountains than the sugar-sweetened versions. A number of effects probably combine to give these results, but ultimately www.JCE.DivCHED.org
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a higher fountain is produced when the rate of bubble formation increases relative to the rate of bubble collapse. We can make some generalizations about the data in Table 1 based upon the effect solution properties have on the rate of bubble formation and collapse. The formation of cavities in solution by ultrasound is favored in solvents with low viscosity, low surface tension, and high vapor pressure (8). Compared to pure water, a solution containing 10% w兾w sucrose (the approximate concentration of sugars in a regular soda) has a significantly higher viscosity (1.336 versus 1.002 cP at 20 ⬚C ; ref 9), a slightly higher surface tension (72.50 versus 71.97 mN兾m at 25 ⬚C; ref 10), and a lower vapor pressure. These data are consistent with a diet soda producing a higher
Table 1. Fountain Heights Obtained with Various Carbonated Beverages Beverage
Size
T/°C
Height/ m
Golden Crown Seltzer Water
1L
5
—
No fountain
Golden Crown Soda Water
1L
5
—
No fountain
Golden Crown Tonic Water
1L
5
—
Foamed over side of unrestricted opening
Harp Lager (Beer)
350 mL
5
4.5
Dr. Pepper
710 mL
5 21
3.2 3.8
Diet Dr. Pepper
710 mL
5 21
2.5 3.9
Mountain Dew
710 mL
5 21
0.9 2.5
Diet Mountain Dew
710 mL
5 21
1.5 3.3
Diet Pepsi
710 mL
5 21
3.0 3.9
Sierra Mist
710 mL
5 21
2.3 2.8
710 mL
5 21
3.5 3.8
Diet Sierra Mist
Comments
Vigorous; stopper ejected in each case
NOTE: Results are the average of three replicates. Except where noted, a reduced-size opening was used.
Figure 4. Time-lapse images of Diet Sierra Mist in the ultrasonic cleaner: (A) Initial solution, (B) ~100 ms after energizing the ultrasound, (C) ~250 ms after energizing the ultrasound, and (D) after termination of the ultrasound and collapse of the foam.
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rate of bubble formation and thus a higher fountain. The origin of the temperature effect on the fountain is not straightforward. Solution properties favor cavity formation at higher temperatures (viscosity and surface tension decrease and vapor pressure increases with temperature), and the rate of bubble growth also increases with temperature (6). These effects must combine to produce a higher rate of degassing at elevated temperatures and thus generate a higher fountain. Seltzer water and soda water produce no fountain, probably because they contain no ingredients that stabilize the bubbles. While the rate of bubble formation in these beverages greatly increases in the ultrasound, much larger bubbles are produced, indicating high rate of coalescence. However, if a small quantity of surfactant is added (1 drop of common dishwashing detergent in 1 L of the beverage will suffice), both seltzer water and soda water produce fountains. Beer, which has a low rate of bubble formation but high concentrations of surfactants that stabilize the foam (7, 11), produces a fountain. These observations further demonstrate the importance of the rates of bubble formation and collapse. Finally, it should be recognized that CO2 does not ideally follow Henry’s law because it undergoes a chemical reaction with the solvent. After dissolution, which is governed by Henry’s law: CO2(g)
CO2(aq) K CO2 = 3.4 × 10−2mol L−1atm−1 (1)
the carbon dioxide can react with water to form carbonic acid: CO2(aq) + H2O
H2CO3 −3 K = 1.7 × 10
(2)
However, because the equilibrium constant is small, CO2(aq) is the predominate species. The carbonic acid can dissociate to form bicarbonate and carbonate according to the following reactions: H2CO3(aq) + H2O
− HCO3 + H2O
+ − H3O + HCO3 −4 Ka1 = 2.7 × 10
(3)
2− + H3O + CO3 −11 Ka2 = 4.69 × 10
(4)
Reactions 2 and 3 are commonly combined into a single expression for Ka1: CO2(aq) + 2H2O
+ H3O + HCO3−
Ka1 = 4.45 × 10
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(5)
The pH values of the carbonated beverages used were well below the combined pKa1 (6.35), and therefore nearly all carbon dioxide is present as CO2(aq). In fact, the solution that remains in the container still has CO2(aq) as the predominant form, as the pH was nearly the same as in the newlyopened beverage (on average the pH increased 0.05 ± 0.07 units). Additionally, the solution is still supersaturated with CO2, as the carbonation is still perceptible by sensation on tongue (though noticeably diminished) and bubbles continue to form in the remaining solution (but at a much lower rate). This demonstration vividly illustrates the large volume of dissolved CO2 contained in an ordinary soda. It can also be an effective tool for stimulating discussions of gas solubility, solution properties, nucleation, and kinetics. Acknowledgments The authors wish to thank Claire and Ben Baur for experimental assistance and Otis Rothenberger and James Webb for their valuable comments on the manuscript. We also acknowledge Robert Turner for helpful discussions. Note 1. This calculation ignores the effect of other solutes on the CO2 solubility and also assume that only CO2 is in the headspace.
Literature Cited 1. Levy, J. B.; Hornack, F. M.; Levy, M. A. J. Chem. Educ. 1987, 64, 260–261. 2. Halpern, A. M. Experimental Physical Chemistry: A Laboratory Textbook, 2nd ed.; Prentice Hall: Upper Saddle River, NJ, 1997. 3. Silberberg, M. S. Chemistry, The Molecular Nature of Matter and Change, 2nd ed.; McGraw Hill: Boston, MA, 2000. 4. Spiro, T. G.; Stigliani, W. M. Chemistry of the Environment, 2nd ed.; Prentice Hall: Upper Saddle River, NJ, 2003. 5. Deamer, D. W.; Selinger, B. K. J. Chem. Educ. 1988, 65, 518. 6. Liger-Belair, G.; Vignes-Adler, M.; Voisin, C.; Robillard, B.; Jeandet, P. Langmuir 2002, 18, 1294–1301. 7. Liger-Belair, G. Sci. Am. 2003, 288, 80–85. 8. Thompson, L. H.; Doraiswamy, L. K. Ind. Eng. Chem. Res. 1999, 38, 1215–1249. 9. CRC Handbook of Chemistry and Physics, 85th ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 2004; p 8-82. 10. CRC Handbook of Chemistry and Physics, 63rd ed.; Weast, R. C., Astle, M. J., Eds.; CRC Press: Boca Raton, FL, 1982; pp F-34–F-35. 11. Shafer, N. E.; Zare, R. N. Phys. Today 1991, 44, 48–52.
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