If you know me, you know that I love the Diet Coke and Mentos reaction. It’s so simple to carry out, but yields incredible results! Just drop a few Mentos candies into a bottle of a carbonated beverage and watch the magic take place! See video 1.
Video 1: Speedy Science Clips: Diet Coke and Mentos in Slow Motion, Tommy Technicium YouTube Channel, August 29, 20201
Very cool, huh? I’ve recently learned how to conduct this reaction in a way that involves a color change. To understand how to pull this off, it is first instructive to briefly describe how this impressive geyser forms. If you’d rather just skip to the experiments, scroll down to the videos in the Experiment section below.
Background
Sodas contain large amounts of dissolved CO2, and Mentos cause this dissolved CO2 to be released as gas bubbles.2-10 The Mentos induce rapidly expanding bubbles that push the beverage out of the bottle as they rise.9-10 The process of CO2 being released from the soda can be described in the following equation:
CO2(aq) → CO2(g) Equation 1
No new chemical compounds are created during the process outlined in Equation 1. While CO2 is in different phases, the same chemical compound is both the reactant and product of Equation 1. Because no new chemical compounds are formed, the escape of CO2 from a soda is known as a physical change. Thus, the process represented in Equation 1 – which powers the Coke and Mentos fountain – is a physical change. However, the Coke and Mentos experiment does not only involve a physical change.6 The physical process of gas escape induces chemical changes. To see this, recall that dissolved CO2 reacts with water to form carbonic acid (H2CO3):
CO2(aq) + H2O(l) ← → H2CO3(aq) Equation 2
Carbonic acid is a weak acid, so it dissociates into a proton and bicarbonate ion (HCO3-):
H2CO3(aq) ← →H+(aq) + HCO3-(aq) Equation 3
Upon addition of Mentos candies, CO2 escapes sodas and the amount of dissolved CO2 in the soda becomes depleted (Equation 1). This causes Equation 2 to shift to the left by the Principle of Le Châtelier. But notice that this causes a depletion of H2CO3, which forces Equation 3 to shift to the left as well – also by the Principle of Le Châtelier. Interestingly enough, as Equation 1 proceeds it drives two chemical processes: the conversion of H2CO3 to CO2 and water, and also the reaction of H+ and HCO3- to form H2CO3.
By reversing Equations 2 and 3 and then adding them together, we see that the sum total of Equations 2 and 3 shifting to the left cause the net consumption of a proton:
H+(aq) + HCO3-(aq) → CO2(aq) + H2O(l) Equation 4
Thus, the escape of CO2 from a carbonated beverage should cause loss of protons, a decrease in acidity, and an increase in pH.6
Experiment
I have found it is possible to use acid-base indicators to detect an increase in pH when Mentos candies are added to carbonated beverages. To see the color change, it was necessary to use a colorless beverage rather than colas or other colored drinks when doing so. You can see me carrying out this experiment along with other explorations in the video below:
Video 2: The Science of Diet Coke and Mentos, Tommy Technicium YouTube Channel, August 26, 2020 11
It is fair to ask if it is not the escape of CO2, but rather basic compounds in the Mentos that is responsible for the shift to higher pH and concomitant color change in the indicator. I tested this by adding Mentos to club soda that was boiled to remove the CO2. Universal indicator was added to test for difference in pH upon adding Mentos. When doing these experiments, I also learned that one can observe almost all the colors of the rainbow when boiling club soda to which universal indicator has been added:
Video 3: Color Changing Coke and Mentos Experiments, Tommy Technicium YouTube Channel, August 30, 202012
It looks as if there is no increase in pH upon adding Mentos to a carbonated beverage that has been degassed, indicating it is indeed CO2 escape that drives the color change when Mentos are added to sodas containing indicator. If anything, addition of Mentos decreases the pH! That Mentos addition does not increase the pH of boiled carbonated beverages can also be demonstrated using a pH meter.6
Discussion
I think it is interesting to note that removing CO2 from club soda by boiling causes an increase in pH from 3 to 8-9, while CO2 removal using Mentos only causes an increase in pH from 3 to 4-5. Why the difference? Two reasons could account for this difference. First, in Video 3 it was observed that Mentos addition causes a slight decrease in pH in a process that takes several minutes. Another reason could be that the pores on the Mentos can only support bubble growth in solutions that have CO2 concentrations that exceed a critical value.10 As the CO2 degasses upon Mentos addition, the CO2 concentration in the soda decreases. However, once the CO2 concentration drops below the critical value, the CO2 can no longer degas. Therefore, because boiling removes all dissolved CO2 but Mentos addition does not, the former causes a larger pH shift than the latter.
The rainbow of colors observed when boiling club soda to which universal indicator was added was a real treat for me to observe. What a simple experiment: add the indicator to club soda, boil the resulting mixture, and watch the rainbow slowly unfold over time! Most sodas do not display such a drastic color change, given that they have acidic additives such as citric acid, instead of the potassium bicarbonate and potassium citrate in club soda.
Connections to the Curriculum
These simple demonstrations can be used to talk about a variety of concepts: chemical vs. physical changes, acid-base chemistry, pH, and the Principle of Le Châtelier. Also, these experiments lend themselves to talking about the impact of increased atmospheric CO2 concentration (from the burning of fossil fuels) on the pH of the world’s oceans. As CO2 in the atmosphere increases, the amount of CO2 dissolved in the ocean increases (Equation 1 gets driven to the left). This in turn causes Equations 2 and 3 to be driven to the right, increasing the acidity of the oceans. Indeed, the pH of the oceans has been observed to drop in an effect known as ocean acidification. As you can imagine there is much concern over the impact that ocean acidification has on marine life. In addition, the rainbow observed upon boiling club soda + universal indicator can be used to introduce students to the fact that our oceans – which store about half of the CO2 emitted by fossil fuel use - will not be able to store as much CO2 as they continue to warm due to the effects of global warming.
Summary
As you can see in Video 2, I used bromocresol green and club soda to get a green-to-blue color change during the Mentos-induced degassing of soda. I also used a home carbonatioin system and bromocresol green to observe a green-to-yellow color change upon pumping CO2 into water with a home carbonation system. In Video 3, universal indicator was used to cause a red-to-orange color change. I’m looking forward to trying other combinations of sodas and indicators to see if other color changes can be generated. Please drop me a line in the comments if you try this demonstration on your own – or learn how to produce other color changes!
Happy experimenting!
References
1. Kuntzleman, T. S., Speedy Science Clips: Diet Coke and Mentos in Slow Motion, Tommy Technicium YouTube Channel, August 29, 2020
2. 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.
3. Coffey, T.S. Diet Coke and Mentos: What is really behind this physical reaction? Am. J. Phys. 2008, 76, 551–557.
4. Gardner, D. E.; Patel, B. R.; Hernandez, V. K.; Clark, D.; Sorensen, S.; Lester, K.; Solis, Y.; Tapster, D.; Savage, A.; Hyneman, J.; Dukes, A. D. Investigation of the Mechanism of the Diet Soda Geyser Reaction. Chem. Educator 2014, 19, 358–362.
5. Huber, C. J.; Massari, A. M. Quantifying the Soda Geyser. J. Chem. Educ. 2014, 91, 428–431.
6. 3. Sims, T. P. T.; Kuntzleman, T. S. Kinetic Explorations of the Candy-Cola Soda Geyser J. Chem. Educ., 2016, 93, 1809–1813.
7. Kuntzleman, T. S.; Davenport, L. S.; Cothran, V. I.; Kuntzleman, J. T.; Campbell, D. J. New Demonstrations and New Insights on the Mechanism of the Candy-Cola Soda Geyser J. Chem. Educ. 2017, 94, 569–576.
8. Kuntzleman, T. S.; Nydegger, M. W.; Shadley, B.; Doctor, N.; Campbell, D. J. Tribonucleation: A New Mechanism for Generating the Soda Geyser. J. Chem. Educ. 2018, 95, 1345–1349.
9. Kuntzleman et. al, Kinetic Modeling of and Effect of Candy Additives on the Candy-Cola Soda Geyser: Experiments for Elementary School Science through Physical Chemistry J. Chem. Educ. 2020, 97, 283–288.
10. Kuntzleman, T. S.; Johnson, R. J. Probing the Mechanism of Bubble Nucleation in and the Effect of Atmospheric Pressure on the Candy-Cola Soda Geyser J. Chem. Educ. 2020, 97, 980–985.
11. Kuntzleman, T. S., The Science of Diet Coke and Mentos, Tommy Technicium YouTube Channel, August 26, 2020
12. Kuntzleman, T. S., Color Changing Coke and Mentos Experiments, Tommy Technicium YouTube Channel, August 30, 2020
Safety
General Safety
General Safety
For Laboratory Work: Please refer to the ACS Guidelines for Chemical Laboratory Safety in Secondary Schools (2016).
For Demonstrations: Please refer to the ACS Division of Chemical Education Safety Guidelines for Chemical Demonstrations.
Other Safety resources
RAMP: Recognize hazards; Assess the risks of hazards; Minimize the risks of hazards; Prepare for emergencies
NGSS
Analyzing data in 9–12 builds on K–8 and progresses to introducing more detailed statistical analysis, the comparison of data sets for consistency, and the use of models to generate and analyze data.
Analyzing data in 9–12 builds on K–8 and progresses to introducing more detailed statistical analysis, the comparison of data sets for consistency, and the use of models to generate and analyze data. Analyze data using tools, technologies, and/or models (e.g., computational, mathematical) in order to make valid and reliable scientific claims or determine an optimal design solution.
Asking questions and defining problems in grades 9–12 builds from grades K–8 experiences and progresses to formulating, refining, and evaluating empirically testable questions and design problems using models and simulations.
Asking questions and defining problems in grades 9–12 builds from grades K–8 experiences and progresses to formulating, refining, and evaluating empirically testable questions and design problems using models and simulations.
questions that challenge the premise(s) of an argument, the interpretation of a data set, or the suitability of a design.
Scientific questions arise in a variety of ways. They can be driven by curiosity about the world (e.g., Why is the sky blue?). They can be inspired by a model’s or theory’s predictions or by attempts to extend or refine a model or theory (e.g., How does the particle model of matter explain the incompressibility of liquids?). Or they can result from the need to provide better solutions to a problem. For example, the question of why it is impossible to siphon water above a height of 32 feet led Evangelista Torricelli (17th-century inventor of the barometer) to his discoveries about the atmosphere and the identification of a vacuum.
Questions are also important in engineering. Engineers must be able to ask probing questions in order to define an engineering problem. For example, they may ask: What is the need or desire that underlies the problem? What are the criteria (specifications) for a successful solution? What are the constraints? Other questions arise when generating possible solutions: Will this solution meet the design criteria? Can two or more ideas be combined to produce a better solution?
Constructing explanations and designing solutions in 9–12 builds on K–8 experiences and progresses to explanations and designs that are supported by multiple and independent student-generated sources of evidence consistent with scientific ideas, principles, and theories.
Constructing explanations and designing solutions in 9–12 builds on K–8 experiences and progresses to explanations and designs that are supported by multiple and independent student-generated sources of evidence consistent with scientific ideas, principles, and theories. Construct and revise an explanation based on valid and reliable evidence obtained from a variety of sources (including students’ own investigations, models, theories, simulations, peer review) and the assumption that theories and laws that describe the natural world operate today as they did in the past and will continue to do so in the future.
Constructing explanations and designing solutions in 9–12 builds on K–8 experiences and progresses to explanations and designs that are supported by multiple and independent student-generated sources of evidence consistent with scientific ideas, principles, and theories.
Constructing explanations and designing solutions in 9–12 builds on K–8 experiences and progresses to explanations and designs that are supported by multiple and independent student-generated sources of evidence consistent with scientific ideas, principles, and theories. Construct and revise an explanation based on valid and reliable evidence obtained from a variety of sources (including students’ own investigations, models, theories, simulations, peer review) and the assumption that theories and laws that describe the natural world operate today as they did in the past and will continue to do so in the future.
Planning and carrying out investigations in 9-12 builds on K-8 experiences and progresses to include investigations that provide evidence for and test conceptual, mathematical, physical, and empirical models.
Planning and carrying out investigations in 9-12 builds on K-8 experiences and progresses to include investigations that provide evidence for and test conceptual, mathematical, physical, and empirical models. Plan and conduct an investigation individually and collaboratively to produce data to serve as the basis for evidence, and in the design: decide on types, how much, and accuracy of data needed to produce reliable measurements and consider limitations on the precision of the data (e.g., number of trials, cost, risk, time), and refine the design accordingly.
Students who demonstrate understanding can construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.
*More information about all DCI for HS-PS1 can be found at https://www.nextgenscience.org/dci-arrangement/hs-ps1-matter-and-its-interactions and further resources at https://www.nextgenscience.org.
Students who demonstrate understanding can construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.
Assessment is limited to chemical reactions involving main group elements and combustion reactions.
Examples of chemical reactions could include the reaction of sodium and chlorine, of carbon and oxygen, or of carbon and hydrogen.
Students who demonstrate understanding can refine the design of a chemical system by specifying a change in conditions that would produce increased amounts of products at equilibrium.
*More information about all DCI for HS-PS1 can be found at https://www.nextgenscience.org/dci-arrangement/hs-ps1-matter-and-its-interactions and further resources at https://www.nextgenscience.org.
Students who demonstrate understanding can refine the design of a chemical system by specifying a change in conditions that would produce increased amounts of products at equilibrium.
Assessment is limited to specifying the change in only one variable at a time. Assessment does not include calculating equilibrium constants and concentrations.
Emphasis is on the application of Le Chatelier’s Principle and on refining designs of chemical reaction systems, including descriptions of the connection between changes made at the macroscopic level and what happens at the molecular level. Examples of designs could include different ways to increase product formation including adding reactants or removing products.
Analyze a Major Global Challenge is a performance expectation related to Engineering Design HS-ETS1.
Analyze a major global challenge to specify qualitative and quantitative criteria and constraints for solutions that account for societal needs and wants.
Students analyze a major global problem. In their analysis, students: Describe the challenge with a rationale for why it is a major global challenge; Describe, qualitatively and quantitatively, the extent and depth of the problem and its major consequences to society and/or the natural world on both global and local scales if it remains unsolved; and Document background research on the problem from two or more sources, including research journals. Defining the process or system boundaries, and the components of the process or system: In their analysis, students identify the physical system in which the problem is embedded, including the major elements and relationships in the system and boundaries so as to clarify what is and is not part of the problem: and In their analysis, students describe* societal needs and wants that are relative to the problem. Defining the criteria and constraints: Students specify qualitative and quantitative criteria and constraints for acceptable solutions to the problem.