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) à 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) ß à H₂CO₃(aq)               Equation 2

Carbonic acid is a weak acid, so it dissociates into a proton and bicarbonate ion (HCO₃^-):

H₂CO₃(aq) ß àH⁺(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) à 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