Love explosive science experiments? The Diet Coke and Mentos eruption is sure to thrill kids and adults alike! This fun and fizzy activity is a perfect example of a physical reaction rather than a chemical reaction. Many people speculated that the geyser was the result of an acid-base reaction, given the low pH of soda. However, the eruption is caused by a physical reaction, rather than any chemical reaction.
What You Need for the Experiment
No matter how messy or sticky the experiment is, there are only two ingredients required to make this geyser. One bottle of 2-liter fizzy drink, preferably Diet Coke, and Mentos are needed in an adequate quantity to give a spectacular reaction. For a 2-liter bottle of Coke, at least five Mentos are good enough. Moreover, all Mentos must be added to the drink simultaneously, giving each of them equal time to create an effect.
- Diet Coke (2-liter bottle is recommended)
- Mentos (at least one roll)
- Index cards
- Tape
- An outdoor area
- Eye protection (safety goggles or glasses)
- Video camera with either a tripod or a helper to take the images (optional)
Setting Up the Experiment
💡 This experiment is best done in an outdoor area! If you want to experiment and take measurements, try it against an exterior wall of a building and use the blue painter’s tape to record the height of the spout.
- Roll up an index card into a tube and secure it with tape. The tube should be large enough to hold whole Mentos candies but small enough to allow them to fall out easily.
- Tape the tube to the top of your bottle of Diet Coke, leaving one side open.
- Place the second index card underneath the tube.
Performing the Eruption
- Slowly and carefully open a new bottle of Diet Coke.
- With the string in hand, step back and pull the string, which will release the index card and cause the Mentos to drop into the Diet Coke.
💡 Tip: Use a tape measure or mark a wall with tape to record the fountain height (the height of the eruption).
The Science Behind the Fizz: It's All About Physics, Not Chemistry
What causes Coke to explode when Mentos are added to it? One would think that there must be a chemical reaction that causes the Coke and Mentos reaction to be so attractive and satisfying. It’s actually not chemical! The carbonated drinks’ fizz comes from carbon dioxide added to the bottles at high pressure. 2-liter Diet Coke contains around 12-15 grams of dissolved carbon dioxide. The candies simply catalyze the release of gas from the Coke bottle.
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Inside Diet Coke (or any carbonated beverage), there is dissolved carbon dioxide gas. This gas creates gas bubbles, which give soda its characteristic fizz. The conversion of dissolved carbon dioxide to gaseous carbon dioxide forms rapidly expanding gas bubbles in the soda, which pushes the beverage contents out of the container. Bottled sodas are kept under pressure so that more carbon dioxide can be forced into solution. When the pressure is released, the carbon dioxide is forced out of solution and makes little gas bubbles. So, if you open a bottle of soda gently, you get a pleasant beverage. If you shake the can first, you disrupt the solution and get a face full of soda. And if you add a big enough surfactant, you get a geyser. It's the same chemistry, but a different magnitude.
Nucleation Sites: The Key to the Eruption
The gas tries to escape and form bubbles around any irregular surface, called a nucleation site. The carbon dioxide molecules attach to the surfaces of the Mentos like they did in the cup of soda. All those Mentos in a lot of soda make a lot of bubbles that rise to the surface and push the soda out in a big woosh!
Mentos candies are not as smooth as they appear to the naked eye. They are covered in bumpy craters, which increases the total surface area. Mentos also have nucleation sites because they are not as smooth as they appear. When you add Mentos to the bottle of soda, the rough surface of the candy accelerates the release of gas. As more gas bubbles form on the Mentos’ surface, they push the liquid out in an explosive eruption, converting the dissolved carbon dioxide gas into a rapidly expanding gas.
The two biggest factors affecting the geyser are the roughness of the candy used and the rate at which it sinks to the bottom of the soda bottle. A rougher candy surface translates to more places for bubbles to grow, or more nucleation sites. Since the Mentos are also heavy enough to sink, they react with the soda all the way to the bottom.
Surfactant Action: Breaking the Surface Tension
A Mentos dropped into a bottle of soda acts as a surfactant, meaning it reduces the surface tension of the soda. Water molecules are polar and attracted to each other. Anything that breaks them apart allows for bubbles of carbon dioxide gas to form in the solution. Surfactants are compounds that lower the surface tension (or interfacial tension) between two liquids or between a liquid and a solid. This is essentially speeding up the process that makes sodas fizzy. Low surface tension also helps bubbles grow quickly. Another factor is that the coatings of Mentos contain gum arabic, a surfactant that further reduces surface tension in the liquid.
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Why Diet Coke? The Role of Aspartame and Sugar
You might be wondering, Why do we use Diet Coke over regular Coke for this experiment? Regular Coke contains sugar, while Diet Coke uses an artificial sweetener called aspartame. Here’s where it gets interesting: Aspartame, the sweetener used in Diet Coke, facilitates the formation of bubbles more easily when Mentos are added. 💡 Fun Fact: The sugar in regular Coke can make the soda more viscous (thicker), which slows down the formation of bubbles.
Experimenting with Variables
The Diet Coke and Mentos eruption is an excellent opportunity to apply the scientific method.
- Form a Hypothesis: What do you think will happen when you drop Mentos into Diet Coke?
- Variables: Change one thing at a time, such as the type of soda, the Mentos flavor (like mint or fruit Mentos), or the temperature of the soda.
- Conclusion: Did your hypothesis match the outcome?
Experiment Ideas
- Try crushing the Mentos into small pieces before adding them to the soda.
- Does Diet Coke create the biggest eruption, or does regular Coke, Root Beer, or Sprite make a higher fountain? Experiment and find out!
- What happens when you use cold Diet Coke versus room temperature?
- Can you replace Mentos with something else?
Real-World Applications and Further Exploration
Mixing mentos and soda makes a great science demonstration for students studying gases, thermodynamics, fluid dynamics, surface science, and the physics of explosions among other chemistry and physics concepts. The force exerted by the soda stream can be harnessed to do work. One enterprising group made a mentos and diet coke-powered rocket.
Tonya Coffey, a professor at Appalachian State University, used the experiment to give her undergraduate physics class a real-world research experience as one of their laboratory assignments. The project was eventually published in the American Journal of Physics.
Safety and Common Misconceptions
Have you ever noticed that when you put a straw in soda pop, the straw gets a lot of bubbles on it? Why does that happen? Also, the things you put in the soda aren’t really as smooth as they look with just your eyes.
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There is an urban legend that eating mentos while drinking soda could cause a person's stomach to burst. However, most of the carbonation is released from the soda as it is being drunk, so the pressure is lower and carbon dioxide is less likely to nucleate. Additionally, the stomach has a couple ways of expelling excess gases.
Historical Context and Notable Experiments
In the 1910s, Wint-O-Green Life Savers were used to create soda geysers. The tubes of candies were threaded onto a pipe cleaner and dropped into the soft drink to create a geyser. At the end of the 1990s, the manufacturer of Wintergreen Lifesavers increased the size of the mints, and they no longer fit in the mouth of soda bottles.
Lee Marek and "Marek's Kid Scientists" performed the Diet Coke and Mentos experiment on the Late Show with David Letterman in 1999. In March 2002, Steve Spangler, a science educator, did the demonstration on KUSA-TV, an NBC affiliate, in Denver, Colorado. The Diet Coke and Mentos geyser experiment became an internet sensation in September 2005.