The seemingly simple act of dropping Mentos into Diet Coke unleashes a spectacular geyser, captivating audiences and sparking curiosity. This phenomenon, popularized through countless online videos, isn't a magic trick, but a demonstration of basic scientific principles at play. It’s a physical reaction, not a chemical one, and understanding the factors involved can turn a fun stunt into an educational experience.
The Anatomy of a Soda Geyser
A soda geyser is a physical reaction between a carbonated beverage, usually Diet Coke, and Mentos mints that causes the beverage to be expelled from its container. 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.
A Brief History
The phenomenon of creating soda geysers isn't new. As early as 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. However, the Diet Coke and Mentos experiment gained widespread attention in the late 1990s and early 2000s, thanks to demonstrations by Lee Marek and Steve Spangler on television. The Diet Coke and Mentos geyser experiment became an internet sensation in September 2005.
The Science Behind the Eruption
The Diet Coke and Mentos eruption is caused by a physical reaction, rather than any chemical reaction. Carbonated sodas contain elevated levels of carbon dioxide under pressure. The solution becomes supersaturated with carbon dioxide when the bottle is opened, and the pressure is released.
Nucleation: The Key to Bubble Formation
The activation energy for bubble nucleation (formation of bubbles) depends on where the bubble forms. It is very high for bubbles that form in the liquid itself (homogeneous nucleation), and much lower if bubble growth occurs within tiny bubbles trapped in some other surface (heterogeneous nucleation). Bubble nucleation and growth in carbonated beverages almost always occur by heterogeneous nucleation: diffusion of carbon dioxide into pre-existing bubbles within the beverage. When dissolved gas diffuses into bubbles that already exist in a liquid, it is called Type IV bubble nucleation.
Read also: Exploding Soda Geyser
When the pressure is released from a soda bottle upon opening it, dissolved carbon dioxide can escape into any tiny bubble located within the beverage. These ready-made bubbles (which are nucleation sites) exist in things such as tiny fibers or non-wettable crevices on the sides of the bottle. Because there usually are very few such pre-existing bubbles, the degassing process is slow. The physical characteristics of Mentos (surface roughness) have the effect of drastically reducing the activation energy for carbon dioxide bubble formation so that the nucleation rate becomes exceedingly high. The nucleation reaction can start with any heterogeneous surface, such as rock salt, but Mentos have been found to work better than most.
Mentos: The Perfect Nucleation Site
Mentos candies contain millions of cavities, roughly 1-3 μm in size, that remain unwetted when added to a soda. Because of this, the addition of Mentos candies to a carbonated beverage provides enormous numbers of pre-existing bubbles into which dissolved carbon dioxide can escape. Thus, adding Mentos candies to a carbonated beverage introduces millions of nucleation sites into the drink, which allows for degassing that is rapid enough to support a jet of foam out of a bottle. Pre-existing bubbles provide a way for the reaction to occur without requiring bubbles to form within the liquid itself (homogeneous nucleation).
Diet Coke vs. Regular Coke: Why Diet Coke Works Best
While Diet Coke and Mentos are the most common way to make a soda geyser, they are not the only options. Many consider Diet Coke to be the optimal option. Some suggest that aspartame in diet drinks lowers the surface tension in the water and causes a bigger reaction, but that caffeine does not accelerate the process. However, experiments have shown that some dissolved solids that increase the surface tension of water (such as sugars) also increase fountain heights. Furthermore, it has also been demonstrated that addition of certain concentrations of alcohol (which lowers surface tension) to carbonated beverages decreases fountain heights. These results suggest that additives serve to enhance geyser heights not by decreasing surface tension, but rather by some other mechanism. Additional explanations for why diet sodas outperform regular sodas in this experiment have been proposed.
Regular Coke contains sugar, while Diet Coke uses an artificial sweetener called aspartame. Aspartame, the sweetener used in Diet Coke, facilitates the formation of bubbles more easily when Mentos are added. The sugar in regular Coke can make the soda more viscous (thicker), which slows down the formation of bubbles.
Factors Influencing the Eruption
Several factors can influence the height and intensity of the Diet Coke and Mentos geyser:
Read also: Mentos and Soda: What Happens?
- Roughness of the Candy: A rougher candy surface translates to more places for bubbles to grow, or more nucleation sites.
- Rate at Which the Candy Sinks: 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.
- Temperature: Temperature is an important role in the reaction. The warmer the liquid, the less gas can be dissolved in that liquid. The colder the liquid, the more gas can be dissolved in that liquid. This is because as the liquid is heated, the gas within that liquid is also heated, causing the gas molecules to move faster and faster. As the molecules move faster, they diffuse out of the liquid, leaving less gas dissolved in that liquid. In colder liquids the gas molecules move very slowly, causing them to diffuse out of the solution much more slowly. More gas tends to stay in solution when the liquid is cold.
- Surface Tension of the Soda Other factors that affect the growth rate or total number of carbon dioxide bubbles also changed the geyser's height, such as temperature and the original 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.
Conducting Your Own Experiment
The Diet Coke and Mentos experiment 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?
Here are some experiments you can try:
- 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?
Measuring the Height of the Geyser
To make any of these tests meaningful, you need to find a way to measure the height of the eruption. A friend or parent with a video camera is a great way to watch and document the results of your experiment, but you’ll also need some specific measurements or data. Try placing the soda bottle next to the wall of a brick building (after getting permission from the building’s owner). Measure the height of the geyser by counting the number of bricks that are wet once the geyser stops. If you want a more specific measurement, use chalk to mark off 1-foot increments on the brick wall before you drop the Mentos into the bottle of soda. Make comparisons, create a chart with your data, and draw some conclusions.
Measuring the Volume of the Geyser
If you want to examine the volume of the geyser instead of the height, make note of the volume of a full bottle of soda before you drop the Mentos into it. (Okay, it’s a trick question because a 2-liter bottle of soda holds . . . 2 liters!) Once the geyser stops, pour out the remaining contents of the bottle and measure how much liquid is left. You could use a beaker or a graduated cylinder to measure the remaining liquid in milliliters. Remember that 1 liter is equivalent to 1000 mL. Subtract the remaining amount of liquid from the original volume of the bottle to calculate the volume of the geyser.
Determining the Optimal Number of Mentos
This has to be the number one question everyone asks about this experiment. What is the best number of Mentos to use to make the highest-shooting geyser? Be sure that the soda bottles are all the same brand and type. Line up a row of ten 2-liter bottles against a brick wall (see “Measuring the Height of the Geyser”). Each bottle will receive a different number of Mentos. Drop one Mentos into the first bottle and record the height by counting the wet bricks (or set up your own scale behind each soda bottle). Of course, this could go on forever, but you’ll start to see a trend in your data that shows the maximum height of the geyser for a certain number of Mentos. Many soda geyser-ologists believe that seven Mentos produce the highest-shooting geyser. Using any more than seven Mentos is just a waste, according to these soda-soaked science enthusiasts.
Read also: Mentos and Soda Reaction
Comparing Soda Brands
You guessed it . . . it’s time to put your favorite soda to the test. Does one brand produce higher-flying geysers? How does generic soda stack up against the big name brands? Use your data from the previous test to determine the standard number of Mentos to use for this test. The only variable you’ll change in this test is the brand of soda while everything else remains the same (the number of Mentos and the amount of soda). Again, make sure all of the soda is at the same temperature because temperature plays an important role in the reaction.
Investigating Temperature's Effect
Just think . . . What is the effect of temperature on the height of the geyser? Does warm soda shoot up higher than cold soda? The key is to keep every launch fair and to make sure the only variable is the temperature of the soda. To enforce the fairness factor, you must stick with one brand of soda for the entire test. Let’s use Diet Coke in this example. You’ll want to purchase three bottles of Diet Coke and two rolls of Mentos. You’re going to set up three tests-warm soda, room temperature soda, and cold soda. Place one bottle of Diet Coke in the refrigerator and let it sit overnight. Place the second bottle in a place where it can reach room temperature overnight. There are two safe ways to warm the other bottle of soda. The simplest method is to let the unopened bottle sit in the sun for several hours. You can also place the bottle of unopened soda in a bucket of warm water. It’s time to return to your launching site. Check to make sure your measuring scale is in place (counting bricks or using an alternative scale against the wall). Let’s start with the bottle of cold Diet Coke. Open the bottle and dip the thermometer down into the soda. Record the temperature. Load seven Mentos into your paper roll and drop them into the soda. Immediately record the data for the cold soda test. Repeat the same procedure for the bottle of soda at room temperature and for the bottle of warm soda. No matter which brand of soda you tested, the warm bottle probably produced the highest-shooting geyser. Warm soda tends to fizz much more than cold soda.
Safety Considerations
While the Diet Coke and Mentos experiment is generally safe, it's essential to take precautions:
- Eye Protection: Always wear eye protection (safety goggles or glasses) to prevent soda from splashing into your eyes.
- Outdoor Area: Perform the experiment in an outdoor area, away from buildings and other structures.
- Clothing: Wear clothes that you wouldn't mind getting splashed with soda.
- Avoid Consumption: Do not attempt to drink the soda after the Mentos have been added.
Dispelling Myths
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. The MythBusters showed that your stomach won’t explode, but it still wouldn’t be a lot of fun. Do not, repeat, do not be stupid and test the limits of your stomach.
From Simple Stunt to Science Fair Project
Simply dropping Mentos into a bottle of soda to make a geyser isn’t really science-it’s just a fun trick to do in the backyard. The original reason I invented the Geyser Tube toy was to find a way to standardize the actual drop of the Mentos. The biggest challenge in the Mentos Geyser experiment is finding a consistent way to drop the Mentos into the soda every time. The winning Geyser Tube design was a clear plastic tube with a special fitting that twisted onto any soda bottle. The trigger pin at the bottom of the tube prevented the Mentos from falling into the bottle until you pulled the string attached to the pin. The moment the pin was pulled, a slider ring resting above the pin fell into place and covered the holes where the trigger pin once was, and the Mentos dropped into the soda. Fortunately, the maker of Mentos (Perfetti Van Melle) also liked the design, and we launched the Mentos Geyser Tube toy at the New York Toy Fair in February 2007.
After completing all of these tests, you’ve become somewhat of a Mentos Geyser expert who has the research to support the answer to the question, “How can you make the highest-shooting Mentos geyser?” Each test isolated an independent variable, and combining all of the information you discovered into one launch is a great way to wrap up your science fair project. You might ask yourself, “Can I use the Mentos Geyser for my science fair project?” The answer is YES, but you’ll need to learn how to turn a cool science activity into a real science experiment. To get the best results in a science experiment, you need to standardize the test conditions as much as possible.
The Mentos Geyser: A Versatile Tool for Education
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. 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 force exerted by the soda stream can be harnessed to do work. One enterprising group made a mentos and diet coke-powered rocket.