The seemingly simple act of dropping Mentos into Diet Coke results in a spectacular geyser, a phenomenon that has captivated audiences and inspired countless educational demonstrations. While often perceived as a chemical reaction, the eruption is primarily a physical process involving the rapid release of dissolved carbon dioxide.
A Brief History of Soda Geysers
The creation of soda geysers isn't new. As early as the 1910s, Wint-O-Green Life Savers were used. These candies were threaded onto a pipe cleaner and dropped into a soft drink to create a geyser. However, a change in the size of Wintergreen Lifesavers at the end of the 1990s made them unsuitable for fitting into soda bottles. The Diet Coke and Mentos experiment gained prominence through demonstrations on television shows like the Late Show with David Letterman in 1999, performed by Lee Marek and "Marek's Kid Scientists," and later on KUSA-TV in Denver, Colorado, in 2002 by Steve Spangler. The experiment truly became an internet sensation in September 2005.
The Eruption: A Physical Reaction
The eruption is caused by a physical reaction, not a chemical one. Carbonated sodas contain high levels of carbon dioxide under pressure. When the bottle is opened, the pressure is released, and the solution becomes supersaturated with carbon dioxide. 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.
Nucleation: The Key to Bubble Formation
Bubble nucleation, the formation of bubbles, is central to understanding the geyser. The activation energy for bubble nucleation 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). In carbonated beverages, bubble nucleation and growth almost always occur by heterogeneous nucleation: diffusion of carbon dioxide into pre-existing bubbles within the beverage. This is known as Type IV bubble nucleation, where dissolved gas diffuses into bubbles that already exist in a liquid.
When pressure is released from a soda bottle upon opening, dissolved carbon dioxide escapes into any tiny bubble within the beverage. These nucleation sites exist in things such as tiny fibers or non-wettable crevices on the sides of the bottle. Because there are usually very few such pre-existing bubbles, the degassing process is slow.
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Mentos: The Perfect Nucleation Site
Mentos candies contain millions of cavities, roughly 1-3 μm in size, that remain unwetted when added to soda. 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). 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.
Factors Influencing the Geyser's Height
Several factors can influence the height and intensity of the Diet Coke and Mentos geyser:
Surface Tension
Tonya Coffey, a physicist at Appalachian State University, suggested 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. Lower surface tension facilitates bubble growth and expansion, potentially leading to a more vigorous eruption. 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.
Soda Type
Diet sodas, particularly Diet Coke, are often preferred due to their composition. Diet Coke and Coke Zero tend to go a bit higher than regular soda, because they have a little more carbonation and the sweeteners help make the reaction a little bigger. Most importantly, Coke Zero and Diet Coke aren’t sugary and sticky. Additional explanations for why diet sodas outperform regular sodas in this experiment have been proposed.
Candy Properties
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. 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. A rougher candy surface translates to more places for bubbles to grow, or more nucleation sites.
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Temperature
Warm soda tends to fizz much more than cold soda. 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. This is why at the bottling plant CO2 is pumped into the cans or bottles when the fluid is just above freezing-around 35 degrees Fahrenheit.
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. It’s important that all Mentos must be added to the drink simultaneously, giving each of them equal time to create an effect.
Turning a Demonstration into an Experiment
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. To get the best results in a science experiment, you need to standardize the test conditions as much as possible. The biggest challenge in the Mentos Geyser experiment is finding a consistent way to drop the Mentos into the soda every time.
Safety Precautions
Safety should always be a priority when conducting this experiment. It is advised to perform the experiment outdoors to avoid sticky messes and accidents. It's best to set up on a flat surface with plenty of space around you. Eye protection is important to guard against unexpected soda sprays, and the area should be clear to prevent slips from spilled liquids. It's also a good idea to wear appropriate clothing or an apron.
Real-World Applications and Educational Value
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.
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Urban Legends and Misconceptions
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.