The Diet Coke and Mentos experiment is a classic demonstration that involves dropping Mentos mints into a bottle of Diet Coke, resulting in a rapid and impressive eruption of soda. This phenomenon, popularized through numerous online videos, is a captivating example of a physical reaction.
The Soda Geyser Phenomenon
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. This experiment has a history, with early versions using Wint-O-Green Life Savers in the 1910s 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 manufacturer of Wintergreen Lifesavers increased the size of the mints at the end of the 1990s, and they no longer fit in the mouth of soda bottles.
The Diet Coke and Mentos experiment gained significant attention, with demonstrations performed on television shows like the Late Show with David Letterman in 1999 by Lee Marek and "Marek's Kid Scientists," and on KUSA-TV in Denver, Colorado, in March 2002 by Steve Spangler, a science educator. The experiment became an internet sensation in September 2005.
Unveiling the Science Behind the Eruption
The eruption is caused by a physical reaction, rather than any chemical reaction. 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. The key to understanding this phenomenon lies in the concepts of carbonation, nucleation, and surface area.
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. When pressure is released from a soda bottle upon opening it, dissolved carbon dioxide can escape into any tiny bubble located within the beverage.
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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. 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.
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). 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.
The Role of Mentos
Mentos candies have a unique surface texture. Although they appear smooth to the naked eye, they are covered in tiny pores and irregularities. These imperfections act as nucleation sites, providing locations where carbon dioxide molecules can easily come out of solution and form bubbles.
Why Diet Coke?
Diet Coke is often preferred in this experiment due to the presence of aspartame, an artificial sweetener. 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. Aspartame, the sweetener used in Diet Coke, facilitates the formation of bubbles more easily when Mentos are added. The Mentos create tiny bumps on the surface, which act as places for the carbon dioxide to escape quickly. 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. The sugar in regular Coke can make the soda more viscous (thicker), which slows down the formation of bubbles.
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Step-by-Step Explanation
- Carbon Dioxide in Soda: Carbonated drinks like Diet Coke contain dissolved carbon dioxide gas under pressure. This gas is what gives soda its fizz.
- Opening the Bottle: When you open a bottle of Diet Coke, you release the pressure, allowing the carbon dioxide to escape from the solution.
- Mentos Introduction: When Mentos are dropped into the Diet Coke, 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.
- Rapid Bubble Formation: The carbon dioxide molecules attach to the surfaces of the Mentos. 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!
- The Geyser Effect: The rapid formation of bubbles creates a large volume of gas that quickly pushes the liquid out of the bottle, resulting in the characteristic geyser effect.
Factors Influencing the Eruption
Several factors can influence the height and intensity of the Diet Coke and Mentos eruption:
- Number of Mentos: The more Mentos added, the more nucleation sites are available, leading to a larger eruption. However, the number of Mentos that will make a difference is limited.
- Type of Soda: Diet Coke generally produces a more significant eruption than regular Coke due to the presence of aspartame and the lower viscosity of the liquid.
- Temperature: The temperature of the soda can also affect the reaction. Some experiments suggest that warmer soda may produce a slightly larger eruption.
- Surface Area of Mentos: Crushing the Mentos into small pieces before adding them to the soda may affect the reaction.
- Simultaneous Drop: All Mentos must be added to the drink simultaneously, giving each of them equal time to create an effect.
Conducting the Experiment
To conduct the Diet Coke and Mentos experiment, you will need:
- A 2-liter bottle of Diet Coke (or other carbonated beverage)
- A roll of Mentos mints
- A safe outdoor location
Procedure
- Open the Diet Coke bottle and place it on a level surface.
- Stack the Mentos candies (approx.
- Quickly drop the Mentos into the bottle.
- Step back and observe the eruption.
For a 2-liter bottle of Coke, at least five Mentos are good enough.
Tips for a Successful Eruption
- Use fresh Mentos for the best results.
- Ensure the soda is not flat or expired.
- Drop all the Mentos into the bottle at the same time for a more dramatic eruption.
- Conduct the experiment outdoors to avoid making a mess indoors.
Real-World Applications and Further Exploration
The Diet Coke and Mentos experiment is a great way to illustrate scientific principles in an engaging and memorable way. It can be used to teach concepts such as:
- Nucleation: The formation of bubbles on a surface.
- Solubility: The ability of a gas to dissolve in a liquid.
- Physical Reactions: Reactions that do not involve the breaking or forming of chemical bonds.
- Surface Tension: The property of a liquid that causes it to minimize its surface area.
Further Experiments
- Try different types of soda (regular Coke, Sprite, etc.) to see which produces the largest eruption.
- Experiment with different types of Mentos (mint, fruit) or other candies.
- Vary the temperature of the soda to see how it affects the reaction.
- Test other materials with rough surfaces, such as rock salt or sand, to see if they produce a similar effect.
Safety Precautions
- Wear eye protection to prevent soda from splashing into your eyes.
- Conduct the experiment outdoors in a safe area away from people and objects.
- Do not attempt to drink the soda after the experiment, as it may contain dissolved Mentos and a large amount of carbon dioxide.
Addressing Misconceptions
Remember growing up with the advice that eating Mentos while drinking soda can burst a person’s stomach? Actually, it’s not that dangerous because most of the carbonation is released as a person drinks the soda. The pressure is lower and carbon dioxide does not nucleate. 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.
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