The Mentos and Coke experiment is a classic demonstration that has captured the curiosity of people of all ages. When Mentos mints are dropped into a bottle of carbonated soda, a dramatic geyser erupts. But what causes this seemingly simple reaction to produce such a spectacular result? This article delves into the science behind the Mentos and Coke experiment, exploring the physical and chemical factors that contribute to this impressive display.
A Brief History of Soda Geysers
The phenomenon of creating soda geysers isn't new. As early as the 1910s, Wint-O-Green Life Savers were used to create similar reactions. People would thread the candies onto a pipe cleaner and drop them into a soft drink to produce a geyser. However, the size of Wintergreen Lifesavers was increased at the end of the 1990s, which meant they no longer fit in the mouth of soda bottles.
The Diet Coke and Mentos experiment gained widespread popularity in the late 1990s and early 2000s. Lee Marek and "Marek's Kid Scientists" demonstrated the reaction on the Late Show with David Letterman in 1999. In March 2002, Steve Spangler, a science educator, performed the demonstration on KUSA-TV in Denver, Colorado. By September 2005, the Diet Coke and Mentos geyser experiment had become an internet sensation.
The Science: A Physical Reaction, Not Chemical
The eruption caused by mixing Mentos and Coke is a physical reaction, not a chemical one. Carbonated sodas contain high levels of carbon dioxide dissolved under pressure. When the bottle is opened, the pressure is released, and the solution becomes supersaturated with carbon dioxide. The rapid conversion of dissolved carbon dioxide to gaseous carbon dioxide forms expanding gas bubbles, which forces the beverage out of the container.
Bubble Nucleation
Bubble nucleation, or the formation of bubbles, plays a crucial role in this reaction. The activation energy required 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 on another surface (heterogeneous nucleation).
Read also: Exploding Soda Geyser
In carbonated beverages, bubble nucleation and growth almost always occur through heterogeneous nucleation. When the pressure is released from a soda bottle, dissolved carbon dioxide can escape into any tiny bubble located within the beverage. These ready-made bubbles (nucleation sites) exist in things like tiny fibers or non-wettable crevices on the sides of the bottle.
The Role of Mentos
Mentos candies contain millions of microscopic cavities, roughly 1-3 μm in size, that remain unwetted when added to a soda. The addition of Mentos candies to a carbonated beverage provides enormous numbers of pre-existing bubbles into which dissolved carbon dioxide can escape. This introduces millions of nucleation sites into the drink, allowing for rapid degassing that supports a jet of foam out of a bottle.
The physical characteristics of Mentos, particularly their surface roughness, drastically reduce the activation energy for carbon dioxide bubble formation, resulting in an exceedingly high nucleation rate. While the nucleation reaction can start with any heterogeneous surface, such as rock salt, Mentos have been found to work better than most.
Factors Affecting the Reaction
Several factors can influence the height and intensity of the soda geyser.
Type of Soda
Diet Coke is often considered the optimal choice for this experiment, although many carbonated beverages can be used. Some suggest that aspartame in diet drinks lowers the surface tension in the water, leading to a bigger reaction. However, experiments have shown that dissolved solids, such as sugars, that increase the surface tension of water can also increase fountain heights. Furthermore, adding certain concentrations of alcohol, which lowers surface tension, to carbonated beverages decreases fountain heights. These results suggest that additives enhance geyser heights through a mechanism other than decreasing surface tension.
Read also: Mentos and Soda: What Happens?
Surface Tension
Surfactants are compounds that lower the surface tension between two liquids, a liquid and a solid, or between a gas and a liquid. Mentos act as surfactants, reducing 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.
Roughness of the Candy
A rougher candy surface translates to more places for bubbles to grow, or more nucleation sites. The candies are covered in bumpy craters, which increases the total surface area. The surface of a Mentos is sprayed with over 40 microscopic layers of liquid sugar.
Temperature
Higher ambient temperature can expedite the release of the dissolved gas.
Pressure
Pressure is inversely proportional to the rate of reaction. When the bottle is still unopened, the interior is pressurized. Opening it rapidly equalizes the pressure.
Diet Coke vs. Regular Coke
Diet Coke and Coke Zero tend to produce a higher geyser than regular soda. This is attributed to the absence of sugar, which makes the soda mixture less viscous. The presence of sweeteners, like aspartame, further lowers the surface tension, leading to a more rapid release of carbon dioxide gas and a faster nucleation process. Soda mixtures with more sugar in them are more viscous, making their reactions less powerful.
Read also: Mentos and Soda Reaction
Step-by-Step Guide to the Mentos and Coke Experiment
To conduct the Mentos and Coke experiment, you will need:
- A 2-liter bottle of Diet Coke (or other carbonated beverage)
- A roll of Mentos candies (at least five)
- A safe outdoor area
Procedure:
- Find a location where the floor and a wall meet.
- Use duct tape to attach the metric tape measure to the wall around 25 cm from the floor.
- Carefully open the bottle of Diet Coke.
- Stack the Mentos candies (approx.
- Drop all the Mentos into the bottle simultaneously.
- Step back quickly and observe the eruption.
Variations on the Experiment
There are many ways to modify the Mentos and Coke experiment to explore different scientific principles.
Varying the Opening
Vary the opening that the soda erupts from. For example, cut the bottle top to increase the diameter an inch or more. Alternatively, dispense the soda into a vase or pitcher. Or, attach plastic piping or tubing to extend the opening’s neck.
Different Candies and Liquids
Test different kinds of carbonated beverages, different kinds of candies with different shapes and textures, or using other things to start the reaction, like rock salt, pennies, or dice.
Warm vs. Cold Soda
Compare warm versus cold Diet Coke. Does temperature affect the eruption height?
Educational Applications
The Mentos and Coke experiment is a great science demonstration for students studying gases, thermodynamics, fluid dynamics, surface science, and the physics of explosions. 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.
Safety Considerations
While the Mentos and Coke experiment is generally safe, it is important to take precautions:
- Perform the experiment outdoors to avoid making a mess indoors.
- Wear eye protection to prevent soda from getting into your eyes.
- Do not attempt to consume large amounts of soda and Mentos simultaneously, as this could cause discomfort.
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 of 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.
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