The Diet Coke and Mentos experiment is a classic demonstration that has captivated audiences for years. It involves the seemingly simple act of dropping Mentos mints into a bottle of Diet Coke, resulting in a dramatic soda geyser. This phenomenon, often perceived as a chemical reaction, is actually a fascinating example of a physical reaction.
The Soda Geyser Phenomenon: A Brief History
The creation of soda geysers isn't new. As far back as the 1910s, people used Wint-O-Green Life Savers to create similar eruptions. These candies were threaded onto a pipe cleaner and dropped into soft drinks to produce a geyser effect. However, the size of Wintergreen Life Savers increased at the end of the 1990s which meant that they no longer fit in the mouth of soda bottles.
The Diet Coke and Mentos experiment gained prominence through appearances on television. Lee Marek and "Marek's Kid Scientists" showcased it on the Late Show with David Letterman in 1999. Later, in March 2002, Steve Spangler, a science educator, demonstrated the experiment on KUSA-TV in Denver, Colorado. By September 2005, the Diet Coke and Mentos geyser experiment had become an internet sensation.
The Science Behind the Eruption: A Physical Reaction
The eruption that occurs in the Diet Coke and Mentos experiment is not a chemical reaction, but a physical one. Carbonated sodas, like Diet Coke, contain high levels of carbon dioxide under pressure. When the bottle is opened, this pressure is released, making the solution 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.
Bubble Nucleation: The Key to the Geyser
Bubble nucleation, or the formation of bubbles, is crucial to understanding the experiment. 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 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 process is known as Type IV bubble nucleation, where dissolved gas diffuses into bubbles that already exist in a liquid.
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When a soda bottle is opened, dissolved carbon dioxide can escape into any tiny bubble located within the beverage. These ready-made bubbles, which act as 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.
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. By introducing millions of nucleation sites into the drink, Mentos allow for degassing that is rapid enough to support 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, leading to 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 Influencing the Eruption: Variables at Play
Several factors can influence the magnitude of the Diet Coke and Mentos eruption.
Diet Soda vs. Regular Soda
Many consider Diet Coke to be the optimal option for creating a soda geyser. 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. 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.
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Surface Tension
The Mentos candy gelatin and gum arabic create an energy that breaks the surface tension of the soda. The pits on the candy coating act as conduits for carbon dioxide bubbles that form immediately when the candy hits the soda, increasing its fizziness.
Temperature
Temperature plays an important role in the reaction. 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
The number of Mentos used also affects the height of the geyser. 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.
Type of Soda
Different brands of soda can produce varying geyser heights. Generic soda may stack up differently against the big name brands. To ensure a fair comparison, it is important to keep the number of Mentos and the amount of soda consistent.
The Role of Carbon Dioxide
Carbon dioxide is the chemical compound that consists of two oxygen atoms bonded to a carbon atom. Carbon dioxide creates the bubbles in soda. The candies simply catalyze the release of gas from the Coke bottle. 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.
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Conducting Your Own Experiment: A Step-by-Step Guide
To conduct your own Diet Coke and Mentos experiment, you will need:
- A 2-liter bottle of Diet Coke
- A roll of Mentos candies
- An outdoor area
- Eye protection (safety goggles or glasses)
- Index cards
- Tape
Procedure:
- Find an outside area that is easy to clean up and can get a little messy.
- Place a Diet Coke bottle in an outdoor area, at least two meters from any buildings or anything hanging above the area, such as eaves, overhangs or wires. Make sure that the bottle is on a level surface and stably standing straight.
- Make a Mentos cartridge to hold the candies for you before you drop them into the Diet Coke bottle by rolling an index card into a tube, slightly larger than the diameter of a Mentos candy. Tape the tube together on the side.
- Carefully remove the cap from the bottle and place the flat index card on top, covering the hole.
- Add four whole Mentos candies to your cartridge, put on your eye protection, and start the video camera.
- Place your full cartridge on top of the flat index card. Line up where the opening of the bottle is with the opening of your cartridge. Quickly pull out the flat index card, releasing the Mentos candies into the bottle. Then step back without tipping the bottle over or disturbing the reaction.
Variations and Further Explorations
There are numerous ways to modify the Diet Coke and Mentos experiment to explore different scientific principles. Here are a few ideas:
- Crushed vs. Whole Mentos: Compare the eruption caused by whole Mentos candies with that of crushed Mentos. The crushed Mentos candies are not as dense as the whole ones, which causes them to sink more slowly, creating a relatively small cola fountain, which should also leave more liquid in the bottle than the larger eruption with whole Mentos candies did.
- Different Liquids: Test different types of soda, such as regular Coke, Sprite, or other carbonated beverages. Which beverages cause the largest and smallest fountains? Why do you think this is?
- Different Candies: Experiment with different kinds of candies with different shapes and textures.
- Temperature Variation: Compare warm versus cold Diet Coke. Does temperature affect the eruption height?
- Varying the Opening: Repeat the process, varying 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.
Addressing Common Misconceptions
One common misconception is 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.