The Mentos & Diet Coke Craze
© Copyright 2007, American Chemical Society
Canadian GST Reg. No. 127571347
Printed in the USA
Q: I saw a video on the Internet in which this guy drops a bunch of Mentos candies into a 2-L bottle of Diet Coke, and the soda erupts all over the place. It looks like a geyser. I tried it myself, and it really does work. What’s going on?
A: In the past six months, I’ve been asked this question more often than any other in my career... even by people I’ve just met. So I went online to learn all I could, to see what kind of research has been done and to read what various “experts” say on the matter. A Google search of “Diet Coke and Mentos” (quotation marks included) turns up 305,000 links! After reading all 305,000 links (yeah right), I learned that very little has actually
been written to explain the phenomenon and even less has been done to research the question. So I decided to conduct a little back yard research of my own and found some intriguing answers. On the basis of this study and the few authoritative Web sites I could find, here is what I believe to be the explanation: The amount of CO2 one can dissolve in a liter of solution depends directly on the pressure of CO2 pushing down on the solution’s surface. In the bottling plant, the CO2 is pumped in at very high pressures, and there- fore a rather high CO2 concentration is established in the bottles of soda. Once a bottle is opened and the pressure on the soda’s surface drops, it creates what is known as a supersaturated solution—under the diminished pres- sure in the room, the solution is holding more CO2 than it is supposed to be able to hold. This is unstable, but it is only a temporary situation. The effervescing (undissolving and bubbling out of the excess dissolved CO2) begins right away, but it is a rather slow process. A newly opened soda bottle will eventually go completely flat, but it can take several hours.
One thing that makes this effervescing go faster is temperature. The higher the temperature, the more quickly the soda will lose its fizz. With this in mind, you might think that adding an ice cube into a cup of freshly poured, room-temperature soda would cool it down and make the fizzing much slower. But
give it a try, and you’ll discover just the opposite. As soon as the ice cube hits the soda, the effervescing increases dramatically, especially if the ice cube is one right out of the ice cube tray with lots of sharp edges and rough surfaces. This is because of a second factor that can have a huge impact on the rate of effervescing—nucleation sites.
Nucleation site is just a fancy name for “a place to get things started.” Small, uneven surfaces give the CO2 molecules a place to come together and form into bubbles. The bottling companies know about nucleation sites: they make the inside walls of soda cans and bottles extremely smooth and free of any nucleation sites. If these walls were at all rough, the soda would spray out the moment the bottle was opened. In fact, nucleation sites are responsible for the mess that is made by opening a soda bottle right after it has been shaken. The shaking creates millions of microscopic bubbles. These micro-bubbles serve as nucleation sites and cause a quick release of CO2 the moment the can is opened. Because these bubbles are spread throughout the bottle, the quick effervescing of CO2 pushes all of the soda above it out of the bottle. If, however, the shaken bottle is allowed to sit undisturbed for 20–30 seconds, these tiny bubbles have time to float their way to the surface, and the bottle can be opened without the mess!
Now, take a close look at a Mentos mint. Its surface is rather dull, not shiny like some candies and, though it feels very smooth to the touch, if we could look at it under high enough magnification, it would show a very rough, pitted surface, with millions of
exposed bumps and edges—in short, a nucleation bonanza! And as the Mentos sink to the bottom and degas the soda, the out-rushing gas brings a fresh plume of gassy soda in contact with the Mentos, producing more gas, and so on. The explanation offered above is a purely physical one. Some Web site authors, how- ever, take a more chemical approach. They argue that the geyser effect is the result of the chemical make-up of Mentos—specifically,
the ingredient known as “gum arabic,” which supposedly acts as a sort of catalyst to the effervescing. The idea being that the gum arabic (and also coconut oil in the Mentos) is a nonpolar, oily gum that helps the carbon dioxide bubbles to get free because oil and water don’t mix. The water molecules in the matrix become more attracted to each other because they are repelled by the oil, and so it takes less energy for the carbon dioxide to break free. But consider this: When the mints Mentos are made smooth by rubbing them repeatedly with a wet paper towel, the fountain effect is cut in half. Also, when the Mentos are dissolved in water and the solution is then poured into a freshly opened bottle of
Diet Coke, there is almost no fountain effect at all. Furthermore, the fruit-flavored Mentos have pretty much the same ingredient list as the mint ones, but their surface is much more smooth and shiny, and they produce almost
no fountain effect with Diet Coke. Likewise, for the sugar-free mint Mentos. This all leads me to believe that if the explanation does have a chemical component to it, it is a relatively small one and that the greatest factor influencing the Diet Coke and Mentos geyser is that of a pitted surface rich with nucleation sites.
The fact is, I really do enjoy watching this phenomenon and showing it to my students, but perhaps what I like best about it is how easy and fun it is to explore these issues on your own. The opportunities for creative, well-designed experiments are almost endless.
© Copyright 2007, American Chemical Society
Canadian GST Reg. No. 127571347
Printed in the USA
Q: I saw a video on the Internet in which this guy drops a bunch of Mentos candies into a 2-L bottle of Diet Coke, and the soda erupts all over the place. It looks like a geyser. I tried it myself, and it really does work. What’s going on?
A: In the past six months, I’ve been asked this question more often than any other in my career... even by people I’ve just met. So I went online to learn all I could, to see what kind of research has been done and to read what various “experts” say on the matter. A Google search of “Diet Coke and Mentos” (quotation marks included) turns up 305,000 links! After reading all 305,000 links (yeah right), I learned that very little has actually
been written to explain the phenomenon and even less has been done to research the question. So I decided to conduct a little back yard research of my own and found some intriguing answers. On the basis of this study and the few authoritative Web sites I could find, here is what I believe to be the explanation: The amount of CO2 one can dissolve in a liter of solution depends directly on the pressure of CO2 pushing down on the solution’s surface. In the bottling plant, the CO2 is pumped in at very high pressures, and there- fore a rather high CO2 concentration is established in the bottles of soda. Once a bottle is opened and the pressure on the soda’s surface drops, it creates what is known as a supersaturated solution—under the diminished pres- sure in the room, the solution is holding more CO2 than it is supposed to be able to hold. This is unstable, but it is only a temporary situation. The effervescing (undissolving and bubbling out of the excess dissolved CO2) begins right away, but it is a rather slow process. A newly opened soda bottle will eventually go completely flat, but it can take several hours.
One thing that makes this effervescing go faster is temperature. The higher the temperature, the more quickly the soda will lose its fizz. With this in mind, you might think that adding an ice cube into a cup of freshly poured, room-temperature soda would cool it down and make the fizzing much slower. But
give it a try, and you’ll discover just the opposite. As soon as the ice cube hits the soda, the effervescing increases dramatically, especially if the ice cube is one right out of the ice cube tray with lots of sharp edges and rough surfaces. This is because of a second factor that can have a huge impact on the rate of effervescing—nucleation sites.
Nucleation site is just a fancy name for “a place to get things started.” Small, uneven surfaces give the CO2 molecules a place to come together and form into bubbles. The bottling companies know about nucleation sites: they make the inside walls of soda cans and bottles extremely smooth and free of any nucleation sites. If these walls were at all rough, the soda would spray out the moment the bottle was opened. In fact, nucleation sites are responsible for the mess that is made by opening a soda bottle right after it has been shaken. The shaking creates millions of microscopic bubbles. These micro-bubbles serve as nucleation sites and cause a quick release of CO2 the moment the can is opened. Because these bubbles are spread throughout the bottle, the quick effervescing of CO2 pushes all of the soda above it out of the bottle. If, however, the shaken bottle is allowed to sit undisturbed for 20–30 seconds, these tiny bubbles have time to float their way to the surface, and the bottle can be opened without the mess!
Now, take a close look at a Mentos mint. Its surface is rather dull, not shiny like some candies and, though it feels very smooth to the touch, if we could look at it under high enough magnification, it would show a very rough, pitted surface, with millions of
exposed bumps and edges—in short, a nucleation bonanza! And as the Mentos sink to the bottom and degas the soda, the out-rushing gas brings a fresh plume of gassy soda in contact with the Mentos, producing more gas, and so on. The explanation offered above is a purely physical one. Some Web site authors, how- ever, take a more chemical approach. They argue that the geyser effect is the result of the chemical make-up of Mentos—specifically,
the ingredient known as “gum arabic,” which supposedly acts as a sort of catalyst to the effervescing. The idea being that the gum arabic (and also coconut oil in the Mentos) is a nonpolar, oily gum that helps the carbon dioxide bubbles to get free because oil and water don’t mix. The water molecules in the matrix become more attracted to each other because they are repelled by the oil, and so it takes less energy for the carbon dioxide to break free. But consider this: When the mints Mentos are made smooth by rubbing them repeatedly with a wet paper towel, the fountain effect is cut in half. Also, when the Mentos are dissolved in water and the solution is then poured into a freshly opened bottle of
Diet Coke, there is almost no fountain effect at all. Furthermore, the fruit-flavored Mentos have pretty much the same ingredient list as the mint ones, but their surface is much more smooth and shiny, and they produce almost
no fountain effect with Diet Coke. Likewise, for the sugar-free mint Mentos. This all leads me to believe that if the explanation does have a chemical component to it, it is a relatively small one and that the greatest factor influencing the Diet Coke and Mentos geyser is that of a pitted surface rich with nucleation sites.
The fact is, I really do enjoy watching this phenomenon and showing it to my students, but perhaps what I like best about it is how easy and fun it is to explore these issues on your own. The opportunities for creative, well-designed experiments are almost endless.
Mythbusters Video - Diet Coke & Mentos
Dr. Coffey - Mentos Diet Coke Geyser
HOW DOES IT WORK?
Here's the question of the day... Why does mixing Mentos with soda produce this incredible eruption? You should know that there is considerable debate over how and why this works. While we offer the most probable explanations below, we also understand and admit that other explanations could be possible... and we welcome your thoughts.
As you probably know, soda pop is basically sugar (or diet sweetener), flavoring, water, and preservatives. The thing that makes soda bubbly is invisible carbon dioxide gas, which is pumped into bottles at the bottling factory using tons of pressure. Until you open the bottle and pour a glass of soda, the gas mostly stays suspended in the liquid and cannot expand to form more bubbles, which gases naturally do.
But there's more... If you shake the bottle and then open it, the gas is released from the protective hold of the water molecules and escapes with a whoosh, taking some of the soda along with it. What other ways can you cause the gas to escape? Just drop something into a glass of soda and notice how bubbles immediately form on the surface of the object. For example, adding salt to soda causes it to foam up because thousands of little bubbles form on the surface of each grain of salt. Many scientists, including Lee Marek, claim that the Mentos phenomenon is a physical reaction, not a chemical one.
Water molecules strongly attract each other, linking together to form a tight mesh around each bubble of carbon dioxide gas in the soda. In order to form a new bubble, or even to expand a bubble that has already formed, water molecules must push away from each other. It takes extra energy to break this "surface tension." In other words, water "resists" the expansion of bubbles in the soda.
When you drop the Mentos into the soda, the gelatin and gum arabic from the dissolving candy break the surface tension. This disrupts the water mesh, so that it takes less work to expand and form new bubbles. Each Mentos candy has thousands of tiny pits all over the surface. These tiny pits are called nucleation sites - perfect places for carbon dioxide bubbles to form. As soon as the Mentos hit the soda, bubbles form all over the surface of the candy. Couple this with the fact that the Mentos candies are heavy and sink to the bottom of the bottle and you've got a double-whammy. When all this gas is released, it literally pushes all of the liquid up and out of the bottle in an incredible soda blast. You can see a similar effect when potatoes or pasta are lowered into a pot of boiling water. The water will sometimes boil over because organic materials that leach out of the cooking potatoes or pasta disrupt the tight mesh of water molecules at the surface of the water, making it easier for bubbles and foam to form.
When a scoop of ice cream is added to root beer, the float foams over for essentially the same reason. The surface tension of the root beer is lowered by gums and proteins from the melting ice cream, and the CO2 bubbles expand and release easily, creating a beautiful foam on top.
Next question... Why should you use diet Coke or diet Pepsi? The simple answer is that diet soda just works better than regular soda. Some people speculate that it has something to do with the artificial sweetener, but the verdict is still out. More importantly, diet soda does not leave a sticky mess to have to clean up. Hey, that's important.
Diet Coke & Mentos Eruption
What's happening?
Although there are a few different theories around about how this experiment works, the most favoured reason is because of the combination of carbon dioxide in the Diet Coke and the little dimples found on Mentos candy pieces.
The thing that makes soda drinks bubbly is the carbon dioxide that is pumped in when they bottle the drink at the factory. It doesn't get released from the liquid until you pour it into a glass and drink it, some also gets released when you open the lid (more if you shake it up beforehand). This means that there is a whole lot of carbon dioxide gas just waiting to escape the liquid in the form of bubbles.
Dropping something into the Diet Coke speeds up this process by both breaking the surface tension of the liquid and also allowing bubbles to form on the surface area of the Mentos. Mentos candy pieces are covered in tiny dimples (a bit like a golf ball), which dramatically increases the surface area and allows a huge amount of bubbles to form.
The experiment works better with Diet Coke than other sodas due to its slightly different ingredients and the fact that it isn't so sticky. I also found that Diet Coke that had been bottled more recently worked better than older bottles that might have lost some of their fizz sitting on shop shelves for too long, just check the bottle for the date.
Why Do Diet Coke and Mentos React?
Combine Diet Coke and Mentos, and the result is explosive—Diet Coke shoots out of the bottle like a miniature, sticky Old Faithful. The reaction is so intense, you can make a rocket propelled by the resulting geyser. But what's the science behind this reaction?
In June 2008, Dr. Tonya Coffey of Appalachian State University and her physics students published a paper on the phenomenon in the American Journal of Physics. They were inspired by a 2006 MythBusters episode that, according to the paper, "did a wonderful job of identifying the basic ingredients in this reaction ... [but] did not sufficiently explain why those ingredients affect the explosion, nor did they provide direct proof of the roughness of the Mentos—a tall order for an hour-long television program." Coffey and her students decided to dig deeper.
IT'S ALL ABOUT TEXTURE
Coffey and company discovered that the ingredients in the Mentos and Diet Coke and, more importantly, the structure of the Mentos, allow carbon dioxide bubbles to form extremely rapidly. When this happens fast enough, you get a nice Diet Coke fountain. (It’s not just Diet Coke and Mentos that react; other carbonated beverages will also readily respond to the addition of Mentos.)
Each Mentos candy has thousands of small pores on its surface which disrupt the polar attractions between water molecules, creating thousands of ideal nucleation sites for the gas molecules to congregate. In non-science speak, this porous surface creates a lot of bubble growth sites, allowing the carbon dioxide bubbles to rapidly form on the surface of the Mentos. (If you use a smooth surfaced Mentos candy, you won’t get nearly same the reaction.) The buoyancy of the bubbles and their growth will eventually cause the bubbles to leave the nucleation site and rise to the surface of the soda. Bubbles will continue to form on the porous surface and the process will repeat, creating a nice, foamy geyser.
In addition to that, the gum arabic and gelatin ingredients of the Mentos, combined with the potassium benzoate, sugar or (potentially) aspartame in diet sodas, also help in this process. In these cases, the ingredients end up lowering the surface tension of the liquid, allowing for even more rapid bubble growth on the porous surface of the Mentos—higher surface tension would make it a more difficult environment for bubbles to form. (Compounds like gum arabic that lower surface tension are called “surfactants”).
Diet sodas produce a bigger reaction than non-diet sodas because aspartame lowers the surface tension of the liquid much more than sugar or corn syrup will. You can also increase the effect by adding more surfactants to the soda when you add the Mentos, like adding a mixture of dishwasher soap and water.
SIZE MATTERS
Another factor that contributes to the size of the geyser is how rapidly the object causing the foaming sinks in the soda. The faster it sinks, the faster the reaction can happen, and a faster reaction creates a bigger geyser; a slower reaction may release the same amount of foam overall, but will also create a much smaller geyser. This is another reason Mentos works so much better than other similar confectioneries: The candies are fairly dense objects and tend to sink rapidly in the soda. If you crush the Mentos, so it doesn’t sink much at all, you won’t get a very dramatic reaction.
The temperature of the soda also factors into geyser size. Gases are less soluble in liquids with a higher temperature, so the warmer your soda is, the bigger your Mentos-induced geyser will be. This is because the gases want to escape the liquid, so when you drop the Mentos in, the reaction happens faster.
WHAT DOESN'T WORK
While caffeine is often cited as something that will increase the explosive reaction with the soda, this is not actually the case, at least not given the relatively small amount of caffeine found in the typical 2-liter bottle of soda generally used for these sorts of Diet Coke and Mentos reactions.
You’ll also sometimes read that the acidity of the soda is a major factor in the resulting geyser. This is not the case either. In fact, the level of acidity in the Coke before and after the Mentos geyser does not change, negating the possibility of an acid-based reaction—though you can make such an acid-based reaction using baking soda.
The Science of Coke and Mentos
How Does This Work?
Why do Diet Coke & Mentos and Coke Zero & Mentos create such exciting geysers?
It’s mostly due to a process called nucleation, where the carbon dioxide in the soda is attracted to the Mentos (they are awfully cute). That creates so much pressure that the soda goes flying. We then built nozzles that make the opening smaller and that makes the geysers go even higher.
So what is nucleation about and why do Mentos release all this pressure so spectacularly? Read on…
Making Lots of Bubbles
After a lot of debate, scientists are now saying that the primary cause of Coke & Mentos geysers is a physical reaction, not a chemical reaction. Their explanation is this process called nucleation.
All the carbon dioxide in the soda – all that fizz – is squeezed into the liquid and looking for a way out. It’s drawn to any tiny bumps that it can grab onto. Those tiny bumps are called nucleation sites: places the gas can grab onto and start forming bubbles.
Nucleation sites can be scratches on a glass, the ridges of your finger, or even specks of dust – anywhere that there is a high surface area in a very small volume.
The surface of a Mentos is sprayed with over 40 microscopic layers of liquid sugar. That makes it not only sweet but also covered with lots and lots of nucleation sites.
In other words, there are so many microscopic nooks and crannies on the surface of a Mentos that an incredible number of bubbles will form around the Mentos when you drop it into a bottle of soda.
Since the Mentos are also heavy enough to sink, they react with the soda all the way to the bottom. The escaping bubbles quickly turn into a raging foam, and the pressure builds dramatically.
All that pressure has got to go somewhere, and before you know it, you've got a big geyser happening!
Try This at Home!
This is a great thing to try yourself – if you’re careful. Here’s how. Make sure you wear your goggles and lab coats, so that you are not only protected, you also look cool as you run away from the flying soda.
The Big Question
What happens if you drink soda and then eat Mentos? Well, a lot of the fizz goes away as you drink. Then when bubbles are released in your stomach, your stomach can expand a bit. And your stomach also has ways of, umm, releasing excess gas… 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. Don't even think about it.
What Else Works?
We've tested everything from Mountain Dew & Lifesavers to Moxie & M&M's. Shockingly enough, dropping just about anything into just about any kind of soda creates at least a little fizz. Even some pocket change made a bottle of root beer bubble up a bit. But the combination of Coke Zero & Mentos is particularly potent!
Does It Have to be Diet?
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. Every time we set off a big geyser display, we get soaked to the skin, so it’s nice not to get covered in sugary goo.
Interesting Links
Thanks to Tonya Coffey at Appalachian State University in North Carolina for publishing a scientific study of Coke & Mentos in the American Journal of Physics. New Scientist has this great summary of the explanation.
Fun with Nucleation
You can learn more about nucleation sites in action if you coat the inside of a small glass with vegetable oil. Move the glass around to get a nice smooth coating of oil and then pour in some soda. What happens? No fizz. Why? No nucleation sites. Now sprinkle in some granulated sugar. What happens? Lots of fizz! Why? Lots of nucleation sites!
Science of Mentos-Diet Coke explosions explained
The startling reaction between Diet Coke and Mentos sweets, made famous in thousands of YouTube videos, finally has a scientific explanation. A study in the US has identified the prime factors that drive the fizzy plumes from Coke bottles: the roughness of the sweet and how fast it plummets to the bottle's base.
"If you drop a pack of Mentos into a bottle of Diet Coke, you get this huge fountain of spray and Diet Coke foam coming out," says Tonya Coffey, a physicist at Appalachian State University in Boone, North Carolina. "This was a good project for my students to study because there was still some mystery to it."
When mint or fruit Mentos are dropped into a fresh bottle of Diet Coke, a jet of Coke whooshes out of the bottle's mouth and can reach a height of 10 metres. Theories abound as to why this happens, with some bloggers speculating that it is an acid-base reaction because Coke is acidic.
Experiments in a 2006 edition of the Discovery Channel programmeMythbusters suggested the chemicals responsible for the reaction are gum arabic and gelatine in the sweets, and caffeine, potassium benzoate and aspartame in the Coke. But there have been no rigorous scientific studies of the reaction until now.
Fizzy liquids
To find out more, Coffey and a team of students tested the reactions between Diet Coke and fruit Mentos, mint Mentos, and various ingredients such as other mints, dish-washing detergent, table salt and sand. They also compared reactions using other fizzy liquids such as caffeine-free and sugary colas, as well as soda water and tonic water.
All the reactions took place in a bottle angled at 10° off vertical and the fountain trajectories were recorded on video. The team also investigated the total mass lost in the fountain and the influence of the sweet's surface roughness.
The results showed that Diet Coke created the most spectacular explosions with either fruit or mint Mentos, the fountains travelling a horizontal distance of up to 7 metres.
But caffeine-free Diet Coke did just as well, suggesting that caffeine does not accelerate the reaction, at least at the normal levels in the drink.
Measurements of the pH of the Coke before and after the experiments showed that its acidity did not change, ruling out the idea that a simple acid-base reaction drives the fountains.
Instead, the vigour of the jets depends on various factors that affect the growth rate of carbon dioxide bubbles.
The rough, dimply surfaces of Mentos encourage bubble growth because they efficiently disrupt the polar attractions between water molecules, creating bubble growth sites.
Rough candy
"Water molecules like to be next to other water molecules, so basically anything that you drop into the soda that disrupts the network of water molecules can act as a growth site for bubbles," Coffey told New Scientist. "And if you have rough candy with a high ratio of surface area to volume, then there's more places for the bubbles to go."
Low surface tension also helps bubbles grow quickly. Measurements showed that the surface tension in water containing the sweetener aspartame is lower than in sugary water, explaining why Diet Coke creates more dramatic fountains than sugary Coke.
Another factor is that the coatings of Mentos contain gum arabic, a surfactant that further reduces surface tension in the liquid. Rough-surfaced mints without the surfactant did not create such large fountains.
Mentos are also fairly dense and sink rapidly, quickly creating bubbles that seed further bubbles as they rise. Crushed Mentos that fell more slowly created puny fountains that only travelled about 30 centimetres.
"Middle-school teachers are getting their students out onto the baseball field next to their school and doing this reaction, and their students love it," says Coffey. "It's a great way to get students excited about science and learn something new."