There was once that I ate a pack of Mentos after drinking a can of Coca-Cola Light.Suddenly I remembered in horror that I read somewhere before that eating coke and Mentos together would cause stomach to explode. For a moment I indeed had a bubbling sensation in my stomach. Then for the subsequent few minutes I sat there, waiting nervously for something to happen, feeling pretty much doomed. I remembered myself thinking: “Am I going to die from explosion?”
The Diet Coke and Mentos reaction has been a popular demonstration in chemistry and physics classes. It was performed numerous times on television and the Internet. Upon dropping Mentos into a bottle of Diet Coke, a jet of coke spray will shoot out from the mouth of the bottle, producing a ‘geyser’ effect.
So what goes on between these two common food items, which seem to be completely harmless on their own?
Well, the reaction is actually physical rather than chemical in nature. And the process of this reaction is called nucleation.
When Diet Coke is manufactured, a large amount of carbon dioxide gas dissolves in the water due to high pressure inside the bottle. The moment when we open the bottle, the high pressure is released and dissolved carbon dioxide starts to change back into the gaseous form.
CO2 (aq) ——-> CO2 (g) When pressure decreases, CO2 changes from aqueous to gaseous form
This process occurs faster when the dissolved CO2 has a site for this change to occur. These sites are called the nucleation sites. Scratches on glass can be one example of nucleation site, and that’s why we always see bubbles form on the sides of the bottle or glass.
Now, we take a look at the surface of Mentos. Although they may look smooth at first glance, they are actually quite rough. It has numerous small bumpy sites, which act perfectly as nucleation sites. Hence CO2 gas bubbles form very quickly, causing a sudden rise of pressure inside the bottle, hence pushing the liquid out of the mouth of the bottle. That’s why we observe a ‘geyser’ effect.
But one question still remains: why Diet Coke in particular? Can other kinds of soda drink (e.g. Classic Coke, Sprite) work equally well?
Let’s watch the experiment video below to observe the reaction between Mentos and different types of soda drinks.
The experiment shows that other kinds of soda drinks can also produce ‘geyser’ effect. However, those produced by Diet Coke (Sold under the name Coca-Cola Light in Singapore) and Coke Zero are more dramatic. When we take a closer look at the ingredient list, we will realize that Diet Coke and Coke Zero contain aspartame (an artificial sweetener) to replace the natural sugar in the Classic Coke. Diet Coke contains an additional additive named potassium benzoate, which functions as a preservative (check out pictures below for the full ingredient lists).
Top to bottom: Classic Coke, Diet Coke (Coca-Cola Light), Coke Zero.
A group of physics students from Appalachian State University conducted a study to investigate the reaction. They varied the various experiment parameters such as the ingredients in the candy and soda, the roughness of the candy, the temperature of the soda, and the duration of the reaction.
Their study proves that aspartame and potassium benzoate are key ingredients in the reaction. The contact angle measurement shows that they reduce the work required for bubble formation, and hence allow the carbon dioxide gas to escape quickly from the bottle. Also, they identified the surface roughness as one important cause of the reaction. In addition, reaction happens faster when the beverage is less viscous (so the candy can fall faster to the bottom), and when it is hotter.
Let’s go back to the story at the beginning. After about ten minutes of nervous wait, I was relieved that I was still intact. No dangerous gassy rebellion was formed in my stomach. Reason? Because when the Coke was traveling down towards my stomach, it got warmed up and most of the bubbles already changed into gaseous form. Therefore, the reaction would not happen even when I swallowed down a pack of Mentos.
Phew! What a relief!
1.Coffey, T.S. (2008). Diet Coke and Mentos: What is really behind this physical reaction? Am.J.Phys. 76 (6).