Why Does the Coke Erupt?

 by Yuanzheng Mao

When you drop a Mento into a bottle of Coke, quickly run away! (Figure 1) This is a well-known phenomenon that Coke erupts with the presence of Mentos in it. However, I am wondering, what is going on in the Coke, that causes it to undergo such a tremendous and funny change?

 

Figure 1 Mentos + Coke messing people up. You can find a lot of such videos online or even try it out yourself! Image source: https://tenor.com/en-GB/view/mentos-grandma-cola-coke-explode-gif-16300198

Coke is a type of fizzy drink. This means that much gas, called carbon dioxide, is dissolved in it, just like you put some salt in water and the salt dissolves. Because of the special technique during the production of Coke, it contains TOO MUCH carbon dioxide. Therefore, the carbon dioxide tends to form bubbles and escape from the Coke if you leave it open, which is why, if you leave the Coke overnight without the cap, it is not fizzy anymore. You can sometimes see bubbles on the wall of the container of the fizzy drink. These bubbles are made of carbon dioxide.

 

This formation of bubbles requires energy to drive it, and the amount of energy needed is high for the carbon dioxide. So, the number of bubbles in the drink isn’t very high because it is too hard for the carbon dioxide to aggregate and form bubbles. However, if you have a rough surface, it’s a different story. Although a Mentos looks smooth, there are a lot of tiny dents, scratches, lumps, etc. on its surface. So, if you drop a Mentos into the Coke, the rough surface of the sweet lowers the amount of energy needed to for the dissolved carbon dioxide to gather and form bubbles. Therefore, so many bubbles quickly appear on the surface of the Mentos and escape from the drink at the same time, making the drink look like a fountain. [1]

 

In a sentence, it is the formation of bubbles from the dissolved carbon dioxide on the rough surface of Mentos that causes the Coke to squirt. This phenomenon is not limited between Coke and Mentos. It can also occur between other fizzy drinks and rough surfaces. [1] For example, if you drop some sands into a bottle of Sprite, the Sprite should theoretically erupt too, although I haven’t tried this before (I will try it out later). The height of the fountain is subject to many factors. It is actually a very complicated system. [1]

References

[1] Wikipedia (2022), Diet Coke and Mentos eruption [online] Last accessed 9 December 2022: https://en.wikipedia.org/wiki/Diet_Coke_and_Mentos_eruption