Why do Mentos fizz so much when you put them into Coke?

Bohuntin Mentos by Paul Appleyard, on FlickrIt’s the perfect kid’s party trick. Drop a Mentos mint into a bottle of Diet Coke and a volcano-like eruption of sweet fizz is guaranteed. It is also guaranteed to delight kids and horrify parents in equal measure. However, the cause of this confectionary quirk has alluded even the best minds for a surprisingly long time….

It was once thought that the acid in coke caused the explosive mint-soda reaction that we see. It doesn’t: experiments show that the acidity of the coke doesn’t change after a mint-induced eruption. Some have speculated that it might be the caffeine or the sweeteners that cause the reaction. Neither is true: caffeine-free and sugary fizzy drinks also react. The real reason for foamy eruptions has less to do with the chemicals in the cola and more to do with the texture of the mint.

Unseen to our eyes, all water has gases dissolved in it. Take river water, for example: if it didn’t have oxygen dissolved in it then the fish couldn’t survive. Coke is a carbonated drink, meaning that it has a particularly large amount of carbon dioxide dissolved in it. This is done by ‘squeezing’ the carbon dioxide gas into the liquid under high pressure in the factory. When you open the bottle before drinking it, the pressurised gas is given the chance to escape – and it does… in the form of bubbles.

All the pressured gas doesn’t escape in one go, for bubbles are fussy creatures and don’t like to form anywhere. No, for all bubbles must start their life at what are called ‘nucleation sites’: microscopic irregularities, bumps or pieces of grit on the surface of the container they are in. If there are more nucleation sites available then more bubbles can form. You can check this out by pouring a fizzy drink (champagne, say) into two glasses: one a clean glass and the other a glass with a sprinkle of sugar at the bottom. The glass with the sugary sprinkling will fizz more vigorously because each grain of sugar offers dozens more nucleation sites from which the bubbles can form.

As it happens, Mentos mints aren’t all that smooth. On a microscopic level, they are punctuated with countless tiny pores. Coke can flood into these holes and, in doing so, it gives the coke access to millions of tiny minty nooks and crannies – and millions of potential nucleation sites. Put simply: a Mentos gives the dissolved carbon dioxide gas a million more reasons to escape. And boy, does it!

Interestingly, any mint will do this – although ones with very smooth, waxy coatings aren’t very good. It’s not just mints either – lots of things will do it.

Feel free to experiment at your own leisure…

Answer by Dr Stu

Question from ‘JJ’ via Facebook

Article by Stuart Farrimond

March 26, 2014

Doctor Stu is editor of Guru Magazine. He originally trained as a medical doctor before deciding to branch out into lecturing, writing, editing and science communication. He drinks far too much coffee, eats lots of ice cream and has a bizarre love of keeping fit.
You can check out Doctor Stu’s blog at realdoctorstu.com or his poncy personal website stuartfarrimond.com. Here's his .

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