The Science Behind the Mentos-Soda Geyser

published Oct 13, 2015
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(Image credit: Mikael Nyberg)

Remember when it seemed like just about everyone was sharing videos of bottled soda eruptions caused by Mentos candy? Even David Letterman featured it at some point — he had guests come on the show to set off 122 bottles of Diet Coke on a street in New York City.

There was a lot of hype over this phenomenon, but few discussed why a bottle of soda erupts when you drop a Mentos candy in it. Let’s look at the science behind the Mentos-soda geyser.

It’s Not an Acid/Base Reaction

Many people assume that the geyser forms when a Mentos candy is dropped into a bottle of soda because of an acid/base reaction, just like when you mix baking soda (sodium bicarbonate) and a vinegar solution (acetic acid in water) to create the ever-popular “volcano” science project.

While the volcano project is most certainly acid-base chemistry, the Mentos-soda geyser is not. But they do have one thing in common: carbon dioxide.

In the case of the baking soda volcano, carbon dioxide is formed when sodium bicarbonate reacts with an acid. On the other hand, when you drop a Mentos candy into a bottle of cola, the carbon dioxide gas was already present in the soda to begin with, yet it seems to suddenly erupt.

There’s an Equilibrium of Gas Inside Every Bottle of Soda

In a bottle of soda, there is gas dissolved in the soda, and there’s gas floating above the surface of the soda when the bottle is closed. The pressure (courtesy of the cap) keeps the gas in place (whether the gas is in solution or in the air trapped above), and the amount of dissolved carbon dioxide is proportional to the amount of carbon dioxide gas above, as they are in equilibrium.

When you unscrew the bottle cap, the carbon dioxide gas escapes, which means more carbon dioxide is dissolved than in gas form. The dissolved gas rises out of solution in an attempt to regain that balance. That’s one of the reasons why when you open a bottle of soda, the gas bubbles suddenly rise.

Dropping “Something” into a Bottle of Soda Will Cause More Gas Bubbles to Suddenly Form

While Mentos seems to work best, even pieces of rock salt can cause a small eruption of gas from soda. The rough surface of these provides little nooks where dissolved gases can gather together to form bubbles more easily. As those bubbles rise, more dissolved gas bubbles out of solution.

Mentos work particularly well because they are denser, so they sink faster. When a piece of Mentos candy is dropped into a soda bottle, just after the bottle is opened, the candy sinks right down to the bottom quickly, and the amount of gas and the speed at which it’s released causes an impressive geyser effect.

The Ingredients in Diet Coke Help, Too

Sure, there’s carbon dioxide trapped in all different kinds of sodas, whether sugar-free, caffeine-free, or regular, but according to a study published in 2008 in the American Journal of physics, it turns out that the Mentos-soda geyser is particularly impressive if you use Diet Coke because of the sweetener additives in Diet Coke: aspartame and potassium benzoate. These two ingredients make it easier for gas bubbles to form in the soda, causing a faster, more explosive geyser to occur. Using Diet Coke has other advantages, too, like a less sticky cleanup job afterwards, according to most Mentos-soda geyser experimentalists.

Did you or your kids ever try to reproduce the Mentos-soda geyser at home?