The science behind the soda geyser

You've seen it. Diet Coke + Mentos = cola fountain. Here's why the combination has such a messy result


Mentos, soda geysers and the science of nucleation have captured Americans' fancy this summer - if hundreds of goofy, homemade videos posted on the Web are any indication.

The formula is simple: Plop a few Mentos candies into a two-liter bottle of soda and behold! A fountain of fizz shoots high into the air - with a force and speed that surprises even grizzled chemists.

"I'm not sure one could have predicted it ... that's the serendipity of science," said John P. Toscano, head of the chemistry department at the Johns Hopkins University.

Kids, don't try this at home - at least not inside the house. Mom will be really mad.

Instead, check out a popular video online of two Maine men, dressed in lab coats and goggles, who used 200 liters of Diet Coke and more than 500 Mentos to reproduce the fountain display from the Bellagio casino in Las Vegas. The men were rewarded for their fizzy artistry with a June appearance on the Late Show with David Letterman (which was rerun Tuesday night).

What makes Mentos and soda such an explosive combination?

After confirming and recording the phenomenon in The Sun's outdoor test laboratory (OK, the parking lot), it was time to solve the mystery.

It turns out that Mentos dropped into a bottle of soda provoke an extreme version of the same process that produces bubbles in champagne, a frothy head on beer, and a painful case of "the bends" in unfortunate scuba divers.

It's evanescence - a gas bubbling out of a fluid suspension - taken to the extreme.

"It's not a chemical reaction," Toscano said. "It's physical, the same idea as when you shake a bottle of soda." The key ingredient in the geysers is carbon dioxide, or CO2. Carbonated beverages get their "bite" when manufacturers combine the gas with water, or H2O, under pressure to produce carbonic acid, or H2CO3, better known as soda water, or seltzer.

Apparently, any carbonated soda will produce a geyser, but Diet Coke seems to be the drink of choice for Mento-istas. Aficionados say it is easier to clean up than regular soda because it lacks sticky sugar.

Normally, CO2 remains dissolved in soda for two reasons: pressure inside the closed bottle and a net of water molecules clinging together.

Opening a bottle of Diet Coke reduces the pressure inside, allowing some CO2 to escape. Leave the bottle open too long and all the gas escapes - so you are left with flat soda.

A similar process takes place in the blood of scuba divers who ascend too quickly from high-pressure depths. As divers near the surface and water pressure drops, nitrogen gas bubbles can form in their blood, causing a potentially lethal condition knows as decompression sickness, or "the bends." But if pressure were the only thing keeping CO2 in a carbonated drink, soda ads showing bubbles drifting appetizingly toward the top of a glass would be inaccurate.

That's because the gas would instantly rush out of a bottle of Diet Coke, Perrier or ginger ale as soon as it was opened - leaving a flat, liquid mess all over the room.

Lucky for soda lovers, nature provides another way to keep the CO2 from escaping. In this case, the surface tension of liquid prevents the big whoosh from happening. The water molecules cling together, creating a net-like structure that holds the CO2 molecules in.

This clinginess explains why insects such as water striders can walk on the surface of lakes, and why rain beads on the outside of camping tents - until someone inside touches the canvas wall, breaking the surface tension and allowing the water to seep in.

Shaking a bottle of soda or beer breaks the fluid's surface tension, too, allowing some of the CO2 molecules to group together and form gas bubbles. Open that bottle, and the gas rushes out - taking some of the liquid with it and spritzing anybody in range.

If you want to induce this extreme evanescence artificially, Mentos turn out to be way better than shaking, or anything else for that matter.

"If you throw anything into soda, say salt, it causes it to fizz," Toscano said. "But there is something special about the Mentos, probably their large surface area, that releases all the CO2, really fast."

Although they feel smooth, the candies are covered with microscopic nooks and crannies that act as seeds from which gas bubbles can grow.

Those imperfections - known as "nucleation sites" in physics - break the tension between the water molecules. They also reduce the energy needed for the CO2 to escape the net of water molecules and form bubbles.

Dust particles have a similar nucleation effect on the freezing temperature of water, raising it so that snowflakes form at warmer temperatures. Similarly, kids making rock candy often use string as a nucleation site for sugar crystals.

Mentos may be the candy of choice for geysers because they just fit inside the neck of a two-liter bottle but still provide a large surface area for bubble formation.

"M&Ms wouldn't do the trick," Toscano said.

The makers of Mentos are thrilled by the attention the online videos have brought to their product - not to mention a recent surge in sales they attribute to the publicity.

"One of the estimates I heard is that there are close to 3,000 geyser videos online," said Pete Healy, the vice president of marketing for Perfetti Van Melle, the Italian company that makes Mentos.

He said the company is planning future projects with the two video makers from Maine - and just this week it launched a Web site where experimenters can post their own Mentos geyser videos.

Healy thinks at least some of the public has been aware of the geyser effect since the early 1990s, when Mento ads first appeared on TV.

"They bubble up every four or five years," he said.


EXTREME DIET COKE AND MENTOS EXPERIMENTS // Two Maine men take Mentos geyers to the next level at

MENTOS WEB SITE // aspiring geyser video makers can enter their production in a contest:

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