How Does It Work
Soda pop is basically water, sugar (or artificial sweetener), flavoring, and preservatives. The thing that makes soda bubbly is carbon dioxide gas (CO2) which is pumped into the brew at the bottling plant. It takes tons of pressure and very low temperatures to do it, too. Until you open the bottle and pour a glass, the gas stays mostly suspended in the liquid and cannot expand to form bubbles, which a gas naturally does. If you shake the bottle and then open it, the gas is released from the protective hold of the water molecules. It escapes as a foaming whoosh! taking some of the liquid along with it. What other ways can you cause the gas to escape? Just drop something into a glass of soda and notice how bubbles immediately form on the surface of the object. For example, adding salt to soda causes it to foam because thousands of tiny bubbles form on the surface of each grain of salt.
Water molecules strongly attract each other, linking together to form a tight mesh around each bubble of CO2 in the soda. In order to form a new bubble, or to expand one already formed, water molecules have to be forced away from each other. It takes extra energy to break this “surface tension.” In other words, water “resists” the expansion of bubbles in the soda.
When you drop the Mentos into the soda, the gelatin and gum arabic from the dissolving candy surface break the surface tension around the dissolved gas. This disrupts the water mesh, so that it takes less work for the gas to expand and form new bubbles. Each Mentos candy has thousands of tiny micro-pits all over its surface, too. These tiny pits are called nucleation sites and they’re perfect places for CO2 bubbles to form. As soon as the Mentos hit the soda, bubbles form all over the surface of the candy.
Mentos candies are heavy for their size and sink to the bottom of the bottle. That’s a double-whammy for making a geyser erupt. With the candy on the bottom of the bottle and the gas being released, it pushes all of the liquid up and out of the bottle in an incredible gas and liquid blast. (You can see a similar effect when pasta is lowered into a pot of boiling water. The water will sometimes boil over because organic materials that leave the pasta disrupt the tight mesh of water molecules at the surface of the pot. This makes it easier for the water to bubble and foam over.)
When a scoop of ice cream is added to root beer, it foams over for basically the same reason. The surface tension of the root beer is lowered by ingredients and proteins in the ice cream. The CO2 bubbles rapidly expand and are easily released creating that beautiful foam on top of the float.
Why should you use diet soda? The simple answer is that it just works better than regular soda. Some people speculate that it has something to do with artificial sweeteners. More importantly, diet soda does not leave a sticky mess to have to clean up. Hey, that’s important for your clean up!
What’s the record for the biggest Mentos fountain? The official Spangler Science geyser was an 18-foot (5.5 m) blast that shot up and almost took out a half-million dollar, high definition TV camera. (You’ll find videos of some of our favorite eruptions online at www.SteveSpanglerScience.com.)
