Just about everyone knows what happens when you drop Mentos mints into a Diet Coke.
Students at Appalachian State University have documented why the reaction occurs by studying the physics responsible for the fizzy result. Their results have been published in the June 2008 issue of the American Journal of Physics.
Tonya Coffey, an assistant professor of physics at Appalachian, developed the research project to as a way for sophomore-level students to build on skills they learned in their freshmen physics courses.
Through a series of experiments, the students found that a reaction between the rough surface of the Mentos, and the potassium benzoate and aspartame contained in Diet Coke were responsible for the famous geyser reaction, in which the liquid can spew up to 30 feet.
In the process, they also learned about the principles of thermodynamics, fluid mechanics, surface science and the physics of eruptions.
“We try to teach students what real experiments are like,” Coffey said. “I thought it would be good for the students to work on an experiment that doesn’t have a known outcome—because that’s what research is.”
Coffey asked her students to find out everything they could about the Diet Coke and Mentos reaction, develop a question about the reaction and design an experiment to answer their question. The students’ only restrictions were to design an experiment that could be accomplished on a tight budget and to use existing equipment at the university.
“We discussed what a real researcher has to do when designing an experiment to answer a question,” Coffey said. Students studied what makes a good experiment, how complications can arise, the need to narrow the number of unknowns in an experiment, and the importance of designing an experiment that tests for one variable at time.
The students measured the volume of liquid displaced and the distance it traveled when a variety of items were added to Diet Coke – including Mentos, Wint-O-Green Lifesavers, rock salt, table salt and sand.
They also studied the surface roughness of the candy and other materials by using a scanning electron microscope and an atomic force microscope.
So why does the reaction occur? In an opened container of soda, carbon dioxide gas bubbles out over the course of minutes or hours until the concentration of carbon dioxide left in the soda is proportional to the carbon dioxide in the surrounding air. This de-fizzing reaction is slow because the surface tension of the liquid is very high, which keeps the gas bubbles trapped.
But when a Mentos is dropped in the beverage, it breaks the surface tension and as it falls the candy’s surfactant coating further reduces the surface tension of the liquid. The candy’s rough surface also provides growth sites for the gas, making it easier for carbonation to escape as a foam geyser.
The geyser also occurs when sand, salt or lifesavers were added to the Diet Coke, but the mass lost and volume traveled is much less spectacular.
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