Experiential Science: Diet Coke & Mentos

LESSON PURPOSE

Present science in a fun format

Share the reaction details and vocabulary behind the science

Encourage you to try your own experiments with alternate variables

Mentos have a strange chalky color and texture and they do taste rather like antacid tablets. If the chalkiness comes from carbonates, the fizzing might be explained as a neutralization reaction that produces CO2.

But the ingredients don't include carbonates- or any other significantly alkaline material. All variations of Mentos contain sugar, glucose syrup, hydrogenated coconut oil, gelatin, dextrin, "natural flavor", corn starch, and gum arabic. They're basically just a big pellet of flavored sugar with gummy stuff added to give them structural integrity and to keep them from sticking together in the package.

Fred Senese of the Department of Chemistry at Frostburg State University in Maryland

So why do Mentos make diet soda foam so violently? It's a physical reaction, not a chemical one.

Ordinarily, water resists the expansion of bubbles in the soda. Water molecules attract each other strongly, and they link together to form a tight mesh around each bubble. It takes energy to push water molecules away from each other to form a new bubble, or to expand a bubble that has already been formed. The phenomenon is called "surface tension".

Now drop a Mentos into the soda. The gelatin and gum arabic from the dissolving candy disrupts the water mesh, so it takes less work to expand bubbles. At the same time, the roughness of the candy surface provides many little nooks and crannies that allow new bubbles to form more quickly (a process called nucleation). As more of the surface dissolves, both processes accelerate, and foam rapidly begins to form.

Fred Senese of the Department of Chemistry at Frostburg State University in Maryland

THE DIET COKE

In summary, if you have a liquid that is supersaturated with gas (like soda, which is pumped full of carbon dioxide), a nucleation site is a place where the gas is able to form bubbles. Nucleation sites can be scratches on a surface or specks of dust – anywhere that you have a high surface area in a very small volume. That's where bubbles can form.

Mentos seem to be loaded with nucleation sites. In other words, there are so many microscopic nooks and crannies on the surface of a Mento that an incredible number of bubbles will form when you drop it in a bottle of soda. Since the Mentos are also heavy enough to sink, they react with the soda all the way to the bottom. The escaping bubbles quickly turn into a raging foam, and the pressure builds dramatically. Before you know it, you've got a big geyser happening!

NEXT STEPS

Test what you've learned

with a word scramble game!

Get supplies for you own geyser at Steve's Spangler's science website

Test alternate combinations: Mountain Dew & Mentos, Diet Coke & Life Savers, etc.

Post your results on BrainHoney!