Ask a Southeastern Idaho Scientist

First of all, to explain to our readers what a Mentos/Diet Coke fountain is, let’s outline the experiment. The experiment is probably best done OUTSIDE in the middle of the lawn. Carefully open the bottle of soda and place it on the ground. Make sure it will not tip over. Unwrap the whole roll of mint-flavored Mentos. Roll a piece of paper into a tube just large enough to hold the Mentos. Position the tube directly over the mouth of the bottle so that all the candy drops into the bottle at the same time. Before you drop the candies in, warn everyone to stand back and get ready to run a safe distance away. Prepare for lots of oohs and ahhhs.

Though there is still some disagreement on why the Mentos/Diet Coke eruption happens, most scientists agree that there are two main reasons.

Soda pop is bubbly because of an invisible gas called carbon dioxide that is pumped into the bottles at the bottling factory. Until you open the bottle, pressure keeps the carbon dioxide dissolved in the pop. Water molecules like to be next to other water molecules and they form a tight ring around each gas bubble and keep the gas bubbles separate.

If you drop anything into a bottle of soda pop, the water molecules are disturbed and move apart, just enough to release more gas bubbles. If you drop a penny into the soda pop, you’ll see tiny bubbles forming on the surface of the penny. If you drop a scoop of ice cream into a cup of root beer, the gas bubbles in the root beer make a lot more gas bubbles and form the foamy top we all love to eat. Try dropping a little salt in a cup of soda pop. What happens? How about sugar?

Now for the second reason Mentos candies create the soda pop eruption. Mentos candies have nucleation sites. These are tiny dents on the surface of the candy that allow an incredible number of bubbles to form. Since the Mentos are also heavy enough to sink, they react with the soda all the way to the bottom of the bottle, allowing many more bubbles to form and quick as a wink you have a huge geyser eruption.