When a chemist sees bubbles like the ones you saw in the experiment, she understands that a gas is being made. In this case, the gas is carbon dioxide. The "fountain" forms because carbon dioxide is leaving the solution, forming a gas. When the gas forms quickly, pressure builds up, and the gas comes out in a rush, pushing some liquid along with it. So the Diet Coke tasted flat because a lot of its carbon dioxide had left the solution, making the fountain. 

Why did the carbon dioxide leave the Diet Coke? The surface of Mentos candies (at least the mint kind) is full of tiny pits. This disrupts the network of water molecules in a carbonated beverage, forcing some of them apart from one another. When that happens, the carbon dioxide molecules are less inclined to stay in solution, because they are no longer surrounded by as much water. The tiny pits on the surface of the Mentos candies are called "nucleation sites," as they are sites where bubbles can form as the carbon dioxide leaves the solution. 

Did you notice that the fountain was not nearly as dramatic in the cold Diet Coke as it was in the room-temperature Diet Coke? That's because temperature affects how well water can dissolve a gas. The warmer the temperature, the less effective water is at dissolving the carbon dioxide. Thus, the combination of warmer Diet Coke and Mentos helped to force even more carbon dioxide out of the Diet Coke, resulting in a better fountain. 

Now interestingly enough, there are lots of different solids that have pits on their surfaces. Salt crystals have pits on their surfaces, as do most mint candies. However, in your experiment, the salt and the other mint candies probably didn't produce as big a fountain. Finally, the regular (non-diet) sodas you used did not produce as impressive a fountain as the diet sodas. In the end, the combination of Mentos and a diet soda probably made the best fountain. Why is that? 

Mentos candies have a lot of gum arabic, which is a type of chemical we call a "surfactant." It tends to reduce the amount of surface tension in water. This makes it even easier for the carbon dioxide to escape. In addition, the aspartame in a diet soda enhances the gum arabic's job. As a result, those two specific ingredients work together to make the fountain spectacular. 

Although you had fun and hopefully learned something about sodas and candies as a result of this experiment, there is a more important thing I want you to learn from all of this. The experiment that you did illustrates the way a chemist does trials to determine what is happening in a process he is observing. Notice how I had you do the experiment. You did the first trial as a "baseline" or a "standard." We call this the control of the experiment. That is the "normal" way the process works. Then, you changed one variable at a time to learn more about the process. First, you replaced the room-temperature Diet Coke with chilled Diet Coke. Thus, you changed only the temperature of the soda—nothing else. That told you the effect temperature had on the process. Next, you went back to room temperature, but then you changed the kind of soda. Once again, compared to the control, only one thing changed: the type of soda. When you were done with that, you went back to room-temperature Diet Coke and then changed what you were dropping into it. 

Each trial, then, represented only one change from the control of the experiment. You changed either temperature, soda, or what was dropped into the soda, but you did not change more than one of those things at a time. That's the best approach to doing chemistry experiments. If you can change only one variable at a time, you have the best chance of learning what is important in the process you are observing. Remember that as you investigate more of the wonders of chemistry! 

*This article published December 2, 2009.

Dr. Jay L. Wile holds an earned Ph.D. from the University of Rochester in nuclear chemistry and a B.S. in chemistry from the same institution. He has won several awards for excellence in teaching and has presented numerous lectures on the topics of nuclear chemistry, Christian apologetics, homeschooling, and creation vs. evolution. He is best known for authoring or co-authoring most of the Exploring Creation with . . . textbooks designed for junior high and high school students who are educated at home. 

Copyright 2009. Originally appeared in The Old Schoolhouse Magazine, Fall 2009.

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