By now I expect you have seen a video of someone putting Mentos candies into an open bottle of Diet Coke. Almost immediately, a plume of foam shoots out of the bottle and into the air. While watching a "Mentos fountain" is interesting, it is significantly more interesting to investigate the process to find out exactly what happens when Mentos and Diet Coke are mixed together.

With that in mind, let's do an experiment. You will need to do it outdoors while wearing old clothes, as it is incredibly messy. To perform the experiment, you will need: 

· Safety goggles (available at a hardware store) 

· A few rolls of Mentos mint candies (You need at least 35 individual pieces of candy.) 

· One package of another brand of mint candy 

· Salt 

· Four 2-liter bottles of Diet Coke 

· One 2-liter bottle of regular Coke 

· One 2-liter bottle of any other kind of diet soda 

· One 2-liter bottle of a regular version of the soda above 

Before you do the experiment, make sure that one of the Diet Coke bottles has been sitting in a refrigerator for several hours so that it is cold. Make sure the other soda bottles are at room temperature. 

The first thing you want to do is get a "baseline" against which you will judge all of your trials. Do this by setting a room-temperature Diet Coke bottle in a clear part of your yard. Take off the lid and set it aside. Open one of the rolls of Mentos and put seven candies in your hand. If possible, hold the candies stacked one on top of another, as if they are still in the roll in which they came. If you can't accomplish this, hold them in a way that will allow you to drop the candies into the bottle very quickly. Quickly drop all the candies into the bottle and move away, keeping your eye on the bottle. Estimate how high the fountain of Diet Coke rises. 

Once the fountain has died down, taste the Diet Coke that is left in the bottle. It is important to note that as a general rule, you should never taste the results of an experiment. No matter how innocent the ingredients are, mixing them can produce unknown, dangerous effects. Thus, the only time you should taste the results of an experiment is when someone who knows a lot of chemistry tells you to do so! 

Using the same number of Mentos candies each time, repeat the experiment with the chilled Diet Coke, the regular Coke, the other diet soda, and the regular version of that same soda. You needn't taste the results of each trial unless you want to. In each case, try to gauge how high the fountain rises compared to the first one you saw.

Once you are done with all those trials, repeat the experiment with one of the bottles of Diet Coke and the other kind of mint. Try to use roughly the same amount of these mints as you used in the experiment with Mentos. Finally, repeat the experiment using salt rather than mints. Don't worry too much about the amount of salt; just use a handful. 

Now that all the hubbub is over, it is time to make sense out of the observations you made. Hopefully you found that the combination of Mentos and room-temperature Diet Coke resulted in a high-rising fountain. While most of the other trials produced some kind of fountain, some were far from impressive. What explains all the data you saw in the experiment? 

First you have to understand something about one of the important ingredients in soda pop: carbon dioxide (CO2). At room temperature, carbon dioxide is a gas. However, like many gases, it can be dissolved in water. When you dissolve carbon dioxide gas in a drink, it acidifies the drink, giving it a "tang" that it otherwise would not have. That's what we mean when we call drinks like Diet Coke "carbonated beverages." They are simply beverages in which carbon dioxide has been dissolved. If a carbonated beverage loses most of its carbon dioxide, we say that it has gone "flat." Do you remember what the Diet Coke tasted like after the fountain occurred? It probably had a hint of mint in it, but the main thing you should have noticed is that it was flat. What does that tell you? It tells you that the carbon dioxide that was dissolved in the Diet Coke was no longer there.