experimental procedure version 1
There are two versions of this experiment - Mentos and Diet Coke vs. Mentos and other diet sodas and Mentos and Diet Coke vs. other candies and Diet Coke. The reason that there are two versions of this experiment is that scientists usually only change one thing (called a variable) at a time. So, one version changes the soda that is used, and the other version changes the candy that is used.
Version 1 - Mentos and Diet Coke vs. Mentos and other diet sodas
experimental procedure version 2
It's all about the bubbles. Have you ever been pranked by someone who secretly shook a can of soda pop before giving it to you to open? Why does the can explode in a fountain of sticky foam? The foam is the liquid from your drink plus lots of bubbles containing carbon dioxide gas added when the soda pop was made, and is what make carbonated drinks so fizzy. This carbon dioxide gas actually dissolves into the liquid soda pop (just like when you stir sugar or salt into water), and manufacturers are able to dissolve much more carbon dioxide- making it even fizzier- by pushing gas into the can or bottle under very high pressure before sealing it. This establishes an equilibrium, with the ratio of the number of carbon dioxide gas molecules at the top of the can to the number of molecules dissolved into the flavored water below remaining constant. When you open your drink, however (without shaking it first), the gas at the top quickly escapes (that's the "pssst" sound you hear) but this also disturbs the equilibrium. This sudden drop in pressure forces a lot of the dissolved carbon dioxide to come out of solution and form gas bubbles in the liquid which quickly float to the surface, pushing some liquid with them and forming a layer of foam. But it takes a lot of energy to initially create a gas bubble inside the liquid, as carbon dioxide molecules must push water molecules- which are strongly attracted to each other- out of the way, so most of the energy available is quickly used up. This means that the bubbles remain fairly small, there isn't much foam, and you don't make a mess.
You can't (usually) see what's happening inside a sealed aluminum can, but when you shake it the energy you add creates lots and lots of tiny bubbles (see the related activity below). Now when you open the can the gas at the top quickly escapes, and just as before, much of the dissolved carbon dioxide comes out of solution as gas molecules. But this time they don't need to waste their limited energy forming the bubbles, as your shaking has already created lots of tiny "starter" bubbles in the liquid, so all this extra carbon dioxide gas easily moves into these little bubbles, making them grow much bigger, much faster. Bigger bubbles float to the surface faster, pushing a lot more liquid with them, and you get a foamy explosion! BTW- if your can of soda is very warm you may not need to shake it to make a mess. Can you guess why?
Shaking the can or bottle is one way to create a bubbly mess, but there are other ways, and some work MUCH better, as you saw in this experiment! There are actually several different reactions happening at the same time to create bubbles when you mix Mentos with Diet Coke, and not all scientists agree about all of them, but here are the basic ideas.
All of these things together combine to produce the reaction that we know and love when we put Mentos and Diet Coke together. However, does that mean that the reaction between Mentos and Diet Coke is the best that it can possibly be? Not necessarily! What if you find a soda that has more of one of these reactants (like caffeine)? Experiment with these variables, and see what you can find out!
variations and related activities
Rock salt (like you put on the sidewalk to melt ice) also reacts well with Diet Coke. Try it. You may also want to try granular salt (i.e. what's in your kitchen salt shaker). Does it work as well (it's exactly the same chemically)? Hint- think about the nucleation sites.
What if the hole that the carbon dioxide and soda came through was smaller? What would happen to the height of the bubbles? Remember, however, that pressure is building up in the bottle, and that it has to get out somewhere.
You can't see what happens inside an aluminum can of soda pop when shaken, but you can see through most plastic bottles. Try shaking a bottle of clear soda pop (like 7-Up, Sprite or Ginger Ale) to see all the tiny bubbles created. Unscrew the cap quickly and you will get a foamy mess. What happens if you wait until all of the bubbles float to the surface and pop before opening the bottle?
The wrinkles in raisins are also very good nucleation sites for carbon dioxide bubbles to form in soda pop. Try adding 5 or 6 raisins (fresh ones work best) to a bottle or tall glass of a clear soda pop like 7-Up, Sprite or Ginger Ale. You probably won't get an explosion, but there will be bubbles and you may be surprised what they do to the raisins! Look for a new experiment page soon that will explain what's happening (or see the links below), but we think you can figure it out if you observe carefully.
References and links to more information
This is the Mythbusters episode that deconstructs what happens when you put Mentos in soda. Parent note: There are a few shots of Kari Byron wearing a swimsuit that are a bit revealing, but other than that, the science explored here is well done and well tested.
The original Eepybird video using Diet Coke and Mentos, which some people say started the craze.
More explanation from Eepybird.
Explanation from the American Chemical Society.
Steve Spangler has some ideas about how to take this further, as well.
A bona fide scientific study of what is happening and why.
More explanations of what happens when you shake a can of soda. Some people think there is a trick to keep a shaken can from exploding, but others think it's a myth. Maybe you should try to confirm or bust the myth! Does this apply to plastic bottles too?
Adding a few raisins to a glass of soda pop is a lot more fun than you may think!