The Science Behind Diet Coke+Mentos Rocketry
OK. Now that we've had some fun, it's time for the science part. You're probably thinking: Why in the world does the combination of Mentos and Diet Coke cause such a spectacular eruption that it can launch a rocket into the air?
The reason rockets blast into the air is that they release energy down, which can be explained by the Third Law of Motion, discovered by legendary scientist Isaac Newton in the 1700s. The Third Law dictates that every action has an equal but opposite reaction. Usually, rocketeers create that action-reaction by burning a chemical fuel to release the energy stored inside in the fuel. (For example, the jetpack we mentioned at the beginning is powered by igniting gasoline, which is what a car engine does.)
But your Diet Coke+Mentos rocket is powered by a slightly different process. The Diet Coke contains molecules of carbon dioxide, which were injected into the drink to make it fizzy. When you open a bottle of Diet Coke in the usual way and pour it into a glass, the water molecules and the carbon dioxide molecules are no longer contained, and they start to separate again. The carbon dioxide molecules come together to form bubbles — a process called nucleation — and then they gradually escape into the air. That's why, if you let a glass of soda sit for a while, the soda will go flat.
But when you put Mentos into the Diet Coke bottle, it greatly speeds up bubble formation, according to a New Scientist article. Why? Well, Mentos have a rough surface filled with tiny dimples you can't see, unless you use a magnifying glass. Those dimples give the carbon dioxide molecules a place to collect quickly and form bubbles. Another thing that speeds up the process is aspartame, the artificial sweetener in Diet Coke. Scientists have found that water with aspartame in it has a lower surface tension than sugary water. That allows the carbon dioxide bubbles to escape more easily.
Take Diet Coke+Mentos Rocketry to a New Level
If you thought your Diet Coke+Mentos rocket was fun, you can try other variations of this experiment as well. If you can persuade your parents to give you a wine cork, you can plug the bottle with that instead of screwing on the lid. Then, instead of bouncing the bottle off the ground, you can set it on a launching pad made from wire coat hangers or in a small hole. Run away and watch from a distance. When enough liberated carbon dioxide builds up inside the bottle, the energy will blow out the cork, and the rocket will soar into the air on its own.
Or, if you're really ambitious, save your allowance to buy 108 bottles of Diet Coke and 648 mentos. Then you can build your own Diet Coke+Mentos-powered car, like these crazy scientists did.