Here we see two people making the biggest mentos and coke explosion I have ever seen, or ever hope to see. They actually set it up so one bottle of coke exploding sets the next one off. By why does candy and soda react so violently? Let's have a look.
These are amazing reactions, and the kinds of things that make you want to head for the grocery store and then out to a clear field. We've all seen coke fizz out of the bottle, and the wide bottled tapering down to a thin neck causes the reaction to blow more than if it were in, say, a bowl. But how does anything cause the coke to fizz that violently.
Tonya Coffey, a physicist at Appalachian State University, took a look at the science that caused this reaction. The most familiar analogous reaction is vinegar and baking soda, so initial thoughts were that there was some kind of acid-base chemical reaction going on. PH testing of the coke before and after the reaction showed no change in acidity. Diet coke and caffeine free coke were tested, and did as well as regular coke. Diet Coke even did a little better. Clearly, the reaction had nothing to do with sugar or caffeine. So what was the deal?
It all has to do with nucleation. We've covered nucleation before. Even when a liquid is dying to bubble, it needs little nucleation sites, either from a rough container or from dust and grit, to get the bubbles started. But how do these sites get bubbles started?
Basically, any water that is about to bubble - like a soda overfilled with carbon dioxide - is in a struggle between two forces. The gas is exerting pressure to rip one water molecule away from another molecule, while the water itself is being held together by a force called cohesion. The two hydrogen molecules huddle at one end of the water molecule with their protons exposed. Meanwhile, the oxygen molecule, which has hold of their electrons, is hugging those electrons close to itself. This makes one end of the molecule slightly positive, and one end slightly negative. Since positive and negative attract, the positive side of one water molecule will attract the negative side of another molecule, holding the entire liquid together. This bonds will form and break quickly, over and over, so the water stays fluid, but they keep forming, and they hold the entire liquid together, even when the gas is exerting a considerable pressure.
When these molecules encounter a rough surface, the tiny bumps and the roughness make it harder for one water molecule to hang on to another molecule, lowering the cohesive force. Mentos has that rough surface. It also is coated with gum arabic, which is a surfactant. A surfactant is a chemical that also disrupts those bonds. Aspartame is also a surfactant. The combination of the surfactant and the rough surface suddenly gave a bottle full of dissolved gas, straining to get out, an opportunity to burst.
Finally, the bottles do better than the cans, because the mentos float down farther. As they move through the soda, more and more water molecules come in contact with it, and so more bubbles are unleashed. The churning from the erupting soda then keeps a fresh supply moving over the mentos.
Now. Who made it to the end, and who is already on their way to buy some candy?
Video via Eepybird.com
Information: Appalachian State University