Cola Explosion When the Mentos come into contact with the Diet Coke, a reaction causes the rapid formation of foam. it is concluded that the potassium benzoate, aspartame, and C02 gas contained in the Diet Coke, in combination with the gelatin and gum arabic ingredients of the Mentos, all contribute to the formation of the foam. The structure of the Mentos is the most significant cause of the eruption due to nucleation.
MythBusters reported that when fruit-flavored Mentos with a smooth waxy coating were tested in carbonated drink there was hardly a reaction, whereas int-flavored Mentos (with no such coating) added to carbonated drink formed an energetic eruption, affirming the nucleation-site theory. The surface of the mint Mentos is covered with many small holes that increase the surface area available for reaction (and thus the quantity of reagents exposed to each other at any given time), thereby allowing C02 bubbles to form with the rapidity and quantity necessary for the “Jet”- or “geyser”-like nature of the effusion.
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Each Mentos candy has thousands of mall pores on its surface which disrupt the polar attractions between water molecules, creating thousands of ideal nucleation sites for the gas molecules to congregate. In non-science speak, this porous surface creates a lot of bubble growth sites, allowing the carbon dioxide bubbles to rapidly form on the surface of the Mentos. (If you use a smooth surfaced Mentos candy, you won’t get nearly same the reaction. ) The buoyancy of the bubbles and their growth will eventually cause the bubbles to leave the nucleation site and rise to the surface of the soda.
Bubbles will continue to form on the porous surface and the process will repeat, creating a nice, foamy geyser. In addition to that, the gum arabic and gelatin ingredients of the Mentos, combined with the potassium benzoate, sugar or (potentially) aspartame in diet sodas, also help in this process. In these cases, the ingredients end up lowering the surface tension of the liquid, allowing for even more rapid bubble growth on the porous surface of the Mentos”higher surface tension would make it a more difficult environment for bubbles to form.
Compounds like gum arabic that lower surface tension are called “surfactants”). Diet sodas produce a bigger reaction than non-diet sodas because aspartame lowers the surface tension of the liquid much more than sugar or corn syrup will. You can also increase the effect by adding more surfactants to the soda when you add the Mentos, like adding a mixture of dishwasher soap and water. Bubble theory: How bubbles form in liquids In most liquids, there is some dissolved gas.
In high surface tension liquids, like water, it is tough for bubbles to orm, because water molecules like to be next to other water molecules (capillary forces). To overcome this, a nucleation site is generally needed. Gas molecules congregate next to nucleation sites, which break up the network of water molecules. When enough are gathered, they form a bubble. Due to capillary forces, the bubble will initially stay at its nucleation site. But usually, the buoyancy of the bubble will eventually cause it to rise, as more and more gas molecules collect in the bubble.
More fun bubble facts… When a soda is bottled, it is bottled under a relatively high pressure of C02 that is opened without shaking high pressure C02 above the liquid escapes, making the familiar hiss. The C02 in the liquid slowly escapes until equilibrium is achieved. When the unopened can is shaken, some of the gaseous C02 gets mixed into the liquid, forming a supersaturated solution. The mixed in gas also provide growth sites for the dissolved C02. The growth sites allow the C02 to escape much more rapidly– hence the “explosive” evolution of C02 gas.