The Real Reason Mentos And Diet Coke Causes An Explosion
When you make a science fair volcano filled with baking soda and vinegar (kids still do this, right?), the mixture bubbles and "erupts" thanks to the chemical reaction that creates a carbon dioxide gas, per Britannica. It's reasonable to think that the eruptive mixture of Mentos and Diet Coke operates similarly — maybe because cola is acidic, while Mentos are chalky and basic. But in fact, according to Scientific American, the explosion is a physical reaction, not a chemical one.
It all goes back to the carbonation of the soda. The reason that soda is bubbly is because it's pumped full of carbon dioxide and then trapped within a can or bottle, where the gas can't escape. When the bottle is uncapped, some bubbles then rise to the surface. But when you uncap a soda, the surface tension of the water means that the gas usually doesn't all rise at once — in fact, this would cause an explosion and leave the remaining soda totally flat. This is why you don't shake up a can of soda before serving it: you'd just be helping the carbon dioxide escape (via Scientific American) and making a mess in the process. Okay — but where do the Mentos fit in?
Coca-Cola Geysers
According to Scientific American, Mentos are covered in small bumps that help the carbon dioxide break free of the water that's keeping it down. As a Mentos candy falls through a bottle of Diet Coke, it helps the carbon dioxide more easily form bubbles — too many bubbles — which then creates a gigantic Coke geyser.
But why Diet Coke in particular? According to Snopes, the Mentos and soda trick works with any carbonated beverage. But Diet Coke has a few superficial advantages — its brown color means that the explosion looks more impressive, and diet soda won't leave the same sticky residue that full-sugar cola would.
The Snopes article also points out that despite rumors that children have died over the years from ingesting Mentos and Coke at the same time, it hasn't actually happened. Nor did Mikey from the Life Cereal commercial die from washing down Pop Rocks with Pepsi (also per Snopes).