Why Cola and Mentos Erupt
Drop a Mentos into cola and a huge fountain shoots up. It is not a chemical reaction but a purely physical surface phenomenon — the microscopic roughness of Mentos provides thousands of nucleation sites at once, releasing dissolved CO2 explosively.
Drop a Mentos into a bottle of cola and a fountain shoots into the air.
Everyone has seen it or tried it once.
Why does such a violent reaction happen?
There must be a chemical reaction between Mentos and cola.
Some acid meeting something and exploding.
It is not a chemical reaction. It is purely a physical surface phenomenon.
Inside cola, [carbon dioxide CO2] is dissolved under pressure. On a smooth glass surface, CO2 escapes slowly. For bubbles to form, they need a "seed." This is called a [nucleation site].
Zoom into the surface of a Mentos and you find it is microscopically rough. This rough surface provides thousands of nucleation sites all at once. Countless bubbles form simultaneously, releasing CO2 explosively in a short moment.
On top of that, the [gum arabic] coating on Mentos lowers the surface tension of cola, accelerating bubble formation. Diet cola erupts even more violently because aspartame lowers surface tension further.
So Mentos does not "react" with cola. It only helps CO2 escape by providing a rough surface.
On the left is a surface-comparison zoom. Smooth glass has few nucleation sites, so CO2 escapes slowly, while the rough surface of a Mentos offers many sites at once. Use the surface toggle to swap between the two and see the difference in site count directly.
On the right is the progression of CO2 inside the cola. Press the stage buttons: ① CO2 dissolved under pressure → ② Mentos enters, nucleation sites activate → ③ bubbles form simultaneously → ④ explosive CO2 release and fountain. Use the cola-type toggle (regular/diet) to compare how surface tension changes the violence of the eruption.
Toggle the surface to compare nucleation-site counts on smooth glass vs. rough Mentos, step through the buttons (①-④) to follow CO2 dissolved → nucleation → simultaneous bubbles → eruption, and switch cola type (regular/diet) to see how surface tension changes the violence of the eruption.
Etched beer glassesBeer glasses with tiny scratches inside foam better. Breweries etch the marks on purpose to supply nucleation points.
Champagne flute nubThe small bump etched at the bottom of a champagne flute does the same thing, gathering nucleation sites so bubbles rise in a fine, steady stream.
Tapping a cola glassTapping the side of a cola glass releases bubbles because new nucleation sites form on the wall. Shaking it works more violently for the same reason.
A handful of sandSalt or sand will also make cola erupt, not just Mentos. Any rough surface becomes a nucleation site. Mentos simply has a large surface area and sinks fast, making it efficient.
Experiment safetyThe Mentos fountain is a physics experiment you can enjoy outdoors with only everyday care about sticky cola on your clothes. Since it is not a chemical reaction, there are no hazardous substances.
Even what looks like an explosion is really thousands of small bubbles forming together.