Why Does Dropping Mentos Into Soda Create a Geyser?

A bottle of carbonated soda contains far more carbon dioxide than the liquid would naturally hold in open air. The sealed bottle keeps the gas dissolved under pressure.

Opening the cap lowers that pressure, but most of the gas cannot escape immediately. To form bubbles efficiently, carbon dioxide needs places where gas molecules can gather. Scratches in a glass, fibers, and tiny particles can provide these starting points, known as nucleation sites.

A Mentos candy supplies an enormous number of them. Although it looks smooth to the unaided eye, its surface contains countless microscopic pits and ridges. Soda entering these irregularities releases bubbles across the candy almost simultaneously.

The candy also sinks rapidly, carrying this bubble production through much of the bottle rather than leaving it only at the surface. Ingredients in the candy and soda reduce forces that normally resist bubble formation, allowing the gas to escape even faster.

Thousands of expanding bubbles push the liquid upward. The narrow bottle opening concentrates the rising foam into a dramatic jet.

The eruption is mainly a physical release of dissolved gas, not a new chemical reaction producing a dangerous substance. The carbon dioxide was already inside the drink. Mentos simply gives it an unusually efficient route out.

Diet soda is often used in demonstrations because it produces a strong eruption and leaves less sticky sugar behind. Results also depend on temperature, bottle shape, carbonation, and how quickly several candies enter.

The geyser is therefore a controlled collapse of an unstable situation. A rough candy suddenly offers billions of gas molecules the surfaces they were waiting for.