We are often asked how this experiment works when we visit schools.
Many students have come to believe that it is a chemical reaction between the Mentos carbonates and the Diet Coke acids that makes the fountain work… however, it turns out that research findings have turned up a quite different explanation; the reaction is largely physical, not chemical.

Diet Coke has a massive amount of dissolved carbon dioxide in the liquid. When you open a Diet Coke bottle, the gas is released instantly, i.e. bubbles form. How does this happen? The solution is actually super-saturated with gas – the gas would escape the liquid if it wasn’t for the high pressure within the unopened bottle. Opening the bottle allows the gas to be released.

Bubbles can also form on tiny points on the surface of a container, known as nucleation points, or site. When you pour a carbonated drink into a glass, the very fine scratches on the glass cause the bubbles to be formed on the tiny nucleation points along the scratches of the glass.

A Mentos has many, many fine scratches on the surface; a perfect place for bubbles form. Dropping a Mentos into Diet Coke allows the dissolved gas within the liquid to come out of the solution extremely quickly, courtesy of the many nucleation points on the Mentos surface.

Variables to test

More on variables here

• Try different numbers of Mentos… how high can you make it go?
• Try different soda drinks. Which carbonated drink make the Mentos eruption go higher?
• What happens if you use cold vs. room temperature soda drinks?
• Does using different flavours of Mentos produce any measurable difference in how high the eruption is?