Why Do Mentos and Coke Explode? The Science of the Soda Fountain

It is one of the most famous science demonstrations on the internet: drop a few mint sweets into a bottle of cola and a fountain of foam blasts out, sometimes several metres high. Many people assume it is a chemical reaction between the sweets and the drink. In fact, it is something simpler, and more interesting: physics.

A bottle full of gas

Fizzy drinks are made by forcing carbon dioxide gas into the liquid under pressure. While the bottle is sealed, the gas stays dissolved. When you open it, the pressure drops, and the gas starts to escape as bubbles. But it does so slowly, which is why a drink can stay fizzy for a while after opening.

Why so slowly? Because a new bubble is hard to start. To form, it needs a tiny pocket where gas can gather, called a nucleation site. Scratches in a glass, dust or ice cubes can act as nucleation sites, which is why bubbles often stream from the same spots.

Mentos: a bubble factory

Under a microscope, the surface of a Mentos sweet looks nothing like the smooth candy it seems. It is covered in microscopic pits and bumps. Each of these is a perfect nucleation site. Drop a Mentos into cola and thousands of bubbles start forming on its surface at once.

Mentos are also dense, so they sink quickly to the bottom of the bottle. As they fall, they release gas all the way down. The bubbles rise, push the liquid upwards and turn it into foam, which has nowhere to go but out of the narrow neck as a powerful jet.

What the experiments showed

In 2008, physicist Tonya Coffey and her students at Appalachian State University tested what makes the fountain work. They tried different drinks, different sweets and other materials, measured how high the fountains went and examined Mentos under an electron microscope.

Their findings included:

• Surface roughness is the key. Smooth, coated sweets produced much smaller fountains, while rough materials, even sand, worked.
• Diet cola gave the biggest eruptions. Its artificial sweetener, aspartame, and the preservative potassium benzoate lower the surface tension of the liquid, making it easier for bubbles to form and grow.
• Warm cola erupted higher than cold, because warmer liquids hold less dissolved gas.
• The amount of caffeine had little effect, and there was no sign of a real chemical reaction: the drink’s ingredients were essentially unchanged afterwards.

Is it dangerous?

The popular rumour that eating Mentos while drinking cola could make your stomach explode is a myth. In your mouth and stomach, the drink is no longer in a sealed bottle, it is warmed and mixed with saliva, and there is no narrow neck to build up a jet. Any extra gas escapes as burps. The demonstration itself is best done outdoors, standing back, because it is messy.

In short

• The Mentos and cola fountain is a physical reaction, not a chemical one.
• Cola contains carbon dioxide dissolved under pressure.
• Bubbles need nucleation sites; a Mentos surface has countless microscopic ones.
• The dense sweets sink fast, releasing gas throughout the bottle.
• Diet cola works best because its ingredients lower surface tension.
• Eating Mentos with cola is not dangerous; the gas comes out as burps.