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Diet Coke & Mentos Geyser: Physical vs Chemical Changes

You will need:

- 1 bottle of Diet Coke
- 1 packet of Mentos
- 1 Nail or small hand drill
- String and scissors
- An area outside of your home!

Copyright

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. 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 solution extremely quickly, courtesy of the many nucleation points on the Mentos surface.

Try different varieties of carbonated drinks.
Do they all work the same?

By the way, yes, we are aware that there are carbonates within the Mentos that will react with the acid in the soda to form carbon dioxide bubbles. This is effect is minimal compared to the actual physical reaction which is far larger.

The moral of the story:
It's not a chemical reaction unless the reactants are changed and a new product is formed.

Instructions

1. Drill a small hole through the bottle cap. Pour out the first 5cm of Diet Coke for extra working room.

2. Tie the 4 Mentos together with string, leaving a 10cm length of string trailing off the mentos bundle.

3. Thread the trailing string length through the bottle cap.

4. Pull the string tight, so that the Mentos bundle is up against the lid. Dont let go of this string yet!

5. Carefully screw the lid onto the bottle, making sure that the Mentos do not touch the Diet Coke.

6. Find an area that can get wet. When you're ready, release the string!

7. Read below to find out how it works.

This original experiment was first designed by Steve Spangler, a science communicator based in the USA.

There are now many different versions of the experiment, the one described is our favourite and is demonstrated in the movie to the right.

We also have the connectors for running this experiment on our online science store.

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