Warning: Diet Coke + Mentos = messy lab

We will never forget the first time we discovered this unique experiment. We learned three particularly important things that we have never forgotten to this day. Firstly, we learned that if you mix Mentos and Diet Coke, you get a violent eruption that will spew a gallon of sticky cola on you before you can even blink an eye. As we passed through the lab animal house, on the way to the kitchen to clean up, we learned something else – rats love Diet Coke! Before we knew it, we had 50 of those little furry critters clinging to our lab coats, licking away at the sticky mixture that had drenched our jackets (and they jumped us from behind knowing good and well that we wouldn’t be able to identify who did it). And finally, we learned that nothing is funnier than watching fifty jittery, burping rats pointing at each other and trying to look innocent.

Read this BEFORE conducting the Diet Coke and Mentos eruption experiment

Before you go any further, please make sure you conduct this experiment outside! This experiment can be very messy. If by some chance, you skipped this paragraph and conducted the experiment in your living room, Geek Slop begs you, please tell your parents you got this experiment from Ben Frye the Science Guy’s web site.

Diet Coke and Mentos produce a physical, not chemical reaction, due to “nucleation sites”

The reaction we’ll describe below is purely physical, not chemical. Some liquids like soda can be supersaturated with gas, in this case, carbon dioxide. The saturation occurs when the liquid is in equilibrium with the gas. When the liquid is opened, the equilibrium is disturbed, and the carbon dioxide comes out of the solution.

The surface of a Mentos candy magnified 500x

A nucleation site is a spot where the gas bubble can form, and it can be any surface that is scratched, has specks of dust, or has a high surface area. When you pour soda into a glass, the carbon dioxide in the soda comes out of solution and forms bubbles due to the nucleation sites. The nucleation sites are usually the sides of the glass, which allow the carbon dioxide to release as bubbles and fizz.

The surface area that the soda contacts when poured into a glass is relatively small, equivalent to the area that the soda touches as it pours into the glass. However, pouring the soda rapidly increases the surface area, causing it to come into contact with more surface area, thus releasing more carbon dioxide and producing more fizz.

This phenomenon is also the reason why shaking a can of soda before opening it causes it to fizz more. The shaking causes the carbon dioxide to come out of the solution, and when the can is opened, the carbon dioxide is released as bubbles due to the nucleation sites.

How to conduct the Diet Coke and Mentos experiment to get the best reaction

Stand the diet coke bottle upright and unscrew the lid. Other soda types will work but Diet Coke works the best because sugar molecules in regular sodas help keep the carbon dioxide in solution, slowing the reaction.
Put a funnel or tube into the spout.
Take a half pack of Mentos and pour it into the funnel.
RUN as fast as you can (screaming while you run adds a bit of excitement to the effect).
A geyser of Diet Coke will come flying out of the bottle – very impressive to see. The world record is almost 3 stories high!

But why Mentos?

The primary reason for the effect has to do with nucleation sites and the extraordinary surface area of Mentos candy. Mentos have a very high surface area for their size and hence, a tremendous number of nucleation sites. The have dimples and hundreds of nooks and crannies that if you could spread them all out, would equal a much larger surface area than just the little piece of candy would seem to have. So with Mentos, you can pack a lot more surface area, or nucleation sites, into a very small space. Also, since Mentos are heavy, the sink to the bottom and react with the soda all the way down.

Dropping anything into the Diet Coke speeds up the release of the carbon dioxide process by both breaking the surface tension of the liquid and also allowing bubbles to form on the surface area of the object. Anything will work but Mentos candy pieces are covered in tiny dimples (a bit like a golf ball), which dramatically increases the surface area and allows a huge number of bubbles to form.

Diet Code and Mentos experiment advanced notes

Coke like many other soft drinks are carbonated. This is what gives the drink it’s fizz and what gives it that lively taste. Coke is bottled under pressure to ensure that the proper levels of carbonation are present when the drink is opened. When you open the bottle the excess gas rushes out of the bottle and a hissing sound is heard. The dissolved gas then slowly bubbles out of the drink. Eventually all of the gas comes out of the solution and we say that the soda is “flat” and tastes “Flat”.

Incidentally, this need to pressurize the soda containers is why it wasn’t until the 1960’s that you say soda come in cans. Many attempts were made before that date, but no one could figure out how to economically make a can like container that could withstand the internal pressure. If you find cans from the 1960’s you will see that they are much thicker than those today. Technology over the years has allowed for the fabrication of much lighter cans that can withstand the pressures.

Likewise, early construction methods for forming glass bottles was not as sophisticated as it is today. Hence, if you look at an old Coke Bottle, you will immediately notice that they are much thicker and heavier that today’s bottle. You will also notice that with today’s plastic bottles, if you happen to get a newly made bottle, will have a lot more gas in it when you open it. This is because the gas can slowly escape through the pores of the plastic bottle. The bottlers charge the bottle with enough gas so that it will yield a sparkling drink even at its expiration date. Keep the bottle in the garage for a year or so and when you open it will be flat. Not so with bottles and cans, unless they start to leak, they will retain their fizz for years.

More about nucleation sites

Nucleation sites are locations where nucleation (the formation of new crystals or particles) is most likely to occur. The presence of nucleation sites is essential for many processes, such as crystal growth, precipitation, and condensation.

Nucleation sites can be of different types, depending on the nature of the process. Some common examples of nucleation sites include:

Surface irregularities: These can be defects, scratches, or impurities on the surface of a material, such as a solid or a liquid. Surface irregularities provide sites for the formation of new crystals or particles by lowering the energy barrier for nucleation.
Heterogeneous particles: These are foreign particles that are present in a liquid or gas. Heterogeneous particles can act as nucleation sites by providing a surface for attachment and growth of new crystals or particles.
Homogeneous nucleation: This occurs in the absence of any foreign particles or surface irregularities. Homogeneous nucleation can occur in supersaturated solutions or in vapor-phase systems, where the concentration of the solute or vapor is high enough to overcome the energy barrier for nucleation.

Required supplies for the Diet Code and Mentos experiment

Supplies: Soda or Coke, Mentos candy

Image Credits

In-Article Image Credits

Diet Coke and Mentos geyser via Wikimedia Commons by Michael Murphy with usage type - Creative Commons License. February 19, 2007
Mentos Geyser with carbonated water (Perrier), Classic Coke, Sprite and Diet Coke via Wikimedia Commons by K. Shimada with usage type - Creative Commons License. September 9, 2007
The surface of a Mentos candy magnified 500x via Wikimedia Commons with usage type - Creative Commons License. July 17, 2017