Most of us have seen a Coke geyser either at a chemistry demonstration or on Youtube. You can easily do this yourself at home although we recommend using the backyard if you are going to be shooting off coke geysers. The idea is to take several Mentos mints and drop them into a bottle of diet Coke. The result is a huge eruption of diet Coke. How does this work? Is this a chemical or a physical process? Let’s investigate what happens when the mints are dropped into a bottle of diet coke. We can use other sodas, but you will get more exciting geysers when using diet Coke–it is also easier to clean up since there is no sticky sugar mess. The height of your geyser will depend on the number of mints you add to the coke. In the picture above we only put 2 mentos in the coke bottle and ended up with a 5 foot geyser. If you add about 10 mints, the geyser can go up to about 30 feet which is why it is best to do this outside. We advise using a rolled up piece of paper to insert the mints into the coke bottle. The mints must be all dropped in at once. You can also purchase tubes for the mints at Steve Spangler’s web site. They also include a package of mentos for about $4.99. The tubes can be used over and over again. Temperature also has to be considered. This works beautifully on a warm day but not on a very cold day. We tried doing these outside on a cold December day, and there was only about a 4 inch geyser–not very impressive.

Now that we have talked about the how, let’s talk about why this works.

Each Mentos mint has many thousands of small pores on the surface which interfere with the polar attractions between water molecules—the surface of the Mentos mint is very porous. This results in thousands of nucleation sites for the carbon dioxide gas molecules to congregate. The CO₂ bubbles begin to rapidly form on the Mento’s surface. Eventually the growth (and buoyancy) of the bubbles will cause the bubbles to leave the sites of nucleation and rise to the surface of the diet Coke. The carbon dioxide bubbles continue to form on the surface of the Mentos mint and this process repeats over and over causing a Coke geyser.

There are additives in the soda, called surfactants, that lower the surface tension of the soda which is why diet Coke is the preferred soda for this demonstration. You will get a more exciting geyser from diet soda than from a soda that contains sugar. In addition you will not have to clean up a sticky mess. If you want to increase the size of your geyser, add a bit of dishwashing soap to the diet Coke.

Can we use other candies for our geyser? Yes, but the geyser will not be anywhere near as dramatic as when you use Mentos. The Mentos mints are very dense and sink to the bottom of the Coke quickly. If you use a less dense candy, it will not sink as low and will result in a very small geyser. Try crushing the Mentos and you will see that it makes a big difference in geyser size.

Again, temperature is important. Recall that gases are not as soluble at higher temperatures because the gas particles have a higher kinetic energy–they move faster. If we attempt our Coke geyser on a very cold day, we will not get much of a geyser. This is because the CO₂ is much more soluble at colder temperatures because of the decrease in kinetic energy. At higher temperatures, the gas wants to escape the liquid resulting in a much more exciting geyser.

Now that we have a basic idea of how it works we can answer the second question. The coke geyser is a physical process because there are no new substances formed.

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