Glow in the Dark Mentos and Tonic Water Fountain

Glow in the Dark Mentos and Tonic Water Fountain

setting up a glowing Mentos and tonic water fountain.
Eric is getting ready to drop a roll of Mentos candies into an open bottle of diet tonic water. We used diet tonic water so the resulting glowing fountain wouldn't be sticky. You can use any black light. Try to avoid getting it wet. Anne Helmenstine

This easy science project works just like the usual Mentos™ and soda fountain except you use tonic water as the soda and you shine a black light (ultraviolet lamp) on the fountain to make it glow. Here's what you do:

Glowing Mentos Fountain Materials

  • roll of Mentos candies
  • bottle of tonic water or diet tonic water

black light

It doesn't matter whether you use regular tonic water or diet tonic water. What is important is that the tonic water lists quinine as an ingredient, since this is the chemical that makes the liquid glow when it is exposed to ultraviolet light. Diet tonic water produces a fountain that is less sticky than the spray from the regular tonic water. The size of the bottle isn't critical. The project works with 20-oz bottles, 1-liter, and 2-liter bottles.

Making Your Glowing Mentos Fountain the Best It Can Be

Glow=in-the-dark fountain!
What do you get when you drop Mentos candies into tonic water that is lit with a black light? Glow=in-the-dark fountain!. Anne Helmenstine

This part is really easy, but it happens fast. The fountain sprays as soon as you slide all of the mentos (at once) into an open bottle of soda. Here's a video of this project, in case you would like to see it in action.

How to Get the Best Glowing Fountain

  1. Remove the label from the bottle of tonic water so that you can see the whole bottle glow.
  2. Turn on your black light. Place it where it won't get drowned in tonic water, which could present an electrical hazard.
  3. The trick to getting a spectacular fountain is to make sure all of the candies drop at once into the bottle. Unwrap your Mentos candies and stack them in a tube made from paper or cardboard. The candies won't fall out of their original wrapper quickly or reliably enough.
  4. Line up the tube containing the candies with the open bottle of soda.
  5. Let the candies fall in.
  6. An alternative is to set a piece of paper or cardboard over the mouth of the bottle. Remove the card when you want the candies to fall.
  7. Use room temperature tonic water. Warm soda seems to fizz a little better than cold soda, plus it is less of a shock when it splashes all over you.

How the Glow in the Dark Mentos & Tonic Water Fountain Works

Photo of the glow-in-the-dark Mentos and tonic water fountain as the spray falls back down.
Here's a photo of the glow-in-the-dark Mentos and tonic water fountain as the spray falls back down. Anne Helmenstine

This fountain works exactly the same way as the original Mentos and soda fountain, except you have a glow from the quinine in the tonic water. The ultraviolet light from the black light excites the electrons in the quinine molecules, bumping them up to a higher energy level. What goes up must come down, which is true of energy as well as liquid from a fountain. As the electrons return to their unexcited state, they release the energy they absorbed from the black light in the form of a photo. Some energy gets lost in the reaction so the emitted photon is less-energetic blue light instead of more energetic ultraviolet light.

As for the fountain itself, before you open the bottle of tonic water the carbon dioxide that makes it fizz is dissolved in the liquid. When you open the bottle, you release the pressure of bottling and some of that carbon dioxide comes out of solution, making your soda bubbly. The bubbles are free to rise, expand, and escape.

When you drop the Mentos candies into the bottle, a few different things happen at once. First, the candies are displacing the tonic water. The carbon dioxide gas naturally wants up and out, which is where it goes, taking some liquid along for the ride. The soda starts to dissolve the candies, putting gum arabic and gelatin into solution. These chemicals can lower the surface tension of the soda, making it easier for bubbles to expand and escape. Also, the surface of the candy becomes pitted, providing sites for bubbles to attach and grow. The reaction is similar to what happens when you add a scoop of ice cream to soda, except much more sudden and spectacular (and less tasty...).