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Glow in the Dark Mentos and Tonic Water Fountain


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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... a lot less tasty).

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