Why does Diet Coke make a higher geyser than carbonated water?

Answer 1

In nearly every case, the warmer a liquid is, the more room it takes up (that is, the volume increases as the temperature rises). So whatever container the soda is in before being released as a geyser is under more pressure if the soda's warm, because there is more volume trying to fit into the same space. The greater the pressure, the faster it will shoot out once released, and the faster it shoots out, the higher it will go.

Answer 2

Mostly because the Mentos provide nucleation sites for the dissolved carbon dioxide in the Diet Coke to escape as a gas. Other active ingredients in the cascade-effect reaction include Aspartame (artificial sweetener), potassium benzoate (preservative), and caffeine in the Diet Coke; and gum arabic and Gelatin in the Mentos. The ingredients seem to have a perfect compatibility with each other and, when mixed together and added to the nucleation, creates a chemical reaction that forces the soda to release all of its dissolved carbon dioxide at once, thus causing a more violent eruption than carbonated water alone.

Answer 3

As a rule of thumb, as you increase the temperature, solids become more soluble (so if you want to dissolve salt in water, heating it helps) but gases become less soluble.

The lowered solubility means it's easier for the dissolved carbon dioxide to escape, causing the soda to "go flat."