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We all might have seen this in some videos or in real life. But how exactly does this work? What is the chemical reaction involved?

And why is the reaction of Mentos with Diet Coke more vigorous?

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From left to right: action of five Mentos candies (per bottle) with Perrier, classic Coke, Sprite and Diet Coke

Image source

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It's actually not a chemical reaction, but a physical reaction. The surface of mentos acts as a nucleation site for the dissolved carbon dioxide in soda, allowing the gas to come out of solution quite rapidly. This rapid release of gas is what causes the coke geyser. Despite mentos appearing to have a smooth surface to the naked eye, there are actually a multitude of microscopic crevices, providing a large amount of nucleation sites. As far as diet coke working better than regular coke, I am unsure (perhaps diet coke has more carbonation? Or perhaps there are chemicals in diet coke which make it form foam much better than the other sodas). I always thought that it was the mint vs fruity mentos that made the largest difference (mint seems to have a lot more nucleation sites which explains the larger geyser in comparison to fruity mentos).

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    $\begingroup$ I will also note that Diet Coke seems to work better than other diet colas. $\endgroup$ – Ben Norris Aug 21 '16 at 0:05
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    $\begingroup$ Diet drinks probably work better for two reasons. One is that they are less viscous than sugary drinks (which can be 10% sugar by mass making the liquid much more viscous than water; diet drinks have fractions of a % sweetners). The other is that some additives (especially aspartame) act as surfactants stabilising bubbles when they form. $\endgroup$ – matt_black Aug 21 '16 at 11:45
  • $\begingroup$ Complete speculation: perhaps one of the ingredients in Diet Coke acts as a catalyst for the conversion of $\ce{H2CO3 -> CO2 + H2O}$, and thus more of the carbonation is able to effervesce to form foam? $\endgroup$ – hBy2Py Sep 1 '16 at 15:04

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