Wednesday, 9 January 2008

Mentos and Coke.... But Why?


Talking to my kids about Mentos and coke eruptions recently, and found this on a web site... this may be the ULTIMATE Mentos and coke performance on the net.

The kids always say wow... and then... WHY? Why does Mentos and Diet Coke make such an explosion, does it have to be diet coke?

The answer is.... well actually there are several possible answers and no one is sure exactly which is the dominant factor. There are some things we think we know, so here is what I think I understand about the eruptions now:

When they bottle pop, the expose the liquid to a highly pressurized Carbon Dioxide (C02) gas, that is what puts the "fizz" in soda pop. When the bottle is capped, the molecules of CO2 are trapped in the liquid and because of the surface tension, they can't expand. Now if you shake one up, and pop the lid, the shaking has broken the surface tension enough to let the molecules of CO2 start to expand and you get a blast all over the place, the expanding molecules take up more space, pushing out the soda pop. But if you open it gently, the surface tension is only mildly disturbed near the top of the bottle or can, and a few thousand molecules expand and rise to the surface or foam up in the bottle.

Two things seem to be the most important factors, surface tension, and the ability of the carbon dioxide molecules to get close together. If you drop salt, marbles, sugar, or most anything else into the pop, it will start to fizz. If the material has soap or fats that break down the surface tension of the liquid, you get more bubbles. And if the material has lots of corners and crevaces for the molecules of CO2 to hook onto, then you get more (bigger) bubbles. This idea is called nucleation If the surface is jagged or irregular, it encourages this nuclaeation, and more tiny molecules of gas expand.

Mentos is good in both aspects, the gum arabic and gelatins in the coating disolve and promote lowering of the surface tension in the liquid. In addition, the candy itself is covered with tiny little depressiions, sort of like a golf ball. They provide a prime source of nucleation sites. Add one more quality to help make a big eruption, the Mentos are heavy. Well heavy enough that they sink quickly to the bottom of the bottle. If they set at the top and expanded, there would be a rush of gas out of the bottle, but mostly invisible. At the bottom, the expansion of the gas is so quick that as they press outward, the cola can not get out of the way and so it is pushed right out the top and up into the sky.

Does it have to be diet. Some say diet is much better, maybe something in the artificial sweetener. Others claim the only difference is that the non-diet soda is a sticky mess to clean up, and there is really no difference in the pressure of the eruptions. Heat seams to make a difference, hot cola shoots higher than cold cola.

So what else might effect it? Go ahead, but a couple of liters of Diet Coke, and a couple of packages of Mentos and have a little experiment. Make a mess, have a ball... and if you learn something along the way... well, that's just the price you pay.
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