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Science Investigations that all students should do Before High School, #5: Mentos in Diet Coke or Diet Pepsi

 #5: Mentos in Diet Coke or Diet Pepsi

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Making a geyser using Mentos and Diet Coke is a classic demonstration. We decided make our demonstration into an experiment by comparing the reactions of Diet Coke with that of Diet Pepsi. We used five Mentos with a 2-liter bottle. I taped a 3-foot measuring stick to the bottle to give us an idea of height comparisons.
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I actually could only get 4 Mentos in the Pepsi before it exploded on me.
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As you can see, Diet Pepsi won out on our test by a landslide.


Why does this reaction happen with diet soda and Mentos?
It is caused by something called nucleation. Bubbles of carbon dioxide "nucleate" shortly after the pressure is released from a container of carbonated liquid. Nucleation often occurs more easily at a pre-existing "interface", or a surface forming a common boundary among two different phases such as using string to make crystals form rock candy from a sugar-water solution. The surface of the mint Mentos  is such an interface, as it is covered with many small holes that increase the surface area available for reaction, thereby allowing CO2 bubbles to form with such a rapidity and in such a quantity that a geyser is formed. A 2006 episode of the television series MythBusters reported that when fruit-flavored Mentos with a smooth waxy coating were tested in carbonated drink there was hardly a reaction, whereas mint-flavored Mentos (with no such coating) added to carbonated drink formed an energetic eruption. 

14 comments:

  1. That looks so fun! We'll have to wait until we get warmer weather to try it out (maybe when we go back to the US to be sure of finding mentos, not sure if they have them here).

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    1. If you would like me to send some Mentos to you, just shoot me an email with your address and I would be happy to do it.

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  2. This is still on my list of things to do, but why does soda have to be diet?

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    1. The jury is still out on that question. Some think it has something to do with diet sweeteners that we don't understand. Some think it is because there is more carbonation in diet sodas. BTW...nucleation occurs with just about anything, not just Mentos, it is just because there are so many tiny surfaces on the Mentos that can be used.

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  3. We have tried this so many times and with absolutely no success! I'm not sure what we do wrong. The fun now is in the teasing that follows every failure!

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    2. Do you use mint Mentos? The fruit flavor ones are coated with wax and don't work.

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  4. Love the videos - they really capture the fun of this demonstration! I read somewhere that you can use regular soda, you just end up with less of a sticky mess everywhere if you use diet.

    Claire - we invested in a Steve Spangler Geyser Tube for £3.50 on Amazon. They do a great job of fast-loading the mentos and maximise your chances of the fun working. :-)

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  5. Love this! But I have to say that I love your kids reactions the best. Can't wait to try this with my boys!!!

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    1. They are enthusiastic, aren't they? LOL

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  6. We've never done this one! I've got to put it on our list of to-dos!

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    1. You will love it, Bethany.

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  7. I really need to do that one. We've done a couple of variations on that, but never the diet coke and mentos.

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    1. What variations have you done? What did you use?

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