back to article Where's the best place to add Mentos to Diet Coke for the most foam? How big are the individual bubbles? Has science gone too far?

Did you know that the popular Diet Coke and Mentos soda geyser experiment works better at higher altitudes? Or that the average size of the bubbles formed on the scotch mints is about 6μm? Now you do, thanks to the wonders of science and those with a bubbling passion for it. A chemistry professor and a high school teacher …

  1. David 132 Silver badge

    Wrong science discipline.

    This isn't chemistry, it's fizz-ics.

    1. Hubert Cumberdale Silver badge

      Re: Wrong science discipline.

      But nobody seems to be investigating the real science issue here: pronunciation. Personally, I insist that it's men-toes, but the marketing fools seem to think it's men-toss. As far as I can see, it's simple: Rolos, Polos, Mentos. The allegedly "correct" pronunciation sounds like a porn genre to me.

      1. Anonymous Coward
        Anonymous Coward

        Re: Wrong science discipline.

        It depends on how you hold it :).

  2. Anonymous Coward
    Anonymous Coward

    The Diet Coke bottles were kept at 27 degrees Celsius (80.6 degrees Fahrenheit)

    Maybe someone who knows a bit of the old science magic stuff could explain the choice of temperature and whether or not different temperatures would change the result.

    1. jonha

      Re: The Diet Coke bottles were kept at 27 degrees Celsius (80.6 degrees Fahrenheit)

      It's probably to do with the fact that you can dissolve more of a given gas in cold water than in warmer water (whereas it's the other way round for solids: the warmer the water the more salt or whatever you can dissolve in it).

      1. bombastic bob Silver badge

        Re: The Diet Coke bottles were kept at 27 degrees Celsius (80.6 degrees Fahrenheit)

        the curve for dissolved gas in water isn't linear. The maximum point is around 2 deg C, with a minimum of around 60C (as I recall), but then it increases again after that, assuming you apply enough pressure to the water to stay liquid, so that very hot water under pressure can hold onto even more dissolved gas than 34 degree water at 1 atmosphere pressure. Or something like that. And according to a few online references, several types of gasses have different curves and minimum points. In general, however, it follows the pattern I described. But none of the online references bothered to extend the curve past the boiling point at sea level atmospheric pressure, because they're not concerned about managing the water chemistry of a pressurized water nuclear reactor...

  3. Antron Argaiv Silver badge

    efficient insertion of Mentos is key

    I built a screw on adapter out of a bottle cap, with a drilled out centre and attached a vertical plastic tube witha a small transverse hole for a metal pin. Screw on adapter, insert pin, load tube with Mentos. Stand clear, pull cord to remove pin.

    The resulting geyser was captured on video for the wife's middle school science class.

    It worked so well, some kid stole it when she demoed it at school!

    1. bombastic bob Silver badge

      Re: efficient insertion of Mentos is key

      if I could do an instantaneous release I could build a soda bottle rocket with the mentos-diet-coke reaction.

      The idea would be to cap the bottle with the mentos [and release mechanism] inside, using a 'rocket engine' cap [of my own design], and use some kind of pull-string arrangement (through the nozzle). Then you pull the string to launch it. I've already got a nice nozzle design, complete with fin/legs to spin it, as OpenSCAD files that can be 3D printed, if anyone wants to look on github for them. Maybe it could help El Reg launch another space plane?

      1. John Brown (no body) Silver badge

        Re: efficient insertion of Mentos is key

        You'll need a very big bottle to get enough usable thrust. As a kid I used washing up liquid bottles partially filled with water and a bicycle valve jammed into the nozzle. Highest I got was over the roof of our two story house.

      2. Neil 32

        Re: efficient insertion of Mentos is key

        They tried the diet coke/Mentos/rocket idea on MythBusters a few years ago. I seem to recall there really wasn't enough thrust to make it work; plus lots of design difficulties in getting something that would work

        1. Dolvaran

          Re: efficient insertion of Mentos is key

          Try an alternative propulsion system. Bleach + Harpic works extremely well.

  4. chivo243 Silver badge

    Cool waste of time

    Cool that they got to see new places, but aren't there lots of variables that were ignored? Moisture content of the mentos in different altitudes and temperatures. Did they buy all of the soda and mentos at the same time, same batches? And as mentioned above, why that specific temperature at different altitudes?

    I know enough about scientific process to ask dumb questions...

    1. bombastic bob Silver badge

      Re: Cool waste of time

      soda fizzing depends on the temperature of the soda due to solubility of CO2 in water and the equilibrium reactions involved. So it needs to be the same in each experiment. The Chem teacher knew this, of course.

  5. Beau

    Diet Coke?

    Yeah well, it doesn't sound as much fun as we had as we had, back around 1958. All we has to do was drop an aspirin in a bottle of coke, to have fun. fun, fun. Oh never mind.

    1. Alan Brown Silver badge

      Re: Diet Coke?

      "All we has to do was drop an aspirin in a bottle of coke"

      not to mention smoking the collected stringy bits that show up when peeling bananas

  6. Mike 16

    Been wondering...

    Why specifically "Diet" coke?

    Is there something about Sugar (or HFCS, or Manitol, or...) that changes the behavior?

    As David 132 remarked, this is more a physics thing than a chemical reaction, so I'd expect, e.g. Seltzer to work as well. So why a particular witch's brew of artificial sweeteners?

    Yes, I can understand the benefit to society at large from one less bottle of that vile brew finding its way into a human digestive system.

    1. Methusalah

      Re: Been wondering...

      My 12 year old grand-daughter recently investigated this as her science project.

      Evidently the type of sweetener used in Diet Coke is Aspartame.

      So she dissolved increasing amounts of a standard aspartame based sweetener into bottles of lemonade.

      Then dropped a standard sized Mento into the mixture and measured the height of the resulting foam plume.

      Her conclusion: The greater the concentration of aspartame in the solution, the higher the resulting plume.

      It maybe that the nucleation is a physics effect, but the actual constituents of the solution can either increase or decrease the speed of the reaction

      1. chivo243 Silver badge

        Re: Been wondering...

        Your 12 year old g-d has more scientific knowledge than most adults, good for her! I consider myself one of most adults too!

      2. Chris G

        Re: Been wondering...

        It may be that because aspartame is not very soluble in water the larger concentrations were mostly particulate and so provided more nucleation sites that aided faster bubble formation once the mento had kicked it off.

    2. GBE

      Re: Been wondering...

      Why specifically "Diet" coke?

      Based on informal comparisons of side-by-side pouring regular and diet sodas into glasses with ice cubes, I'm convinced that diet soda foams quite a bit more than regular.

      I don't know why. Perhaps the high levels of dissolved sugar in regular soda increases surface tension and therefore decreases bubble size? Or perhaps the solubility of CO₂ as a function of temperature differs?

      1. Methusalah

        Re: Been wondering...

        From my grand-daughter

        As far as she understands ---Her caveat, not mine!

        Artificial sweeteners such as Aspartame, act in lowering the suface tension of the liquid they are dissolved in, so producing larger bubbles and more rapidly. So more foam is produced by diet versions of both coke and lemonade as they use artificial sweeteners rather than natural sugar.

        She says the foam is mainly Co2 bubbles surrounded by a thin layer of the liquid, held around the bubble of gas by the liquids own surface tension.

        She also points out that increases in temperature of the drink will also reduce the surface tension.

        To quote her... " The warmer the pop the more the molecules in the liquid bop around, so reducing the attraction between the molecules"

        I hope this helps

        1. bombastic bob Silver badge

          Re: Been wondering...

          take a lesson from a 12 year old - explaining otherwise complex concepts in terms that mortals can understand [and having fun with it]

      2. Neil Barnes Silver badge

        Re: Been wondering...

        Why specifically Diet coke? There's a practical reason: diet coke isn't sticky when you come to clean up the mess, in the way that sugar-sweetened coke is...

        Disturbing lack of coke icon --->

        1. KittenHuffer Silver badge

          Re: Been wondering...

          Are we talking a pint of coke? Or a line?

      3. Alan Brown Silver badge

        Re: Been wondering...

        "I'm convinced that diet soda foams quite a bit more than regular."

        It does.

        Carefully open a can of (unshaken) diet coke

        Carefully open a can of (unshaken) ordinary coke

        Now pick each one up in turn and slam the bottom down _hard_ on a tabletop (keep it upright, you're not trying to spill or crush anything)

        Compare the resulting mess

        By the way, airline cabin staff passionately HATE people who ask for diet coke at 30,000 feet because the stuff is more likely to make a mess and takes longer to pour/settle

        Diet Pepsi can be just as bad. I've had a 2 litre bottle jet away across a carpark after being dropped. The shock and gas release was enough to blow the cap off and it went about 50 metres

    3. Neil 32

      Re: Been wondering...

      Do a search for the MythBusters episode that covered the Diet Coke/Mentos phenomenon. They did tests on the various ingredients and comparisons of different carbonated drinks.

  7. diver_dave

    To protect...

    The victim of his own (suggest suitable word here)

    A colleague of mine once tried dissolving solpadine in diet coke.

    This resulted in much swearing, soaked laptops and three of us announcing we were going for lunch and would, maybe, be back later depending on how long it took him to clean up.

    Effervescent tablets plus coke = wooshh

  8. DJO Silver badge

    Doing the hard way

    Don't they have low pressure chambers in America?

    By climbing a mountain to do this it's tricky to split the effects of lower air pressure and lower gravity at altitude, a pressure chamber would eliminate one of the variables.

    1. martinusher Silver badge

      Re: Doing the hard way

      A lot of the 'intermountain West' is elevated so its actually a lot easier to just go visit someone in Albuqueqeu (5000') than find a pressure chamber, especially as you're going to find supermarkets that sell both Diet Coke and Mentos.

      One quirk of living in the US is that if you buy a box of sponge mix or something like that you'll find 'high altitude' instructions for cooking it along with the normal instuctions. High altitude means anything over 5000' and you really need to adjust those receipies. Even brewing a pot of tea is subtly different -- a decent cuppa needs proper 100C boiling water.

      1. Anonymous Coward
        Anonymous Coward

        Re: Doing the hard way

        If you get all the water in your kettle to 100C it'll probably detonate

    2. Kevin McMurtrie Silver badge

      Re: Doing the hard way

      In California you can easily cover -280 feet to +10000 feet in day of driving, or even more if you don't mind a hike. The Sierra Mountain range is pretty much why all-season tires exist.

  9. G R Goslin

    I suppose....

    I suppose someone had to introduce a bit of greeen propaganda. You really should not add assumptions as data into science, as in the latter part of the comment.

    "Therefore, discussing the results of my Coke and Mentos experiments provides a great platform to introduce students to various impacts such as ocean acidification associated with increased CO2 in the atmosphere from burning fossil fuels.” ®

    1. Anonymous Coward
      Anonymous Coward

      Re: I suppose....

      greeen propaganda

      Why the extra "e"? Is it an evergreen?

    2. phuzz Silver badge

      Re: I suppose....

      None of that is an assumption.

      Dissolved CO2 in water makes it more acidic (in the case of the sea, it's making it less alkali, the average is around pH 8.1, rather than making it acidic per se).

      There is an increase in CO2 in the atmosphere, from burning fossil fuels.

  10. Dr_N

    Old Skool is best

    Grape fireworks in the microwave is way more satifying with less clean-up needed.

    Internet circa 1995 iirc.

  11. Fedup

    Back in the old days

    I seem to remember we used to use Polo mints instead of Mentos .

    1. Is It Me

      Re: Back in the old days

      Mint imperials work very well, in fact from the unscientific testing I did a few years ago they work better than Mentos.

      And cheap diet cola also works at least as well as name brand diet cola

      1. Anonymous Coward
        Anonymous Coward

        Re: Back in the old days

        Mint imperials work very well

        Especially after Brexit. None of this metric stuff..

      2. Anonymous Coward
        Anonymous Coward

        Re: Back in the old days

        cheap diet cola also works at least as well as name brand diet cola

        My preference for using diet coke for this has more to do with the fact that I want to preserve the good stuff, with proper sugar. Not a fan of aspartame and yes, I understand the irony of still wanting to drink coke in that context :).

  12. richdin

    Worthy of ig-noble award...

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