Mentos + 2 liter soda = explosion?

October 13, 2005 3:38 PM

Why does this work?

Take a two-liter bottle of soda. Put 13 Mentos in a test tube. Tip the test tube over the open bottle and put a piece of paper between the test tube and the bottle. Pull out the paper so that all 13 Mentos fall quickly into the bottle of soda. Almost all of the soda will shoot up and out of the bottle. Why does this happen? The guy kind of explains it, but I don't get it. I'm definitely trying it!!

posted by clh to Science & Nature (11 answers total)

according to the site itself: "Mentos contains a chemical known as ARABIC GUM (this is the ingredient that makes the mint "chewy"). This ingredient causes the surface tension of the water molecules to break even more easily, releasing more carbon dioxide gas at an astounding rate!"

I buy it.
posted by allen.spaulding at 3:45 PM on October 13, 2005

Another Pepsi Boom post.
posted by Capn at 3:56 PM on October 13, 2005

This explanation does a better job of explaining the phenomenon.
posted by O9scar at 4:05 PM on October 13, 2005

I seriously doubt that this effect has anything to do with arabic gum, or any other specific ingredient in Mentos. The addition of the mints is just increasing the number of nucleation sites for the formation of CO2 bubbles. Increase the numbers of places that bubbles can form, then bubbles form faster. Although I haven't tried it, any chemically rough surface should do this.

The insides of soda cans these days are coated with something to reduce the 'rough' edges that are present on the surface of metal. This decreases the number of nucleation sites, so that your soda doesn't explode out of the can when you open it. Before those coatings were used, shaking the soda would have a very noticable effect when you opened the can. This is where the myth of 'tapping' the top/bottom of the can before opening it came from. Knock the bubbles stuck the sides and bottom of the can off, and when you open the can, you won't have as much beverage pushed up with those bubbles as they escape the can.

So this likely has nothing to do with Mentos. Try a sugar cube, or just sand - both should increase the fizz factor. Though I wouldn't want the latter in my beverage.
posted by istewart at 4:41 PM on October 13, 2005

Wouldn't baking soda have the same effect? IStewart seems right on.
posted by acoutu at 6:53 PM on October 13, 2005

I've seen this done with a packet of lifesavers. Baking soda would probably have the same effect, and I guess that salt or anything providing nucleation sites would do it.

Every taken a sip of fizzy drink while eating something sugary?
posted by tomble at 8:29 PM on October 13, 2005

I dunno... but it's AWESOME!
posted by spilon at 8:32 PM on October 13, 2005

Baking soda would have the same effect, but for a different reason. Sodium bicarbonate, when added to an acidic solution (like vinegar, or soda) will form CO2 gas via a chemical reaction. The situation with mentos is not technically a chemical reaction; the added nucleation sites just increase the rate that CO2 bubbles can form (from CO2 already dissolved in the pop), no additional CO2 is made in this process. Try adding baking soda to orange juice; it would proably still fizz. If you add mentos to orange juice, all you'll get is that nasty orange juice / toothpaste flavor combination, no bubbles.
posted by istewart at 10:37 PM on October 13, 2005

istewart is correct.

As a chemist, I don't buy the gum/glycerine solution lowering the surface tension argument. While they certainly will have the effect if present in large enough quantities, neither dissolve particularly fast, especially in a fairly strong highly-charged (ionic) solution like soda water. So, I think it's purely a physical nucleation thing.

Btw, aluminum cans are amazing feats of engineering. Scientific American has a great article on them in the September 1994 edition, unfortunately only available at lunatic rates on-line.
posted by bonehead at 6:48 AM on October 14, 2005

I always found that drinking Coke while eating a candy bar with nougat (Snickers, Three Musketeers, et al.), caused it to foam up in your mouth, much to the delight of my high school cafeteria table.
posted by patgas at 6:50 AM on October 14, 2005

IT'S MICROSCOPIC SEED-BUBBLES! I've found that several chem references contain a widespread misconception: the (wrong) idea that bubbles can be nucleated by "roughness." In fact, rough surfaces can only provide nucleation sites if the surface is dry. The actual nucleation sites are gas pockets trapped in the rough spots. They're tiny bubbles. "Seed bubbles." The nucleation sites for bubble formation are always just smaller bubbles.

It makes sense if you think about it: to nucleate a condensing fog droplet we need a solid or liquid seed, and to grow a crystal we need a tiny solid seed crystal. To grow a bubble we need to start out with an existing gas/liquid interface: a tiny seed bubble. "Roughness" doesn't do it. Neither do chemical additives.

This "roughness" misconception is not so benign, since safety concerns involving explosive boiling are present. Especially important is safety involving the explosive boiling of microwaved coffee. If we believe in "roughness" and we don't understand the role of seed bubbles, we can end up injuring ourselves.

Chemists are aware of the hazards present when water is boiled in smooth glass containers, and for this reason they often use some "boiling stones." These small object provoke a roiling boil, and therefore they force the water to boil at the usual temperature, rather than letting the water superheat and then spontaneously boil violently. Yet previously-used "boiling stones" are unsafe. They will not work unless they've been allowed to thoroughly dry. Why? It's because the rough surface of a well-dried boiling stone will trap tiny pockets of air, and these pockets still exist when the boiling stone is immersed. The air acts as "seed bubbles" which provoke normal boiling. The tiny pockets fill with steam during boiling, and if the "boiling stone" is cooled and removed from the water, the steam shrinks and condenses, so the gas pockets collapse and vanish. The wet surface of a used boiling-stone is still rough, but it contains no seed bubbles.

Knowing the role of seed-bubbles, we can easily predict a danger with microwave ovens. If we boil our coffee twice, it will superheat the second time, and superheated coffee can "explode" unexpectedly. The phenomenon resembles that with boiling-stones: by boiling the coffee the first time, all the microscopic air pockets on the walls of the cup will be filled with steam. If the coffee is allowed to cool, these "seed bubbles" collapse and vanish. If the coffee is heated a second time, it might not boil normally. Instead no bubbles appear, and the temperature can rise well over 100C degrees (The curious can easily verify this event with an IR thermometer.) At high enough temperature the unstable liquid will boil explosively when disturbed. To be safe with microwave ovens, avoid boiling coffee twice, and use a wooden stir-stick as a "boiling stone" to prevent dangerous superheating.

Here's some seed-bubble theorizin'.

If we allow some Mentos to sit for awhile in a small bit of warm water, the dissolving sugar should release a huge number of invisibly small seed bubbles. There should be far more seed bubbles than are normally released during the few seconds the Mentos are in the bottle of cola. So, if we use a syringe to collect and inject this Mentos-water into the bottom of a bottle of cola, the "mentos effect" should be much greater than normal.
posted by billb at 8:06 PM on July 3, 2006