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Why Do Diet Coke and Mentos React?

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Combine Diet Coke and Mentos, and the result is explosive—Diet Coke shoots out of the bottle like a miniature, sticky Old Faithful. The reaction is so intense, you can make a rocket propelled by the resulting geyser. But what's the science behind this reaction?

In June 2008, Dr. Tonya Coffey of Appalachian State University and her physics students published a paper on the phenomenon in the American Journal of Physics. They were inspired by a 2006 MythBusters episode that, according to the paper, "did a wonderful job of identifying the basic ingredients in this reaction ... [but] did not sufficiently explain why those ingredients affect the explosion, nor did they provide direct proof of the roughness of the Mentos—a tall order for an hour-long television program." Coffey and her students decided to dig deeper.

It's All About Texture

Coffey and company discovered that the ingredients in the Mentos and Diet Coke and, more importantly, the structure of the Mentos, allow carbon dioxide bubbles to form extremely rapidly. When this happens fast enough, you get a nice Diet Coke fountain. (It’s not just Diet Coke and Mentos that react; other carbonated beverages will also readily respond to the addition of Mentos.)

Each Mentos candy has thousands of small pores on its surface which disrupt the polar attractions between water molecules, creating thousands of ideal nucleation sites for the gas molecules to congregate. In non-science speak, this porous surface creates a lot of bubble growth sites, allowing the carbon dioxide bubbles to rapidly form on the surface of the Mentos. (If you use a smooth surfaced Mentos candy, you won’t get nearly same the reaction.) The buoyancy of the bubbles and their growth will eventually cause the bubbles to leave the nucleation site and rise to the surface of the soda. Bubbles will continue to form on the porous surface and the process will repeat, creating a nice, foamy geyser.

In addition to that, the gum arabic and gelatin ingredients of the Mentos, combined with the potassium benzoate, sugar or (potentially) aspartame in diet sodas, also help in this process. In these cases, the ingredients end up lowering the surface tension of the liquid, allowing for even more rapid bubble growth on the porous surface of the Mentos—higher surface tension would make it a more difficult environment for bubbles to form. (Compounds like gum arabic that lower surface tension are called “surfactants”).

Diet sodas produce a bigger reaction than non-diet sodas because aspartame lowers the surface tension of the liquid much more than sugar or corn syrup will. You can also increase the effect by adding more surfactants to the soda when you add the Mentos, like adding a mixture of dishwasher soap and water.

Size Matters

Another factor that contributes to the size of the geyser is how rapidly the object causing the foaming sinks in the soda. The faster it sinks, the faster the reaction can happen, and a faster reaction creates a bigger geyser; a slower reaction may release the same amount of foam overall, but will also create a much smaller geyser. This is another reason Mentos works so much better than other similar confectioneries: The candies are fairly dense objects and tend to sink rapidly in the soda. If you crush the Mentos, so it doesn’t sink much at all, you won’t get a very dramatic reaction.

The temperature of the soda also factors into geyser size. Gases are less soluble in liquids with a higher temperature, so the warmer your soda is, the bigger your Mentos-induced geyser will be. This is because the gases want to escape the liquid, so when you drop the Mentos in, the reaction happens faster.

What Doesn't Work

While caffeine is often cited as something that will increase the explosive reaction with the soda, this is not actually the case, at least not given the relatively small amount of caffeine found in the typical 2-liter bottle of soda generally used for these sorts of Diet Coke and Mentos reactions.

You’ll also sometimes read that the acidity of the soda is a major factor in the resulting geyser. This is not the case either. In fact, the level of acidity in the Coke before and after the Mentos geyser does not change, negating the possibility of an acid-based reaction—though you can make such an acid-based reaction using baking soda.

Daven Hiskey runs the wildly popular interesting fact website Today I Found Out. To subscribe to his “Daily Knowledge” newsletter, click here.

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Big Questions
Can You Really Go Blind Staring at a Solar Eclipse?
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A total solar eclipse will cut a path of totality across the United States on August 21, and eclipse mania is gripping the country. Should the wide-eyed and unprotected hazard a peek at this rare phenomenon?

NASA doesn't advise it. The truth is, a quick glance at a solar eclipse won't leave you blind. But you're not doing your peepers any favors. As NASA explains, even when 99 percent of the sun's surface is covered, the 1 percent that sneaks out around the edges is enough to damage the rod and cone cells in your retinas. As this light and radiation flood into the eye, the retina becomes trapped in a sort of solar cooker that scorches its tissue. And because your retinas don't have any pain receptors, your eyes have no way of warning you to stop.

The good news for astronomy enthusiasts is that there are ways to safely view a solar eclipse. A pair of NASA-approved eclipse glasses will block the retina-frying rays, but sunglasses or any other kind of smoked lenses cannot. (The editors at MrEclipse.com, an eclipse watchers' fan site, put shades in the "eye suicide" category.) NASA also suggests watching the eclipse indirectly through a pinhole projector, or through binoculars or a telescope fitted with special solar filters.

While it's safe to take a quick, unfiltered peek at the sun in the brief totality of a total solar eclipse, doing so during the partial phases—when the Moon is not completely covering the Sun—is much riskier.

WOULDN'T IT BE EASIER TO JUST TELL YOUR KIDS THEY WILL GO BLIND?

NASA's website tackled this question. Their short answer: that could ruin their lives.

"A student who heeds warnings from teachers and other authorities not to view the eclipse because of the danger to vision, and learns later that other students did see it safely, may feel cheated out of the experience. Having now learned that the authority figure was wrong on one occasion, how is this student going to react when other health-related advice about drugs, alcohol, AIDS, or smoking is given[?]"

This story was originally published in 2012.

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Big Questions
If Beer and Bread Use Almost the Exact Same Ingredients, Why Isn't Bread Alcoholic?
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If beer and bread use almost the exact same ingredients (minus hops) why isn't bread alcoholic?

Josh Velson:

All yeast breads contain some amount of alcohol. Have you ever smelled a rising loaf of bread or, better yet, smelled the air underneath dough that has been covered while rising? It smells really boozy. And that sweet smell that fresh-baked bread has under the yeast and nutty Maillard reaction notes? Alcohol.

However, during the baking process, most of the alcohol in the dough evaporates into the atmosphere. This is basically the same thing that happens to much of the water in the dough as well. And it’s long been known that bread contains residual alcohol—up to 1.9 percent of it. In the 1920s, the American Chemical Society even had a set of experimenters report on it.

Anecdotally, I’ve also accidentally made really boozy bread by letting a white bread dough rise for too long. The end result was that not enough of the alcohol boiled off, and the darned thing tasted like alcohol. You can also taste alcohol in the doughy bits of underbaked white bread, which I categorically do not recommend you try making.

Putting on my industrial biochemistry hat here, many [people] claim that alcohol is only the product of a “starvation process” on yeast once they run out of oxygen. That’s wrong.

The most common brewers and bread yeasts, of the Saccharomyces genus (and some of the Brettanomyces genus, also used to produce beer), will produce alcohol in both a beer wort
and in bread dough immediately, regardless of aeration. This is actually a surprising result, as it runs counter to what is most efficient for the cell (and, incidentally, the simplistic version of yeast biology that is often taught to home brewers). The expectation would be that the cell would perform aerobic respiration (full conversion of sugar and oxygen to carbon dioxide and water) until oxygen runs out, and only then revert to alcoholic fermentation, which runs without oxygen but produces less energy.

Instead, if a Saccharomyces yeast finds itself in a high-sugar environment, regardless of the presence of air it will start producing ethanol, shunting sugar into the anaerobic respiration pathway while still running the aerobic process in parallel. This phenomenon is known as the Crabtree effect, and is speculated to be an adaptation to suppress competing organisms
in the high-sugar environment because ethanol has antiseptic properties that yeasts are tolerant to but competitors are not. It’s a quirk of Saccharomyces biology that you basically only learn about if you spent a long time doing way too much yeast cell culture … like me.

This post originally appeared on Quora. Click here to view.

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