Make instant slushies with supercooling
Marissa Shieh
at 09:48 AM May 29 2017
Make instant slushies with supercooling
Water bottles in freezer
Marissa Shieh/Popular Science
Hacks // 

Sound the alarms: Water doesn't always freeze when it should. In fact, you can chill water and most other liquids below the temperatures at which they usually freeze. Scientists once cooled water down to the frigid temperature of -51°C, and the H2O remained liquid, at least for a fraction of a second, before it flash-froze. But forget scientists—you can use this phenomenon, called supercooling or undercooling, to make instant slushies—even beer slushies.

First, let's look at the science. It sounds crazy, but liquid doesn't automatically become solid when you chill it to its freezing point. Even at cold temperatures, like the average freezer's 0°F, it takes a process to turn water into ice. The two substances are very different, from their structures to their densities (ice is eight percent less dense than water). So kick-starting this transformation process requires that something shake up the status quo.

In other words, the liquid must contain some sort of impurity that water molecules can lightly bind to. Without such an agent, a small crystal of ice might form, but it won't have the energy to grow larger. The nascent crystal of ice will change back into liquid, and the liquid water will just keep getting colder and colder until it's supercool. The difference between supercooled and normal liquid water goes beyond temperature—the colder fluid is actually 20 percent less dense, according chemist Valeria Molinero of the University of Utah. When a liquid is in this state, with its temperature lower than its freezing point, it becomes unstable. Add in one stray speck of dust, an ice cube, a scratch in the container, or even an accidental jostle, and the whole volume will flash freeze before your eyes.

That's because, under normal conditions, an impurity in the liquid—such as that speck of dust—can stabilize small ice crystals as they form. Molecules will lightly bind to this outside object, starting a chain reaction as more and more molecules latch onto the growing mass. The impurity thus triggers a transition from liquid to solid, water to ice. So if you want to make a supercooled liquid yourself, don't try it with tap water or cloudy beers: They have too much stuff in them, which makes them more likely to freeze than to supercool.

Fun fact: In both supercooling liquids and in regular freezing liquids, the process of crystallization begins at just one point. In a normal bottle of water, crystallization is more likely to begin at the bottom of the bottle, according to Tom Hill of Colorado State University. The frigid air first cools down the liquid molecules closest to the surfaces of the bottle, and these cooler molecules fall to the bottom. Within five to ten minutes, the whole bottle is frozen. In supercooled liquid, on the other hand, the freezing process begins around the impurity, which can be anywhere in the bottle, and quickly spreads from there.

Supercooling isn't just scientifically interesting. It also lets you make water freeze into ice seemingly instantaneously. Which makes a pretty good party trick. With some planning, prepare to astound friends and guests with the seemingly magical power of transforming water into ice before their very eyes. And if you're over 21, you can also turn liquid beer into beer slushies! Here's how.

 
Unfrozen water bottles freezer

Water bottles in freezer

I placed a bottle of tap water in the middle. Around this bellwether, I placed four bottles of Poland Spring bottled water. Two of the bottles went into the freezer at the same time as the tap water. Then, 15 minutes later, I added two more bottles in case I waited too long and accidentally froze the first two.

 

Marissa Shieh/Popular Science

Warning: Don't drink the supercooled liquid!

"It might break your teeth off because it'll freeze in between two teeth and push them apart," says Hill. Once it's frozen, however, it will become safe—so feel free to down that beer slushie.

Materials

  • A few bottles of water (not tap water). Tap water, even if it's safe to drink, will likely contain too many impurities to let it supercool without freezing. This trick will work best with distilled water; however, I also achieved the proper effect using bottled water.
  • One bottle of tap water (as an indicator)
  • Optional: A few bottles of beer. Pick a brand that comes in a clear bottle, such as Corona, so you can observe it. Because bubbles can spark freezing, consider opening the beer a little bit, just to relieve the pressure, before you pop it in the freezer. That way, when you try to open the bottle later, the sudden bubbles won't cause the beer to crystallize before you get to show it off.
  • Ice cubes or chunks of popsicle
  • Large bowl
  • Optional: Polystyrene cup
 
Frozen water freezer

After 3.5 hours, the tap water froze—and so did my first set of would-be supercool water. But the second set of bottles still looked liquid.

Marissa Shieh/Popular Science

Instructions

  1. Place the liquid you want to supercool in an undisturbed section of the freezer.
  2. As an indicator, place a bottle of tap water in the freezer as well. When the tap water freezes, you'll know that the distilled water (from here on, I'll call the wannabe supercool water "distilled water" for clarity) is supercooled and ready to go. Note: The beer may take a little longer to supercool because it contains alcohol. However, when the distilled water is ready, you can also test one of the beer bottles just in case.
  3. All told, the distilled water should take around two hours to become supercool, but the timing depends on the strength of your freezer. After an hour and a half has elapsed, start checking on the bottles every 15 minutes. When they're ready, remove the bottles from the freezer. Remember that frozen beer bottles can explode, so take care.
  4. Your super cool supercooled liquid is ready to show off! First, place an ice cube or popsicle chunk (for color contrast) in a large bowl. Then pour the liquid slowly onto the ice. The supercooled fluid should transform into a slush before your eyes. If you've made a beer slushie, go ahead and drink it now.
  5. If you've supercooled a bottle of distilled water, there are a couple other ways you can play with it. Take another bottle and tap it on the table or smack it with your hand. In reaction to this force, ice will appear seemingly out of nowhere. Another trick is to pour the water into a polystyrene cup (you can also prepare this ahead of time by simply supercooling the water in a cling-film-covered cup instead of in a bottle). Dangle or drop an ice cube into the water, and watch ice crystals spread around the cube.
 
Supercooled water popsicle

To show the difference in color between the original ice and the instant slush, I used a popsicle as a catalyst. It worked just as well as an ice cube to reveal that I had indeed created supercooled water!

Marissa Shieh/Popular Science

comments powered by Disqus
Filed under:
Sign up for the Pop Sci newsletter
Australian Popular Science
ON SALE 25 OCTOBER
PopSci Live