Supercooling water is easy and fun. Basically, you chill water below its freezing point and crystallize into ice on command. Here are step-by-step instructions for supercooling water using two different methods.
Method #1: Supercooling Bottled Water
Supercooling bottled water in a home freezer is the easy. The only “trick” is that the bottled water needs to be reasonably pure. Fiji water is one of the most reliable brands to use, but any water purified by reverse osmosis or distillation works well. The reason bottled water works better than tap water is because tap water contains tiny particulates, dissolved minerals, and dissolved gases that provide nucleation sites for ice to form. You can still supercool tap water, but first let it sit for several hours so any bubbles can escape.
- Place an unopened bottle of water in the freezer. Also, fill a bottle the same size with tap water.
- Let the bottles chill undisturbed for around 2-1/2 hours. The exact time needed to supercool water depends on the temperature of the freezer and the size of the water bottle. Don’t disturb either bottle, but look to see if the tap water has frozen. When the tap water freezes, it means the purified bottle water is supercooled. (If the water in both bottles is frozen, you need to melt the ice and start again, but check on the bottles after less time. If both bottles freeze again, either you waited too long again or else the bottled water wasn’t sufficiently pure.)
- Carefully remove the bottle of supercooled water from the freezer.
- Initiate crystallization into ice by shaking the bottle or by opening it and pouring the liquid over a piece of ice or bit of frozen popsicle.
Method #2: Supercool a Glass of Water
You don’t need a freezer to supercool water. All you need is ice and some salt. When you sprinkle salt on ice, the small amount of melted ice (liquid water) dissolves salt and experiences freezing point depression. This lowers the temperature of the ice below 0 °C or 32 °F. It’s a handy way to freeze homemade ice cream that also works for supercooling water.
- Fill a large bowl with ice. Sprinkle a couple of tablespoon of salt over the ice.
- Either chill a water bottle (with bottle of tap water, for reference) in the salted ice or place a glass of purified water in the bowl (with thermometer, so you know when it’s supercooled). Make sure the level of the ice is above the liquid level in the container.
- For a glass containing only a couple of tablespoons of water, it only takes around 15 minutes to supercool the liquid. Bottled water takes longer and you may need to refill the ice in the bowl if too much melts.
- Freeze the supercooled water into ice by disturbing it or by dropping a small piece of ice into the water.
How Supercooling Water Works
Water normally freezes into ice at 0 °C or 32 °F, but it can be supercooled at ordinary pressures down to a temperature as low as −48.3 °C or −55 °F, as long as there are no nucleation sites to support ice crystal growth. At the bottom end of the temperature range, supercooled water is glassy water. In other words, it is an amorphous solid, but not crystalline ice. But, just below the freezing point it’s still a liquid.
As temperatures drop below freezing, supercooled water doesn’t crystallize because of the way water molecules organize when chilled. Normal ice is hexagonal ice (like six-sided snow crystals), but hydrogen bonding gives water other options. Supercooled water has tetrahedral water molecules and pentameric water clusters. So, even though hexagonal ice is more stable than liquid water, it doesn’t form its preferred crystal structure unless it is disturbed because the tetrahedral and pentameric water is “in the way.”
Supercooling vs Freezing Point Depression
Some people confuse supercooling with freezing point depression. While both processes cause a liquid to freeze at a temperature below the normal freezing point, supercooling involves a pure substance. Freezing point depression occurs when dissolved particles interrupt the organization of molecules from the liquid to solid phase.
Examples of Supercooled Water in Nature
Supercooling of water is a natural phenomenon. It occurs in cumulus and stratus clouds. Aircraft employ de-icing systems because otherwise supercooled water crystallizes into ice when the wings disturb it.
Freezing rain is another example of supercooled water. When the rain strikes a surface, it crystallizes into ice.
If you enjoyed this project, another fun example of supercooling is hot ice!
References
- Chaplin, Martin (2004) “Supercooled water.” Water Structure and Science.
- Debenedetti, P. G.; Stanley, H. E. (2003). “Supercooled and Glassy Water.” Physics Today. 56 (6): 40–46. doi:10.1063/1.1595053
- Giovambattista, N.; Angell, C. A.; Sciortino, F.; Stanley, H. E. (July 2004). “Glass-Transition Temperature of Water: A Simulation Study.” Physical Review Letters. 93 (4): 047801. doi:10.1103/PhysRevLett.93.047801
- Moore, Emily; Molinero, Valeria (2011). “Structural transformation in supercooled water controls the crystallization rate of ice.” Nature. 479 (7374): 506–508. doi:10.1038/nature10586