Scientists Say: Supercool

To supercool a liquid means to chill it below its freezing point without it turning solid

These trees are covered in rime ice, which forms when supercooled water droplets freeze onto solid surfaces.

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Supercool (verb, “SOOP-er-kool”)

To supercool a liquid means to chill it below its freezing point without it turning solid.

Supercooling can happen because liquids are loose jumbles of atoms. It’s hard for such disordered atoms to lock themselves into the crystal structure of a solid. Liquids often need tiny bits of solid matter to get them started. By latching onto a solid, it becomes easier for the atoms in a liquid to slot into a crystal structure. Bits of dust or other matter can often seed crystal formation in liquids.

If liquids are cooled enough, some of their atoms may spontaneously order themselves into a crystal structure. That tiny bit of solid can then seed more crystal growth. But a liquid can be cooled far below its freezing point before it seeds its own crystal formation. For instance, pure water has avoided turning into ice down to about –46° Celsius (–51° Fahrenheit). That temperature is much lower than water’s freezing point of 0 °C (32 °F).

High in Earth’s atmosphere, the air is fairly dust-free. Without solid specs to glom onto, water droplets tend to supercool. Such supercooled droplets help form glowing “noctilucent” clouds. Supercooled water is also found in tails behind comets. You can even produce supercooled water at home. All you need is a bottle of pure water and a cooler filled with a salty, icy slurry.

Chemicals can help water stay liquid below freezing temperatures. For instance, some fish have anti-freezing proteins. These proteins help prevent the water in their bodies from freezing. This allows the fish to survive at very low temperatures. Similarly, sea water can stay liquid below 0 °C (32° °F) thanks to its salt.

In a sentence

Supercooling human livers tripled their shelf life, compared to keeping the donated organs on ice at freezing temperatures.

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Maria Temming is the assistant editor at Science News Explores. She has bachelor's degrees in physics and English, and a master's in science writing.

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