Water's Ultimate Freezing Point

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How low can you go?

For water, the answer is -55 degrees Fahrenheit (-48 degrees C; 225 Kelvin). University of Utah researchers found that is the lowest temperature liquid water can reach before it becomes ice.

Back in grade school, we all learned that water under normal atmospheric pressure freezes at 32 degrees Fahrenheit (0 C), but that rule only holds for water with tiny impurities.

"If you have liquid water and you want to form ice, then you have to first form a small nucleus or seed of ice from the liquid. The liquid has to give birth to ice," said chemist and co-author of the study Valeria Molinero in a press release.

Impurities in water serve as those seeds.

But in very pure water, "the only way you can form a nucleus is by spontaneously changing the structure of the liquid," Molinero explains. She and co-author Emily Moore published their study today in the journal Nature.

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Under the right conditions, pure water can get super cold.

When it gets that cold, the liquid passes into another intermediate form of liquid with the properties of both regular liquid water and ice. But this intermediate phase exists for only a short time. Its very existence has been difficult to prove. Discovery News reported on research that used computer models to observe the properties of this elusive liquid.

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The University of Utah researchers also used computer models to observe this weird form of water.

"This intermediate ice has a structure between the full structure of ice and the structure of the liquid," said Molinero, "We're solving a very old puzzle of what is going on in deeply supercooled water."

IMAGE:

Ice over liquid water (Wikimedia Commons)

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