Today In Water

28 Nov

Supercool.

Question: At what temperature does water completely freeze?

Answer: -55 degrees Fahrenheit (-48.3 degrees Celsius)

I know, you think I’m wrong. Water molecules start to crystallize at 32 degrees Fahrenheit/0 degrees Celsius, but do not completely transform into a solid crystal until it’s far colder. This has something to do with water’s weirdness.

Supercooled liquid water must become ice at minus 55 F not just because of the extreme cold, but because the molecular structure of water changes physically to form tetrahedron shapes, with each water molecule loosely bonded to four others, according to the new study by chemists Valeria Molinero and Emily Moore.

The findings suggest this structural change from liquid to “intermediate ice” explains the mystery of “what determines the temperature at which water is going to freeze,” says Molinero, an assistant professor at the University of Utah and senior author of the study, published in the Nov. 24 issue of the journal Nature.

“This intermediate ice has a structure between the full structure of ice and the structure of the liquid,” she adds. “We’re solving a very old puzzle of what is going on in deeply supercooled water.”

However, in the strange and wacky world of water, tiny amounts of liquid water theoretically still might be present even as temperatures plunge below minus 55 F and almost all the water has turned solid — either into crystalline ice or amorphous water “glass,” Molinero says. But any remaining liquid water can survive only an incredibly short time — too short for the liquid’s properties to be detected or measured.

How and at what temperature water must freeze has more than just “gee-whiz” appeal. Atmospheric scientists studying global warming want to know at what temperatures and rates water freezes and crystallizes into ice.

“You need that to predict how much water in the atmosphere is in the liquid state or crystal state,” which relates to how much solar radiation is absorbed by atmospheric water and ice, Molinero says. “This is important for predictions of global climate.”

So, remember, there’s water, there’s ice, and then there’s intermediate ice.

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