How cold water get before it absolutely freezes?

How cold water can get before it absolutely freezes?

8:50 PM, 2nd December 2011
How cold water can get before it absolutely freezes?
Molinero, assistant professor, University of Utah

UTAH, US: According to the new study, super cooled liquid water must become ice at    -55 F because the molecular structure of water changes physically to form tetrahedron shapes, with each water molecule loosely bonded to four others.

We are solving a very old puzzle of what is going on in deeply super cooled water,”

said Molinero, an assistant professor, the University of Utah.

However, tiny amounts of liquid water theoretically still might be present even as temperatures plunge below -55 F and almost all the water has turned solid – either into crystalline ice or amorphous water “glass,” said Molinero. “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, said Molinero. “This is important for predictions of global climate.”

Liquid water as cold as -40 F has been found in clouds. Scientists have done experiments showing liquid water can exist at least down to -42 F.

“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 Molinero.

Yet in very pure water, “the only way you can form a nucleus is by spontaneously changing the structure of the liquid,” she added.

Molinero said key questions include, “under which conditions do the nuclei form and are

Super cooled water has been measured down to about -42 F, which is its “homogenous nucleation temperature” – the lowest temperature at which the ice crystallization rate can be measured as water is freezing. Below this temperature, ice is crystallizing too fast for any property of the remaining liquid to be measured.

To get around the problem, Molinero and chemistry doctoral student Moore used computers at the University of Utah’s Center for High Performance Computing. The behavior of super cooled water was simulated and also modeled using real data.

Computers provide “a microscopic view through simulation that experiments cannot yet provide,” said Molinero.

 It took thousands of hours of computer time to simulate the behavior of 32,768 water molecules to determine how the heat capacity, density and compressibility of water changes as it is supercooled, and to simulate how fast ice crystallized within a batch of 4,000 water molecules.

The computers helped Molinero and Moore determine how cold water can get before it reaches its theoretical maximum crystallization rate and must freeze. The answer: minus 55 F (or minus 48 degrees Celsius).

The computers also showed that as water approaches minus 55 F, there is a sharp increase in the proportion of water molecules attached to four others to form tetrahedrons.

When water approaches -55 F, there is an unusual decrease in density and increases in heat capacity and compressibility. These unusual thermodynamics coincide with liquid water changing to the tetrahedral structure.

“The change in structure of water controls the rate at which ice forms,” said Molinero. “We show both the thermodynamics of water and the crystallization rate are controlled by the change in structure of liquid water that approaches the structure of ice.”

 

© University of Utah News

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