From: Andy Soos, ENN
Published June 17, 2010 04:53 PM
The Oddness of Water and Ice
Water is vital for life and how it freezes is very important. For
years water (ice) has been known to exist in 15 phases. Subjected to
higher pressures and varying temperatures, ice can form in fifteen
separate known phases. With care all these types can be recovered at
ambient pressure. The types are differentiated by their crystalline
structure, ordering and density. There are also two metastable phases of
ice under pressure, both fully hydrogen-disordered; these are IV and
XII. Ice XII was discovered in 1996. In 2006, XIII and XIV were
discovered. Ices XI, XIII, and XIV are hydrogen-ordered forms of ices Ih,
V, and XII respectively. In 2009 ice XV was found at extremely high
pressures and −143 degrees celsius. Now there is another variation.
Most liquids freeze at a higher temperature under pressure, because
the pressure helps to hold the molecules together. However, the strong
hydrogen bonds in water make it different: water freezes at a
temperature below 0 °C under a pressure higher than 1 atmosphere.
Consequently, water also remains frozen at a temperature above 0 °C
under a pressure lower than 1 atmosphere. The melting of ice under high
pressures is thought to contribute to the movement of glaciers.
As well as crystalline forms, solid water can exist in amorphous (non-crystalline)states
as amorphous solid water, low-density amorphous ice, high density
amorphous ice , very high density amorphous ice and hyperquenched glassy
water.
The form most common on Earth is a hexagonal ice crystal. Some of the
other variants are:
Ice Ic - A metastable cubic crystalline variant of ice. The oxygen atoms
are arranged in a diamond structure. It is produced at temperatures
between 130 and 220 K, and can exist up to 240 K when it transforms into
ice Ih. It may occasionally be present in the upper atmosphere.
Ice II - A rhombohedral crystalline form with highly ordered structure.
Formed from ice Ih by compressing it at temperature of 190—210 K. When
heated, it undergoes transformation to ice III.
Ice III - A tetragonal crystalline ice, formed by cooling water down to
250 K at 300 MPa (about 30 atmospheres).
Ice IV - A metastable rhombohedral phase. It can be formed by heating
high density amorphous ice slowly at a pressure of 810 MPa (about 80
atmospheres).
Ice V - A monoclinic crystalline phase. Formed by cooling water to 253 K
at 500 MPa (about 50 atmospheres).
Now, University of Utah chemists have confirmed the coexistence of ice
and liquid after water crystallizes at a new very low temperatures. They
describe their work in the June 21 issue of the Journal of Chemical
Physics, which is published by the American Institute of Physics.
The new ice forms at a temperature of 180 K, which is typical of the
upper atmosphere where water blurs between ice and liquid.
"This blurring is what's interesting," says Valeria Molinero, who led
the research. "Our findings show that what goes on there is important to
the behavior of water and to the formation of clouds."
By targeting this critical temperature zone, the new work might be
important for understanding cloud formations that regulate global
radiation and hence climate change.
The article, "Ice Crystallization in Water's "No-Man's Land" by Emily
Moore and Valeria Molinero appears in the Journal of Chemical Physics.
See: http://jcp.aip.org/
For further information: http://en.wikipedia.org/wiki/Ice or
http://www.eurekalert.org/pub_releases/2010-06/aiop-tsl061410.php
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