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