June 29, 2011
Water can be converted into any one of the 15 known crystalline phases by changing the temperature or pressure.
Most of the liquids freeze under pressure (even at a higher temperature), because the pressure causes the molecules come together. But, the water is different because of the strong hydrogen bonds – water freezes at a temperature below 0°C under a pressure higher than 1 atm (0.10 MPa). Consequently, water also remains frozen at a temperature above 0 °C under a pressure lower than 1 atm.
Such melting of ice under high pressures is thought to contribute to the movement of glaciers. The ice, water, and water vapor can coexist at the triple point, at exactly 0.01 °C at a pressure of 611.73 Pa.
The phases of ice are still being discovered. In addition to the currently discovered 15 phases of ice, we can expect more such phases to be discovered in the future. At an extremely higher pressures, ice is predicted to even become a metal.
Ice XII was discovered in 1996. Ice XIII and XIV were discovered in 2006. In 2009, the ice XV was found at extremely high pressures and −143°C. (Diagram credits – Wikipedia)