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The two states of liquid water

Water is known to be a peculiar substance, as it has more than 70 anomalous properties and behaviors that distinguish it from most liquids. H2O is, for example, the only molecule on Earth that exists naturally and simultaneously in all three states or phases of matter—liquid, solid and gas. As ice, its densest state, it floats in liquid water, whereas most solids will sink. A new research study has now underscored the unique character of this abundant molecule. According to the study, liquid water can exist as two different molecular structures, in high-density and low-density forms (PNAS, June 26, 2017). A group of researchers from Europe and the United States arrived at this finding after examining supercooled water—which is found in most clouds and can transform into ice almost immediately—using two methods of X-ray analysis. The tests revealed the structures and movements of these two molecular forms of liquid water. They also indicated that one form can transform into the other and that water can exist in both states at low temperatures where ice crystallization is slow. “In a nutshell: Water is not a complicated liquid, but two simple liquids with a complicated relationship,” said chemical physicist Lars G. M. Pettersson of Stockholm University in Sweden, a coauthor of the study, in an article announcing the study. These findings will improve our understanding of how water behaves at different temperatures and pressures, and could be useful for developing new techniques to purify and desalinate ocean water.