The usual liquid state of water that we are all familiar with corresponds to liquid water at normal temperatures. However, a new study published in the journal Science shows that water at low temperatures exists in two different liquid states, a low-density liquid at low pressures and a high-density liquid at high pressures; these two liquids have noticeably different properties and differ by 20% in density; the results imply that at appropriate conditions, water should exist as two immiscible liquids separated by a thin interface similar to the coexistence of oil and water.Conceptual representation showing a glass containing two liquids of water separated by a thin interface; the bottom liquid is dense than the liquid at the top and, thus, shows that it is composed of water molecules closely packed. Image credit: Jerker Lokrantz / Anders Nilsson.Theoretical simulations suggest that deeply supercooled water undergoes a transition between high- and low-density forms, but this transition is difficult to study experimentally because it occurs under conditions in which the crystallization of ice is extremely rapid.The possibility that water could exist in two different liquid states was proposed approximately 30 years ago, based on results obtained from computer simulations, said co-author Professor Nicolas Giovambattista, a researcher in the Department of Physics at Brooklyn College and the Graduate Center of the City University of New York.This counterintuitive hypothesis has been one of the most important questions in the chemistry and physics of water, and a controversial scenario since its beginnings.This is because experiments that can access the two liquid states in water have been very challenging due to the apparently unavoidable ice formation at the conditions where the two liquids should exist.Professor Giovambattista and colleagues combined X-ray lasers for rapid structure determination with infrared pulses for rapid heating of amorphous ice layers formed at about 200 K.The heating process created high-density liquid water at increased pressures.As the layer expanded and decompressed, low-density liquid appeared and grew on time scales between 20 nanoseconds and 3 microseconds.The dynamics of these processes provides support for a liquid-liquid transition in bulk supercooled water.It remains an open question how the presence of two liquids may affect the behavior of aqueous solutions in general, and in particular, how the two liquids may affect biomolecules in aqueous environments, Professor Giovambattista said.This motivates further studies in the search for potential applications._____Kyung Hwan Kim et al. 2020. Experimental observation of the liquid-liquid transition in bulk supercooled water under pressure. Science 370 (6519): 978-982; doi: 10.1126/science.abb9385