Structures of water and aqueous electrolyte solutions under sub- and super-critical conditions studied mainly by X-ray diffraction and also by neutron diffraction are reviewed and the experimental results are compared with those reported by using computer simulations. Some Raman spectroscopic data are included for discussing the existence of hydrogen bonds in water at high temperature and high pressures (HTHPs). The authors propose a classification of supercritical water into three categories: (a) low density water, (b) medium density water, and (c) high density water, because density is a very important thermodynamic quantity to describe properties of sub- and super-critical water. From changes in the water-water intermolecular distance and the coordination number of water with temperature and pressure, and especially with density at HTHP, the authors conclude that the compact tetrahedral-like water structure is decomposed and long-distance water-water interactions increase with temperature and pressure, and they propose a model for water: under supercritical conditions water consists of small clusters, much smaller aggregates such as oligomers, and even monomeric gas-like water molecules.
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