It is an intermediate state of water between normal water and supercritical water. Supercritical water is at a temperature and pressure above its critical point. (temperature >374°C, pressure >221 atm) Subcritical water lies below ranges of the critical points. So simply it is a stage under supercritical water with controlled temperature and pressure according to the requirement of application.
Sub – critical water can be used as a cheap, non-toxic, non-flammable, clean solvent in organic chemical research and even commercial applications.
Following research is example usage of sub-critical water as solvent in hydroxymethylfurfural research.
Research: Dehydration of fructose to 5-hydroxymethylfurfural in sub-critical water over heterogeneous zirconium phosphate catalysts by Feridoun Salak and Hiroyuki Yoshida of Osaka Prefecture University.
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