Unsolved mysteries of water in its liquid and glassy phases

Although H2O has been the focus of a considerable amount of research since the beginning of the century, its peculiar physical properties are still not well understood. First we discuss some of the anomalies of this ‘complex fluid’. Then we describe a qualitative interpretation in terms of percolation concepts. Finally, we discuss recent experiments and simulations relating to the liquid–liquid phase transition hypothesis that, in addition to the known critical point in water, there exists a ‘second’ critical point at low temperatures. In particular, we discuss very recent measurements at Tsukuba of the compression-induced melting and decompression-induced melting lines of high-pressure forms of ice. We show how knowledge of these lines enables one to obtain an approximation for the Gibbs potential G(P, T ) and the equation of state V (P, T ) for water, both of which are consistent with the possible continuity of liquid water and the amorphous forms of solid water.