Anomalies
Currently, any explanation for the behavior of water in terms of ordinary theories of the liquid state fails, since water does not conform to the behavior of standard liquids. For instance, the molar volume (H2O) at 1 atm pressure and 0°C contracts from 19.66 cm3 for ordinary ice to 18.0182 cm3 for liquid water [13]. On heating from the supercooled state, the liquid does not expand as would be normal, but its volume shrinks [13,14] until a density maximum is achieved at 3.984°C. Under pressure [15], the melting point as well as the temperature of the density maximum move to lower temperatures. This should be compared with the case of ordinary liquids where pressure promotes freezing and consequently moves the freezing point to higher temperatures. At sufficiently high pressures the anomalous V, T effects in water disappear altogether.
Even more astonishing are some of the other anomalies. At moderate pressures and for temperatures below about 35°C, water possesses a negative pressure coefficient of viscosity [3,16]. This means that increasing the pressure gives rise to a lower viscosity, clearly not an intuitive behavior. Unlike other liquids, the isothermal compressibility of water under atmospheric pressure declines with increasing temperature [13], reaching a minimum near 46.5°C. This also seems very strange, since warmer things are usually more compressible, with the change in volume with pressure at constant temperature becoming larger, not smaller, on increasing the temperature. The heat capacity [3] of the liquid is much higher than that of ice, about double the value expected through contributions from various degrees of freedom using ordinary considerations. Finally, isotope effects on the densities[13,17] and transport properties[18] do not possess the ordinary mass or square-root-mass behavior. For example, the D2O/H2O viscosity ratio rises from about 1.16 at 100°C to around 2.0 in deeply supercooled water.