Advanced #14

A liquid-liquid transition occurs at approximately 330 K

At around 57 degrees Celsius, water shows signs of a transition between two liquid states.

Scientific Explanation

Numerous physical properties of water display conspicuous behavior at a temperature near 330 K (approximately 57 degrees Celsius): inflection points, extrema, or changes in the temperature dependence. These include isothermal compressibility, speed of sound, refractive index, dielectric relaxation, and various transport coefficients. This clustering of anomalies at the same temperature points toward a structural transition within liquid water.

The hypothesis proposes that a liquid-liquid transition occurs at approximately 330 K — a shift between two different structural arrangements of water molecules. Below this temperature, a more open, tetrahedral arrangement dominates (similar to the LDL state); above it, a denser, less ordered structure prevails (similar to the HDL state).

This transition is not a sharp first-order phase transition like the melting of ice, but rather a continuous crossover — a gradual shift in the dominant local structure. X-ray and neutron scattering experiments have shown that the local coordination number and the distribution of nearest neighbors change measurably at this temperature.

If the existence of this transition is confirmed, it would provide an important link between water’s behavior at everyday temperatures and the hypothetical two liquid phases at low temperatures. The crossover at 330 K would then be the “high-temperature echo” of the liquid-liquid coexistence line emanating from the second critical point.

Liquid-Liquid Transition at Approximately 330 K Schematic showing a property change in liquid water around 330 K (57 degrees Celsius). A graph of a measured water property versus temperature shows a crossover or inflection point near 330 K, suggesting a transition between two structural arrangements of liquid water. Temperature (K) Property (a.u.) 280 300 330 360 380 Transition ~330 K (57 °C) More structured (LDL-like) Less structured (HDL-like) Liquid-Liquid Transition at ~330 K
Liquid-liquid transition at approximately 330 K: the measured property shows a clear inflection point, suggesting a structural crossover in liquid water.

Everyday Relevance

The temperature of 57 degrees Celsius is remarkably close to everyday experience. It is the temperature at which hot tap water flows from the faucet, at which tea cools enough to drink comfortably, and at which many biological processes take place. The fact that water’s internal structure changes right in this accessible range could explain why certain physical and biochemical processes display unexpected behavior around this temperature.

For the food industry and pharmaceuticals, where processes frequently occur in aqueous solution between room temperature and 80 degrees Celsius, understanding this transition may prove valuable. If the solvent properties of water undergo a structural shift at 330 K, then solubilities, reaction rates, and protein behavior could change abruptly — not because of the temperature change itself, but because of a change in the nature of the solvent.