Advanced #7

Water has two liquid phases and a second critical point

At approximately -91 degrees Celsius, a second critical point is suspected where two liquid phases coexist.

Scientific Explanation

Most liquids exist in only one liquid phase. Water, however, may behave differently: theoretical models and computer simulations suggest that at very low temperatures and high pressures, two distinct liquid forms of water exist — one with high density (HDL, high-density liquid) and one with low density (LDL, low-density liquid).

These two forms differ in the arrangement of their hydrogen bonds. In LDL, the molecules form an open, tetrahedral network similar to ice, resulting in lower density. In HDL, the molecules are more densely packed with distorted and partially broken hydrogen bonds, yielding a higher density.

The hypothetical second critical point — where the distinction between HDL and LDL vanishes — is predicted to occur at approximately minus 91 degrees Celsius and around 100 MPa of pressure. Reaching this point experimentally is extremely difficult because water in this temperature range normally crystallizes immediately. This region has therefore been dubbed “no man’s land” in water research.

Recent experiments involving ultrafast heating of amorphous ice and measurements in nanopores have provided growing evidence supporting the second critical point hypothesis. If confirmed, it would elegantly explain many of water’s other anomalies as distant effects of this hidden critical point influencing behavior at higher temperatures.

Two Liquid Phases of Water and the Second Critical Point Schematic phase diagram showing the hypothesized second critical point of water at approximately -91 degrees Celsius and 100 MPa, where high-density liquid water and low-density liquid water coexist. The liquid-liquid coexistence line separates the two phases. Temperature Pressure -91 °C 0 °C HDL (High-Density Liquid) LDL (Low-Density Liquid) 2nd Critical Point ~-91 °C, ~100 MPa Ice Hypothesized Second Critical Point of Water
Hypothesized second critical point of water, where the liquid-liquid coexistence line between HDL and LDL terminates.

Everyday Relevance

Although the second critical point lies in a temperature range not encountered in daily life, it indirectly affects our world. The theory suggests that many “everyday” anomalies of water — such as the density maximum at 4 degrees Celsius, the high heat capacity, and the unusual compressibility behavior — are distant manifestations of this hidden critical point.

For cryobiology, understanding water’s two liquid phases is of great importance: when freezing cells or organs for transplantation, researchers must precisely understand how water behaves at low temperatures. The ability to preserve water in a glassy state (vitrification) rather than as crystalline ice is directly related to the interplay between HDL and LDL.