Stockholm University Researchers Prove Existence of Long-Theorized Second Critical Point in Supercooled Water

Researchers confirm a liquid-liquid critical point in supercooled water at -63°C, explaining why water behaves differently than all other liquids.

By: AXL Media

Published: Mar 27, 2026, 7:54 AM EDT

Source: Information for this report was sourced from Stockholm University

Solving the Mystery of Water's Anomalous Behavior

Water is the only substance essential for life, yet it defies the standard laws of physics: it is densest at 4°C, its ice floats, and it becomes more compressible as it gets colder. For decades, physicists have theorized that these "strange" properties are caused by a hidden critical point reachable only in a "no-man's land" of deep supercooling. A study published in Science by Stockholm University researchers has finally confirmed this point exists at approximately $-63$°C ($210$ K) and $1,000$ atmospheres. By using femtosecond X-ray pulses to "photograph" water before it could freeze into ice, the team observed the liquid-liquid transition vanish into a new, singular critical state.

The Tug-of-War Between Two Liquid Phases

The study demonstrates that water is not a simple, uniform liquid. Instead, at low temperatures and high pressures, it can exist as two distinct macroscopic phases: a Low-Density Liquid (LDL) and a High-Density Liquid (HDL). The newly discovered critical point is the specific coordinate where the distinction between these two phases disappears. Professor Anders Nilsson explains that even at ambient temperatures and pressures, water is "supercritical," meaning it constantly fluctuates between these two bonding structures. These rapid, molecular-level fluctuations are what generate the unusual heat capacity, viscosity, and density shifts that make water unique.

The "Black Hole" of Molecular Dynamics

One of the more startling observations made during the experiment at the PAL-XFEL X-ray laser facility in South Korea was the dramatic slowing of molecular movement. As the water entered the critical region, its dynamics slowed down to such an extent that researchers compared the effect to a "Black Hole." According to researcher Robin Tyburski, once the system enters this critical state, the water molecules become so entangled in their fluctuations that they appear unable to escape the transition. This phase had previously been impossible to measure because water typically crystallizes into ice far too quickly at these temperatures.