Researchers have reported direct evidence of a long-theorized phase transition in liquid water, offering an explanation for several of its unusual physical properties. The findings center on the existence of a liquid-liquid critical point, where two distinct forms of liquid water become indistinguishable under specific conditions.
The study, led by Kyung Hwan Kim of Pohang University of Science and Technology in collaboration with Anders Nilsson, provides experimental confirmation of a concept that has remained debated for decades. Scientists have long suspected that water’s anomalies, including its maximum density at 4°C, may arise from the coexistence of two liquid states, according to details highlighted by SciTechDaily.
One of the main challenges in studying this phenomenon has been accessing the extreme temperature range known as “no-man’s-land,” between approximately -40°C and -70°C. In this range, water tends to freeze almost instantly, preventing conventional observation techniques from capturing its liquid state. This limitation has historically made it difficult to verify the theory experimentally.
To overcome this, the research team employed an X-ray free-electron laser, a high-intensity imaging tool capable of capturing molecular motion at extremely short timescales. Using this approach, they were able to observe water in its supercooled liquid state without it crystallizing into ice. Experiments were conducted at the Pohang Accelerator Laboratory’s PAL-XFEL facility.
Over the course of more than a decade, the team refined its methods to probe deeper into this previously inaccessible region. Earlier work demonstrated that water could remain in a liquid state at temperatures as low as -45°C. Subsequent experiments extended this range to approximately -70°C, providing initial evidence that two distinct liquid phases could exist under these conditions.
In the latest study, researchers tracked how water behaves under varying temperatures and pressures and identified a critical point near -60°C. At this point, the two liquid forms merge into a single supercritical phase. This observation is considered the first direct experimental confirmation of the liquid-liquid critical point in water.
The findings help explain several of water’s unusual characteristics, including its density anomaly and its role in supporting life. Understanding these properties at a fundamental level may also influence fields such as climate science, biology, and materials research, where water plays a central role.
The study represents a significant step in resolving a long-standing scientific question. By confirming the existence of a liquid-liquid critical point, researchers have provided a framework for interpreting water’s behavior under extreme conditions and opened new avenues for investigating its role in natural systems.
