Why is water densest at 4°C? Why is it so essential for the emergence and sustenance of life? These fundamental questions can feel like they must have clear answers somewhere in a textbook. Surprisingly, they have long been among the most stubborn puzzles in science.
Now, after a decade of sustained effort, scientists have identified the underlying cause of water's unique properties. The findings could rewrite textbook explanations of water.
A team led by Professor Kyung Hwan Kim in the Department of Chemistry at POSTECH (Pohang University of Science and Technology), in collaboration with Professor Anders Nilsson in the Department of Physics at Stockholm University, has successfully observed water's liquid-liquid critical point (LLCP), one of the most challenging problems in science for decades. The study, which answers a fundamental mystery of water, was published in Science on March 26 (local time).
Water is one of the most extensively studied substances, but also one of the most difficult substances to fully explain. A leading hypothesis proposed to account for water's unique properties is the existence of a liquid-liquid critical point, a special point where two distinct liquid forms of water become indistinguishable.
Scientists have predicted that if the LLCP exists, it would be hidden in a deeply supercooled regime, roughly between -40°C and -70°C, sometimes called "no-man's-land." To test the hypothesis experimentally, researchers must directly measure liquid water that remains unfrozen below -40°C. But in this temperature range, water freezes faster than conventional measurement methods can capture, making direct observation effectively impossible for decades.
Over the last ten years, the research team steadily pursued this problem despite the long-standing experimental barrier. They overcame it by using an X-ray free-electron laser (XFEL), a source often described as "dream light", capable of producing extremely intense X-ray pulses and capturing molecular-scale motion occurring within one ten-trillionth of a second. The experiments were performed using PAL-XFEL at the Pohang Accelerator Laboratory.
In 2017, the team became the first in the world to show that it is possible to probe liquid water without freezing down to -45°C, demonstrating that the previously "inaccessible" region could in fact be explored. In 2020, they advanced their experimental approach by utilizing amorphous ice, extending measurements to liquid water down to -70°C and providing the first evidence that, at ultralow temperatures, water can exist in two distinct liquid states. Both studies were published in Science and drew broad attention.
In the newly published work, the researchers tracked how water changes with temperature and pressure in far greater detail than before. They report the first direct observation of a liquid-liquid critical point, near -60°C, where water transitions from two distinct liquid states into a single supercritical liquid state. With this observation, the team has now identified the fundamental origin of water's extraordinary behavior.
This achievement is not a short-term result, but the culmination of long-term persistence aimed at a foundational scientific question. By turning what had remained largely theoretical into experimentally grounded evidence, the team has pushed our understanding of water into a new phase.
Professor Kyung Hwan Kim said, "The intense debate in the scientific community, spanning many years, over water's unusual properties and a liquid-liquid critical point has finally been brought to a close." He added, "This discovery will serve as a starting point for uncovering the essential roles water plays in living systems and in a wide range of natural phenomena."
This research was supported by the National Research Foundation of Korea (NRF), through the Outstanding Young Scientist Grant program and the Leading Research Center Support Program, and by the Samsung Science and Technology Foundation.
