X-Ray Lasers Reveal Water's Hidden Critical Point, Resolving a Century-Old Mystery About Life's Most Essential Molecule
Stockholm University physicists used ultra-fast X-ray pulses to confirm a second critical point in supercooled water at -63 degrees Celsius and 1,000 atmospheres, explaining why water behaves unlike any other liquid.
Overview
A team of physicists at Stockholm University has experimentally confirmed the existence of a hidden critical point in supercooled water, a finding that resolves a decades-old debate about why water behaves so differently from every other known liquid. The results, published in Science on March 26, 2026, show that at approximately minus 63 degrees Celsius and 1,000 atmospheres of pressure, two distinct liquid forms of water merge into one, producing fluctuations that ripple outward to shape water’s behavior even under everyday conditions.
What We Know
The experiment was conducted at the Pohang Accelerator Laboratory (PAL-XFEL) in South Korea, where the team used ultra-short X-ray pulses to observe supercooled water in the narrow window before it crystallizes into ice. As described by Phys.org, researchers began with tiny samples of amorphous ice, melted them with a short infrared laser blast, and then probed the resulting liquid with X-ray pulses on nanosecond-to-microsecond timescales.
The measurements revealed that water can exist as two competing liquid phases at low temperatures and high pressures: a high-density liquid (HDL) and a low-density liquid (LDL). At the newly confirmed critical point, according to ScienceDaily, the distinction between these two phases vanishes entirely, producing large-scale instabilities that propagate across wide ranges of temperature and pressure.
“Now we have found that such a point exists,” said Anders Nilsson, professor of chemical physics at Stockholm University and a lead author of the study, as quoted by EurekAlert. Robin Tyburski, another member of the research team, noted in ScienceDaily that the dynamics near the critical point behave “almost like a Black Hole” in that once water enters the region, its properties change in ways that are difficult to reverse.
The paper, titled “Experimental evidence of a liquid-liquid critical point in supercooled water,” involved collaborators from POSTECH University in South Korea, the Max Planck Society and Johannes Gutenberg University in Germany, and St. Francis Xavier University in Canada, according to EurekAlert.
Why Water Is Strange
Water’s anomalous properties have puzzled scientists for more than a century. Unlike nearly all other liquids, water expands when it freezes, reaches maximum density at 4 degrees Celsius rather than at its freezing point, and exhibits unusually high heat capacity and surface tension. These behaviors are critical to life on Earth: ice floats, insulating lakes and oceans from freezing solid, and water’s thermal properties stabilize planetary climates.
The newly confirmed critical point provides a unified explanation for these anomalies. As reported by The Debrief, the fluctuations generated by the proximity of two liquid phases at the critical point extend their influence all the way to the temperatures and pressures at which water exists on Earth’s surface. In effect, everyday water is permanently shaped by a phase transition that occurs far below the conditions most organisms ever encounter.
What We Don’t Know
The discovery raises as many questions as it answers. The researchers noted in ScienceDaily the striking coincidence that water is the only known supercritical liquid at ambient conditions where life exists, but whether this relationship is essential to biology or merely coincidental remains an open question.
The experimental conditions required to observe the critical point directly, minus 63 degrees Celsius at 1,000 atmospheres, are extreme. Further work will be needed to map the full phase diagram of supercooled water and to determine how the two liquid phases influence water’s behavior in biological systems such as protein folding and cellular membranes.
Additionally, while the experiment confirms a critical point predicted by theoretical models going back to the 1990s, the precise molecular mechanisms that produce two liquid forms of a single substance remain a subject of active computational and theoretical research.