A new critical point in water has been discovered by Stockholm University physicists, explaining some of water’s strangest properties that have long baffled researchers.
The critical point occurs in water around -63 °C and 1000 atmospheres, as observed in the lab with ultrafast X-ray lasers in past research. The new work was published in Science, achieving a physics feat long believed impossible, with potential implications for areas that include studies involving the origin of life.
The Strange Physics of Water
Despite how common water is on Earth, its physical properties are very strange in comparison to other liquids. The effects of pressure and temperature on its density, heat, viscosity, and compressibility are the opposite of those of other known liquids.
Typically, matter shrinks and becomes denser as it cools. Therefore, ice should be denser than liquid water, yet water actually achieves its highest density at a mere 4 °C. As seen in floating ice cubes, water begins to expand again below this temperature, with the rate increasing as the temperature drops. Compressibility and heat capacity likewise defy normal expectations for matter as water cools.
Water can exist in two macroscopic liquid phases, which bond water molecules differently at low temperature and high pressure. However, beyond the supercritical point, the water forces itself into a single state. This is not an easy transition, though, as the water switches between states and even mixes elements of both at times. It is these fluctuations that give water its strange properties.
Predicting the Supercritical Point of Water
“There has been an intense debate about the origin of the strange properties of water for over a century since the early work of Wolfgang Röntgen”, explained Anders Nilsson, a Professor of Chemical Physics at the Department of Physics at Stockholm University. “Researchers studying the physics of water can now settle on the model that water has a critical point in the supercooled regime.”
The team’s research builds on over a century of preceding work, where they used X-ray pulses to investigate water’s critical point. X-ray pioneer Wolfgang Röntgen was also among the first to speculate on the two states of water, and the supercritical point itself was predicted in a theoretical simulation in a 1992 study published in Nature. Yet, until now, physically observing the supercritical point had been considered impossible.
“The reason is that the critical point is in a part of the phase diagram where ice is formed extremely fast and thereby assumed to be impossible to investigate,” Nilsson told The Debrief. “There are many theoretical predictions and extrapolations of experimental data far from the point, and they vary a lot. Naturally, there are some predictions close to what we now derive from the experiments.”
Finding Water’s Critical Point
Ultra-fast X-ray laser pulses enabled the team to study phenomena long believed too fleeting to be easily observed. “What was special was that we were able to X-ray unimaginably fast before the ice froze and could observe how the liquid-liquid transition vanishes and a new critical state emerges,” Nilsson said.
“For decades, there have been speculations and different theories to explain these remarkable properties, and one theory has been the existence of a critical point. Now we have found that such a point exists,” Nilsson added.
Intriguingly, the system’s dynamics slow as it approaches the critical point. The team compares it to a black hole, where there is no escape once an object crosses the event horizon.
“It’s amazing how amorphous ices, such an extensively studied state of water, happened to become our entrance to the critical region. It’s a great inspiration for my further studies and a reminder of the possibilities of making discoveries in much-studied topics such as water,” said Aigerim Karina, Postdoc in Chemical Physics at Stockholm University.
“I find it very exciting that water is the only supercritical liquid at ambient conditions where life exists, and we also know there is no life without water,” added Fivos Perakis, an associate professor in Chemical Physics at Stockholm University. “Is this a pure coincidence, or is there some essential knowledge for us to gain in the future?”
The researchers say that while at present their work remains purely of scientific interest, in the future it may have a range of potential practical applications in a range of different areas.
The paper, “Experimental Evidence of a Liquid-Liquid Critical Point in Supercooled Water,” appeared in Science on March 26, 2026.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.