Researchers at Stockholm University have used advanced x-ray lasers to uncover a long-suspected feature of water: a critical point that appears when water is deeply supercooled. This occurs at about -63 °C and 1000 atmosphere. Even under everyday conditions, this hidden point influences how water behaves, helping explain many of its unusual properties. The results were published in the journal Science.
Water is everywhere and essential for life, yet it does not act like most other liquids. Properties such as density, heat capacity, viscosity, and compressibility respond to temperature and pressure in ways that are opposite to what scientists see in typical substances.
In most materials, cooling causes them to contract and become denser. Based on this pattern, water should reach its highest density when it freezes. Instead, ice floats, and liquid water is actually most dense at 4 degrees C. That is why colder water remains below warmer water in lakes and oceans.
When water is cooled below 4 degrees, it begins expanding again. If pure water is cooled below 0 degrees (where crystallization happens slowly), this expansion continues and even accelerates as the temperature drops further. Other properties, including compressibility and heat capacity, also behave in increasingly unusual ways as the temperature decreases.
Capturing Water’s Hidden State With X-Ray Lasers
To investigate these strange behaviors, scientists used extremely fast x-ray pulses generated by powerful lasers in South Korea. These pulses allowed them to observe water in a supercooled state just before it turned into ice.
“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,” says Anders Nilsson, Professor of Chemical Physics at the Department of Physics at Stockholm University. “For decades there has 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.”
Two Liquid Forms of Water and a Critical Transition
Under low temperatures and high pressure, water can exist as two distinct liquid phases with different molecular bonding structures. As conditions change, these two forms merge into a single phase at the critical point.
Near this point, the system becomes highly unstable, and water rapidly shifts between the two liquid states or mixtures of them. These fluctuations extend across a wide range of temperatures and pressures, even reaching normal environmental conditions. Scientists believe these constant shifts are what give water its unusual characteristics.
Beyond the critical point, water enters a supercritical state, and under everyday conditions, it already exists in this regime.
A “Black Hole-Like” Effect in Water Dynamics
The researchers also found that molecular motion slows dramatically as water approaches the critical point.
“It looks almost that you cannot escape the critical point if you entered it, almost like a Black Hole,” says Robin Tyburski, researcher in Chemical Physics at Stockholm University.
A Breakthrough Decades in the Making
“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,” says Aigerim Karina, Postdoc in Chemical Physics at Stockholm University.
“It was a dream come true to be able to measure water under such low temperature condition without freezing,” says Iason Andronis, PhD student in Chemical Physics at Stockholm University. “Many have dreamt about finding this critical point but the means have not been available before the development of the x-ray lasers.”
“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. Is this a pure coincidence or is there some essential knowledge for us to gain in the future?” says Fivos Perakis, an associate professor in Chemical Physics at Stockholm University.
Solving a Century-Old Mystery 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,” explains Anders Nilsson. “Researchers studying the physics of water can now settle on the model that water has a critical point in the supercooled regime. The next stage is to find the implications of these findings on waters importance in physical, chemical, biological, geological and climate related processes. A big challenge in the next few years.”
International Collaboration Behind the Discovery
This research involved collaboration between Stockholm University, POSTECH University and PAL-XFEL in South Korea, the Max Planck Society, Johannes Gutenberg University in Germany, and St. Francis Xavier University in Candada. Contributors included Aigerim Karina, Robin Tyburski, Iason Andronis, and Fivos Perakis, along with former members of the Chemical Physics group at Stockholm University