Water has a newfound ‘critical point’ that may help explain its quirks

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POSTECH University, South Korea By Emily Conover March 26, 2026 at 2:00 pm Share this:Share Share via email (Opens in new window) Email Share on Facebook (Opens in new window) Facebook Share on Reddit (Opens in new window) Reddit Share on X (Opens in new window) X Print (Opens in new window) Print A hidden feature of water, long submerged, has finally been brought to the surface. New experiments have revealed supercooled water’s critical point — a specific pressure and temperature at which two distinct phases of water turn into one. The critical point appears at about 210 kelvins (around –63° Celsius) and about 1,000 times the pressure exerted by Earth’s atmosphere at sea level, researchers report in the March 26 Science. The discovery may help explain certain odd properties of the ubiquitous, all-important liquid. Sign up for our newsletter We summarize the week's scientific breakthroughs every Thursday. Water is already known to have a critical point at high temperature. At about 374° C and 218 times atmospheric pressure, the distinction between the liquid and gas phases is erased. Beyond that critical point, water is what’s called a supercritical fluid. Scientists had long predicted a second critical point existed at low temperature, in water that is supercooled, meaning that it temporarily remains liquid below its normal freezing point. “For 20 years or more, many people were waiting to see direct evidence … based on experiments,” says physicist Nicolas Giovambattista of Brooklyn College in New York, who was not involved with the research. “It’s amazing that it finally came.” Certain odd properties of water tipped scientists off to this possibility. For example, most liquids increase in density upon cooling. But water increases in density down to about 4° C where it reaches a maximum. Then it reverses course: Further cooling makes water less dense. And water’s heat capacity, the amount of energy required to increase its temperature a given amount, does a similar about-face. Scientists suspected the flip-flopping properties could be a sign of a critical point lurking at lower temperature. In 2020, experiments provided evidence that supercooled water can take on two different phases: a high-density or a low-density liquid. Those two phases were thought to become one at the critical point, but until now, that hadn’t been observed. Experiments at pressures and temperatures close to the predicted critical point are extremely challenging. That realm is known as “no man’s land” because supercooled water freezes almost instantaneously there. So chemical physicist Anders Nilsson of Stockholm University and colleagues turned to sophisticated tactics. “We have to do everything very quickly,” Nilsson says. The researchers started with tiny samples of special types of ice, called amorphous ice, in which the molecules are jumbled up rather than arranged in a crystalline structure. In experiments at Pohang Accelerator Laboratory in South Korea, researchers hit each sample of ice with a short blast from an infrared laser to melt it. Then, within nanoseconds to microseconds, they probed it with the lab’s X-ray laser. The results revealed the liquid’s structure and density under various pressures and temperatur