At cold temperatures, water has two different liquid phases, which become one at the critical point. The discovery could help explain water’s quirks.
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 and about 1,000 times the pressure exerted by Earth’s atmosphere at sea level, researchers report in the March 26.
The discovery may help explain certain odd properties of the ubiquitous, all-important liquid.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.: 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., 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 temperatures. Snapshots of the just-melted liquid, taken over time as the water expanded, revealed how water behaved as the pressure in the sample decreased. Below the critical point, there was a distinct phase transition from one liquid to another as the pressure dropped. At higher temperatures, there was no such transition between two distinct liquids, indicating that the critical point had been reached. The researchers’ results are impressive, says physicist Greg Kimmel of Pacific Northwest National Laboratory in Richland, Wash. “The data they present shows a pretty clear picture,” matching the critical point hypothesis. He notes, however, that the work assumes that the liquid has reached a state of equilibrium, meaning that flows of matter and energy have settled down. And since the measurements are taken so quickly, it’s unclear if that’s the case. For Giovambattista, who has spent his career performing computer simulations of water and this critical point, just seeing it in the real world is a relief. “It’s kind of inner peace.”
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