Water Has a Newfound ‘Critical Point’ That May Help Explain its Quirks
Arthur T Knackerbracket writes:
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.
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.
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