Water has two liquid states. Discovery could end debate lasting more than a century about the origin of water’s anomalous properties.
With ultrafast X-ray lasers capable of observing water in an extreme state before it freezes, researchers at Stockholm University have finally identified a “critical point” escondido in supercooled water, a discovery that could explain the anomalous behavior of this substance essential to life and reveal its role in natural, biological and climatic processes.
A ubiquitous and indispensable substance for life, water differs from most liquids because it presents unusual physical properties. Its density, heat capacity, viscosity and compressibility do not vary with temperature and pressure in the usual way. Contrary to what happens with most materials, water does not reach its maximum density when freezing: it is densest at 4 °C, which is why ice floats and colder water masses remain under warmer ones in lakes and oceans.
When water cools below 4°C, it begins to expand again. If it is sufficiently pure and cooled below 0°C without immediately crystallizing, this expansion continues and becomes even more pronounced as the temperature drops.
To investigate these features, scientists turned to extremely fast pulses of X-rays generated by advanced lasers in South Korea, according to the study in Science.
With this technology, scientists captured water in a deeply supercooled state, just before ice formed, and observed a phenomenon long theoretically predicted: the existence of two distinct liquid forms of water under conditions of low temperature and high pressure, which end up converging at a common critical point.
This point appears about -63 °C and 1000 atmospheres of pressure. According to the researchers, cited by , even though it is very far from everyday conditions, it continues to influence the behavior of water at normal temperatures and pressures.
Near this point, water becomes extremely unstable and rapidly oscillates between the two liquid states, or between mixtures of both. It is these fluctuations, the authors believe, that could be at the origin of many of the unusual properties observed in the natural environment.
The team also noticed that molecular movement slows down very sharply as the water approaches this critical point. One of the researchers described the effect as something similar to a “black hole”: once inside this critical region, the system tends to become stuck in an extremely slow state of dynamics.
The discovery could end a debate lasting more than a century about the origin of water’s anomalous properties. And it may reveal its impact on physical, chemical, biological, geological and climatic phenomena.
Scientists also admit that the fact that water exists, in Earth’s ambient conditions, in a regime influenced by this supercritical state raises a more complex question: is it a coincidence that life emerged precisely on a planet where water presents these unique characteristics, or is there an essential key to better understanding the existence of life itself?
