Water reacts very unusually to very low temperatures. Let's start with the fact that when it cools, contrary to logic, water does not shrink, but expands (this is why ice has the property of buoyancy). Cold water is less compressible than hot water. Moreover, when frozen, water molecules can change their position in every possible way.
All this is difficult to find an explanation, and the existing theories cause fierce controversy in scientific circles. One of them was formulated almost three decades ago and was that ice water can exist in two different liquid forms, one of which has a less dense structure. In other words, there are two kinds of water, each of which is a separate liquid. It is difficult to prove this theory in a laboratory, but Italian scientists have managed to find evidence to defend it. The study was recently published in the journal Science.
In their study, scientists Pablo Debenedetti and Gul H. Zerze from Princeton University and Francesco Shortico from La Sapienza in Rome suggest that the "second critical point of water" occurs at temperatures from -83 to -100 degrees and at atmospheric pressures almost 2000 times higher. than the pressure above sea level. The critical point is the only temperature and pressure value at which two phases of a substance become indistinguishable, and this happens just before the substance passes from one phase to another. Water, for example, has a well-known critical point in the transition from liquid to vapor.
“Just imagine our joy when we saw that critical fluctuations proceed exactly as we expected, - explains Sortino, - now I can sleep peacefully, because after 25 years my idea has finally found confirmation.”
Until now, experiments using real water molecules to test the second critical point of "supercooling" water could not provide unequivocal evidence of its existence. This is largely due to the fact that icy water usually turns into ice, Debenedetti said.
For this reason, the researchers decided to resort to using computer models. The process is truly laborious: despite the high power of modern supercomputers, scientists spent 18 months doing the necessary calculations to create models.
In simulations, when the temperature was still far from the freezing point, the density of the water began to fluctuate greatly. As a result, the scientists managed to find the critical point that they were looking for in two different computer models of water. At the same time, to find the critical point of water in both models, different computational approaches were applied.
As with the transition from the liquid phase to the gas phase, ice water can change into two different phases, depending on how its molecules are rearranged. Thus, in a low-density liquid, four molecules are grouped around a central molecule to form a tetrahedron. However, in a liquid with a higher density, the sixth molecule comes into play, which leads to an increase in its density.
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In their article, the researchers write that "within the limits of our computational capabilities, the existence of a metastable critical point in the stage of deep cooling of water molecules has been proven."
Naturally, now this conclusion must be confirmed by other experiments, "using more accurate and expensive computational means."