José Manuel NievesSEGUIR Updated: Save Send news by mail electrónicoTu name *
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At very cold temperatures, the water displays a behavior very strange. For a start, and against all logic, to cool down the water of expands rather than contract (that’s why ice floats). The cold water, in addition, it is more difficult to compress than the hottest . And to make matters worse, when it freezes, its molecules can be arranged in several different ways .
it Is difficult to find explanations for all of this, and those that exist are subject to controversy and bitter scientific controversies. One of them, proposed almost three decades ago, is the idea that very cold water can exist in two liquid forms a different, a less dense and structured than the other. In other words, you could have two kinds of water , and every one of them would be a liquid other than. Check it out in the laboratory it is difficult, but a team of researchers, italians and americans just find a strong evidence that it could really be so. The work was just published in the journal Science.
In his study, Pablo Debenedetti and Gül H. Zerze , of Princeton University, and Francesco Sciortico, the Sapienza, in Rome, suggest that the “second critical point of water” takes place at temperatures of between -83, and -100 degrees and an atmospheric pressure almost 2,000 times greater than that at sea level. A critical point is a unique value of temperature and pressure at which two phases of matter become indistinguishable, and it occurs just before the subject passes from one phase to another. The water, for example, has a critical point well-known to the passing of liquid-to-vapor .
“you Can imagine our joy when we begin to see the critical fluctuations behaving exactly as they were supposed to do -explains Sciortino-. Now I can sleep peacefully, because after 25 years, my original idea has been confirmed”.
Until now, experiments using molecules of water real to test if there is a second critical point for “ supercooling ” they have not succeeded in providing unambiguous evidence of its existence. According to Debenedetti this is due in large part to the tendency of the water supercooled to become ice.
that’s why, on this occasion, the researchers decided to use computer models. We can get an idea of the difficulty of simulating the process if we think that, in spite of the tremendous power of the supercomputers of today, it took 18 months to complete calculations to carry out the simulation.
In the simulations, and as the temperatures fell well beyond the freezing point, the density of the water began to show large fluctuations. In the end, the scientists managed to detect the critical point they were looking for in two computer models of different water. In each model underwent the molecules of water to two computational approaches different, and both were successful in finding the desired second critical point of water.
As happens in the passage of the liquid phase to the vapor, the two phases of the water superenfriada happen because the shape of the water molecules can induce two different ways of joining, or packaged. In this way, in the liquid of lower density , four molecules are grouped around a fifth central molecule in a geometric shape called a tetrahedron. In the liquid of greater density, however, comes into play a sixth molecule, which has the effect of increasing the density.
In their paper, the researchers write, “to probe the limits of what is computationally possible at present in this area, we provide a clear evidence of the presence of a critical point is metastable in the region of deeply supercooled of the two molecules of water”.
Naturally, the finding should be confirmed now by other experiments “that use means even more accurate and computationally more expensive.”