Popular Science

Another explanation for the Mpemba effect (this is why boiling water freezes faster than cold water)


Summary: due to the presence of hydrogen bonds in water molecules, the configuration of the O-H covalent bonds changes, with the accumulation of additional energy in them, released during cooling and working as an additional heating that prevents freezing. In hot water, the hydrogen bonds are stretched, the covalent bonds are not strained, the energy storage is low-cooling and freezing is faster. There is a certain characteristic time tau required for the formation of hydrogen bonds; if the cooling process proceeds slowly, then the Mpemba effect will disappear. If the cooling process proceeds relatively quickly (up to tens of minutes), then the effect is pronounced. Probably, there must also be some critical temperature, starting from which the effect appears, but this is not reflected in the article.

The KDPV shows an image from the original article, looking at which the reader should clearly see that energy is stored in covalent bonds, which can then be released in the form of additional heat, preventing cold water from cooling.

History of the issue
Aristotle was the first to note that hot water freezes faster than cold water, but chemists have always refused to explain this paradox. Up to this day.

Water is one of the most common substances on Earth, but at the same time one of the most mysterious. For example, like most liquids, its density increases with cooling. However, unlike the others, its density reaches a maximum at a temperature of 4C, and then begins to decrease up to the crystallization temperature.

In the solid phase, water has a slightly lower density, which is why ice floats on the surface of the water. This is one of the reasons for the existence of life on Earth - if ice were denser than water, then when it freezes, it would sink to the bottom of lakes and oceans, which would make many types of chemical processes that make life possible impossible.

So there is the strange Mpemba effect, named after a Tanzanian student who discovered that hot ice cream concoctions freeze faster than cold ones in the freezer of a school kitchen sometime in the early 1960s. (In fact, this effect has been noted by many scholars in history, starting with Aristotle, Francis Bacon, and René Descartes.)

The Mpemba effect is that hot water freezes faster than cold water. This effect has been measured in a variety of cases, with various explanations set out below. One idea is that hot containers have better thermal contact with the freezer and dissipate heat more efficiently. Another is that warm water evaporates faster, and since this process is endothermic (it goes with the absorption of heat), it accelerates freezing.

None of these explanations seem plausible, so a real explanation has so far been lacking.
New explanation for the effect (now definitely correct)
Today Zi Chang of Nanyang Technological University in Singapore and several of his colleagues provided one. These guys argue that the Mpemba effect is the result of the unique properties of different types of bonds that hold water molecules together.

So what is it about these connections? Each water molecule is made up of a relatively large oxygen atom bonded to two small hydrogen atoms by a conventional covalent bond. But if you put several water molecules next to each other, then hydrogen bonds will also begin to play an important role. This is due to the fact that the hydrogen atoms of one molecule are located near the oxygen of another molecule, and interact with it. Hydrogen bonds are much weaker than covalent bonds, but stronger than the van der Waals forces that geckos use to stick to vertical walls.

Chemists have long known the importance of these connections. For example, the boiling point of water is much higher than other liquids with similar molecules because hydrogen bonds hold the molecules together.

But in recent years, chemists have become increasingly interested in other roles that hydrogen bonds can play. For example, water molecules in thin capillaries form long chains held together by hydrogen bonds. This is very important for plants in which evaporation of water through leaf membranes effectively pulls a chain of water molecules up from the roots.

Now Zee and coauthors argue that hydrogen bonds also explain the Mpemba effect. Their key idea is that hydrogen bonds lead to tighter contact of water molecules, and when this happens, the natural repulsion between the molecules leads to the contraction of the covalent bonds and the accumulation of energy in them.

However, when the liquid heats up, the distance between the molecules increases and the hydrogen bonds stretch. It also allows you to increase the length of covalent bonds and thus give back the energy stored in them. An important element of the theory is the fact that the process in which covalent bonds give up the stored energy is equivalent to cooling!

In fact, this effect is amplified by the normal cooling process. Thus, hot water should be cooled faster than cold water, the authors argue. And this is exactly what we see in the Mpemba effect.
Why is the new explanation better than the previous ones?
These guys calculated the amount of additional cooling, and showed that it exactly corresponds to the observed difference in experiments to measure the difference in the cooling rates of hot and cold water. Voila! It's an interesting look at the complex and mysterious properties of water that still keep chemists awake at night. While the idea of Zee and coauthors is compelling, it could turn out to be another theoretical mistake that other physicists will have to refute. This is because the theory lacks predictive power (at least in the original article).

Zee and co-authors need to use their theory to predict new properties of water that are not deduced from conventional reasoning. For example, if covalent bonds are shortened, this should lead to the emergence of some new measurable properties of water, which should not have been manifested otherwise. The discovery and measurement of such properties would be the last cherry on the cake, lacking in theory in its current form.

So while the guys may have explained the Mpemba effect pretty well, they need to do a bit of work to convince others of this.

Be that as it may, their theory is interesting.

PS in 2016, one of the co-authors - Chang Q. Sun, together with Yi Sun, published a more complete statement of the proposed theory, with consideration of surface effects, convection, diffusion, radiation and other factors - and seem to observe good agreement with experiment ( Springer ).
Ref: arxiv.org/abs/1310.6514: O:H-O Bond Anomalous Relaxation Resolving Mpemba Paradox

why "they explained again" - but because it already happened:

Non-equilibrium Markov processes: they can follow some unusual trajectories much faster than equilibrium ones, so the rapid cooling of boiling water falls on such an "accelerated" trajectory, and overtakes cold water (which cools in more equilibrium conditions).
Clusters (also due to hydrogen bonds) that interfere with crystallization. In boiling water, such clusters are absent, and when it freezes, they do not have time to form, but in water that has been cold outside the freezer for a long time, they have time and do not allow it to freeze normally.
Hypothermia below the freezing point, which is less pronounced in initially hot water, because there is more disorder, and there is not enough time to get organized in the freezer during the freezing process. (but this is clearly a problem - in experiments, the entire cooling curve of hot water goes steeper than cold water, and not only the freezing process, and this "disorder" on thermal conductivity and cooling, if it should influence, is just slowing down the cooling, and acceleration is observed).
Water evaporates from the surface and carries away heat. For hot water, it is faster (only it is not clear why, after equalizing the temperatures, the water that was hot continues to evaporate more actively, although it is already colder than the water that was initially cold).
It's all to blame for convection, which improves heat transfer (convection currents rotate by inertia even after the temperature of the glasses has leveled off and for a long time after that).
American Journal of Physics 77, 27 (2009); https://doi.org/10.1119/1.2996187
Dissolution of impurities (gases?) Is to blame for everything. There are fewer impurities in boiling water, freezing is faster.
Tags: physics
xially 12 march 2021, 16:58
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