Boiled water freezes faster. Why does hot water freeze faster than cold water?

21.11.2017 11.10.2018 Alexander Firtsev

« Which water freezes faster cold or hot?”- try asking your friends a question, most likely most of them will answer that cold water freezes faster - and make a mistake.

In fact, if you simultaneously put two vessels of the same shape and volume in the freezer, one of which will contain cold water and the other hot, then hot water will freeze faster.

Such a statement may seem absurd and unreasonable. Logically, hot water must first cool down to cold temperature, and cold water should already turn into ice at this time.

So why does hot water overtake cold water on its way to freezing? Let's try to figure it out.

History of observations and research

People have observed the paradoxical effect since ancient times, but no one attached much importance to it. So inconsistencies in the rate of freezing of cold and hot water were noted in their notes by Arestotel, as well as by Rene Descartes and Francis Bacon. An unusual phenomenon often manifested itself in everyday life.

For a long time, the phenomenon was not studied in any way and did not arouse much interest among scientists.

The study of the unusual effect began in 1963, when an inquisitive student from Tanzania, Erasto Mpemba, noticed that hot milk for ice cream freezes faster than cold milk. Hoping to get an explanation of the reasons for the unusual effect, the young man asked his physics teacher at school. However, the teacher only laughed at him.

Later, Mpemba repeated the experiment, but in his experiment he no longer used milk, but water, and the paradoxical effect was repeated again.

Six years later, in 1969, Mpemba asked this question to physics professor Dennis Osborne, who came to his school. The professor was interested in the observation of the young man, as a result, an experiment was conducted that confirmed the presence of the effect, but the reasons for this phenomenon were not established.

Since then, the phenomenon has been called Mpemba effect.

Throughout the history of scientific observations, many hypotheses have been put forward about the causes of the phenomenon.

So in 2012, the British Royal Society of Chemistry would announce a competition of hypotheses to explain the Mpemba effect. Scientists from all over the world participated in the competition, in total 22,000 scientific papers were registered. Despite such an impressive number of articles, none of them clarified the Mpemba paradox.

The most common was the version according to which, hot water freezes faster, since it simply evaporates faster, its volume becomes smaller, and as the volume decreases, its cooling rate increases. The most common version was eventually refuted as an experiment was conducted in which evaporation was excluded, but the effect was nevertheless confirmed.

Other scientists believed that the reason for the Mpemba effect is the evaporation of gases dissolved in water. In their opinion, during the heating process, gases dissolved in water evaporate, due to which it acquires a higher density than cold water. As is known, an increase in density leads to a change in the physical properties of water (an increase in thermal conductivity), and hence an increase in the cooling rate.

In addition, a number of hypotheses have been put forward that describe the rate of water circulation as a function of temperature. In many studies, an attempt was made to establish the relationship between the material of the containers in which the liquid was located. Many theories seemed very plausible, but they could not be scientifically confirmed due to a lack of initial data, contradictions in other experiments, or due to the fact that the identified factors were simply not comparable with the rate of water cooling. Some scientists in their works questioned the existence of the effect.

In 2013, researchers at the Nanyang Technological University in Singapore claimed to have solved the mystery of the Mpemba effect. According to their study, the reason for the phenomenon lies in the fact that the amount of energy stored in hydrogen bonds between cold and hot water molecules differs significantly.

Computer simulation methods have shown the following results: the higher the temperature of the water, the greater the distance between the molecules due to the fact that the repulsive forces increase. Consequently, the hydrogen bonds of molecules are stretched, storing more energy. When cooled, the molecules begin to approach each other, releasing energy from hydrogen bonds. In this case, the release of energy is accompanied by a decrease in temperature.

In October 2017, Spanish physicists, in the course of another study, found out that it is the removal of matter from equilibrium (strong heating before strong cooling) that plays a large role in the formation of the effect. They determined the conditions under which the likelihood of the effect is maximum. In addition, scientists from Spain have confirmed the existence of the reverse Mpemba effect. They found that when heated, a colder sample can reach a high temperature faster than a warm one.

Despite exhaustive information and numerous experiments, scientists intend to continue studying the effect.

Mpemba effect in real life

Have you ever wondered why in winter the ice rink is filled with hot water and not cold? As you already understood, they do this because a skating rink filled with hot water will freeze faster than if it were filled with cold water. For the same reason, slides in winter ice towns are poured with hot water.

Thus, knowledge about the existence of the phenomenon allows people to save time when preparing sites for winter sports.

In addition, the Mpemba effect is sometimes used in industry - to reduce the freezing time of products, substances and materials containing water.

Mpemba effect(Mpemba paradox) - a paradox that states that hot water under certain conditions freezes faster than cold water, although it must pass the temperature of cold water in the process of freezing. This paradox is an experimental fact that contradicts the usual ideas, according to which, under the same conditions, a hotter body needs more time to cool down to a certain temperature than a cooler body to cool down to the same temperature.

This phenomenon was noticed at the time by Aristotle, Francis Bacon and Rene Descartes, but only in 1963, the Tanzanian schoolboy Erasto Mpemba found that a hot ice cream mixture freezes faster than a cold one.

Erasto Mpemba was a student at Magambin High School in Tanzania doing practical cooking work. He had to make homemade ice cream - boil milk, dissolve sugar in it, cool it to room temperature, and then put it in the refrigerator to freeze. Apparently, Mpemba was not a particularly diligent student and procrastinated on the first part of the assignment. Fearing that he would not be in time by the end of the lesson, he put the still hot milk in the refrigerator. To his surprise, it froze even earlier than the milk of his comrades, prepared according to a given technology.

After that, Mpemba experimented not only with milk, but also with ordinary water. In any case, already being a student at the Mkvava High School, he asked Professor Dennis Osborne from the University College in Dar es Salaam (invited by the director of the school to give a lecture on physics to the students) about water: "If you take two identical containers with equal volumes of water so that in one of them the water has a temperature of 35 ° C, and in the other - 100 ° C, and put them in the freezer, then in the second the water will freeze faster. Why? Osborne became interested in this issue and soon in 1969, together with Mpemba, they published the results of their experiments in the journal "Physics Education". Since then, the effect they discovered is called Mpemba effect.

Until now, no one knows exactly how to explain this strange effect. Scientists do not have a single version, although there are many. It's all about the difference in the properties of hot and cold water, but it is not yet clear which properties play a role in this case: the difference in supercooling, evaporation, ice formation, convection, or the effect of liquefied gases on water at different temperatures.

The paradox of the Mpemba effect is that the time during which the body cools down to the ambient temperature must be proportional to the temperature difference between this body and the environment. This law was established by Newton and since then has been confirmed many times in practice. In the same effect, water at 100°C cools down to 0°C faster than the same amount of water at 35°C.

However, this does not yet imply a paradox, since the Mpemba effect can also be explained within known physics. Here are some explanations for the Mpemba effect:

Evaporation

Hot water evaporates faster from the container, thereby reducing its volume, and a smaller volume of water with the same temperature freezes faster. Water heated to 100 C loses 16% of its mass when cooled to 0 C.

The evaporation effect is a double effect. First, the mass of water required for cooling is reduced. And secondly, the temperature decreases due to the fact that the heat of evaporation of the transition from the water phase to the vapor phase decreases.

temperature difference

Due to the fact that the temperature difference between hot water and cold air is greater - hence the heat exchange in this case is more intense and hot water cools faster.

hypothermia

When water is cooled below 0 C, it does not always freeze. Under certain conditions, it can undergo supercooling while continuing to remain liquid at temperatures below the freezing point. In some cases, water can remain liquid even at -20 C.

The reason for this effect is that in order for the first ice crystals to begin to form, centers of crystal formation are needed. If they are not in liquid water, then supercooling will continue until the temperature drops enough that crystals begin to form spontaneously. When they start to form in the supercooled liquid, they will start to grow faster, forming an ice slush that will freeze to form ice.

Hot water is most susceptible to hypothermia because heating it eliminates dissolved gases and bubbles, which in turn can serve as centers for the formation of ice crystals.

Why does hypothermia cause hot water to freeze faster? In the case of cold water, which is not supercooled, the following occurs. In this case, a thin layer of ice will form on the surface of the vessel. This layer of ice will act as an insulator between the water and cold air and will prevent further evaporation. The rate of formation of ice crystals in this case will be less. In the case of hot water undergoing subcooling, the subcooled water does not have a protective surface layer of ice. Therefore, it loses heat much faster through the open top.

When the supercooling process ends and the water freezes, much more heat is lost and therefore more ice is formed.

Many researchers of this effect consider hypothermia to be the main factor in the case of the Mpemba effect.

Convection

Cold water begins to freeze from above, thereby worsening the processes of heat radiation and convection, and hence the loss of heat, while hot water begins to freeze from below.

This effect is explained by an anomaly in the density of water. Water has a maximum density at 4 C. If you cool water to 4 C and put it at a lower temperature, the surface layer of water will freeze faster. Because this water is less dense than water at 4°C, it will stay on the surface, forming a thin cold layer. Under these conditions, a thin layer of ice will form on the surface of the water for a short time, but this layer of ice will serve as an insulator protecting the lower layers of water, which will remain at a temperature of 4 C. Therefore, further cooling will be slower.

In the case of hot water, the situation is completely different. The surface layer of water will cool more quickly due to evaporation and a greater temperature difference. Also, cold water layers are denser than hot water layers, so the cold water layer will sink down, lifting the warm water layer to the surface. This circulation of water ensures a rapid drop in temperature.

But why does this process not reach the equilibrium point? To explain the Mpemba effect from this point of view of convection, it would be necessary to assume that the cold and hot layers of water are separated and the convection process itself continues after the average water temperature drops below 4 C.

However, there is no experimental evidence to support this hypothesis that cold and hot layers of water are separated by convection.

gases dissolved in water

Water always contains gases dissolved in it - oxygen and carbon dioxide. These gases have the ability to lower the freezing point of water. When the water is heated, these gases are released from the water because their solubility in water at high temperature is lower. Therefore, when hot water is cooled, there are always fewer dissolved gases in it than in unheated cold water. Therefore, the freezing point of heated water is higher and it freezes faster. This factor is sometimes considered as the main one in explaining the Mpemba effect, although there are no experimental data confirming this fact.

Thermal conductivity

All these (as well as other) conditions have been studied in many experiments, but an unequivocal answer to the question - which of them provide a 100% reproduction of the Mpemba effect - has not been obtained.

So, for example, in 1995, the German physicist David Auerbach studied the influence of water supercooling on this effect. He discovered that hot water, reaching a supercooled state, freezes at a higher temperature than cold water, and therefore faster than the latter. But cold water reaches the supercooled state faster than hot water, thereby compensating for the previous lag.

In addition, Auerbach's results contradicted earlier data that hot water is able to achieve more supercooling due to fewer crystallization centers. When water is heated, gases dissolved in it are removed from it, and when it is boiled, some salts dissolved in it precipitate.

So far, only one thing can be asserted - the reproduction of this effect essentially depends on the conditions under which the experiment is carried out. Precisely because it is not always reproduced.

O. V. Mosin

Literarysources:

"Hot water freezes faster than cold water. Why does it do so?", Jearl Walker in The Amateur Scientist, Scientific American, Vol. 237, no. 3, pp. 246-257; September, 1977.

"The Freezing of Hot and Cold Water", G.S. Kell in American Journal of Physics, Vol. 37, no. 5, pp. 564-565; May 1969.

"Supercooling and the Mpemba effect", David Auerbach, in American Journal of Physics, Vol. 63, no. 10, pp. 882-885; Oct, 1995.

"The Mpemba effect: The freezing times of hot and cold water", Charles A. Knight, in American Journal of Physics, Vol. 64, no. 5, p 524; May, 1996.

The phenomenon of hot water solidifying at a faster rate than cold water is known in science as the Mpemba effect. Such great minds as Aristotle, Francis Bacon and Rene Descartes pondered over this paradoxical phenomenon, but for millennia no one has yet been able to offer a reasonable explanation for this phenomenon.

Only in 1963, a schoolboy from the Republic of Tanganyika, Erasto Mpemba, noticed this effect on the example of ice cream, but none of the adults gave him an explanation. Nevertheless, physicists and chemists seriously thought about such a simple, but so incomprehensible phenomenon.

Since then, different versions have been expressed, one of which was as follows: part of the hot water simply evaporates at first, and then, when a smaller amount remains, the water solidifies faster. This version, due to its simplicity, became the most popular, but scientists were not completely satisfied.

Now a team of researchers from the Nanyang Technological University in Singapore, led by chemist Xi Zhang, says they have solved the age-old mystery of why warm water freezes faster than cold water. As Chinese experts found out, the secret lies in the amount of energy stored in hydrogen bonds between water molecules.

As you know, water molecules are made up of one oxygen atom and two hydrogen atoms held together by covalent bonds, which at the particle level looks like an exchange of electrons. Another well-known fact is that hydrogen atoms are attracted to oxygen atoms from neighboring molecules - this forms hydrogen bonds.

At the same time, water molecules as a whole repel each other. Scientists from Singapore noticed that the warmer the water, the greater the distance between the molecules of the liquid due to the increase in repulsive forces. As a result, hydrogen bonds are stretched, and therefore store more energy. This energy is released when the water cools - the molecules approach each other. And the return of energy, as you know, means cooling.

As the chemists write in their article, which can be found on the arXiv.org preprint site, hydrogen bonds are stretched more strongly in hot water than in cold water. Thus, it turns out that more energy is stored in the hydrogen bonds of hot water, which means that more of it is released when cooled to sub-zero temperatures. For this reason, freezing is faster.

To date, scientists have solved this riddle only theoretically. When they present convincing evidence of their version, then the question of why hot water freezes faster than cold water can be considered closed.

In the good old formula H 2 O, it would seem that there are no secrets. But in fact, water - the source of life and the most famous liquid in the world - is fraught with many mysteries that sometimes even scientists cannot solve.

Here are the 5 most interesting facts about water:

1. Hot water freezes faster than cold water

Take two containers of water: pour hot water into one and cold water into the other, and place them in the freezer. Hot water will freeze faster than cold water, although logically, cold water should have turned into ice first: after all, hot water must first cool down to cold temperature, and then turn into ice, while cold water does not need to cool down. Why is this happening?

In 1963, Erasto B. Mpemba, a senior high school student in Tanzania, while freezing a prepared ice cream mixture, noticed that the hot mixture solidified faster in the freezer than the cold one. When the young man shared his discovery with a physics teacher, he only laughed at him. Fortunately, the student was persistent and convinced the teacher to conduct an experiment, which confirmed his discovery: under certain conditions, hot water really freezes faster than cold water.

Now this phenomenon of hot water freezing faster than cold water is called the Mpemba effect. True, long before him, this unique property of water was noted by Aristotle, Francis Bacon and Rene Descartes.

Scientists do not fully understand the nature of this phenomenon, explaining it either by the difference in hypothermia, evaporation, ice formation, convection, or the effect of liquefied gases on hot and cold water.

Note from Х.RU to the topic "Hot water freezes faster than cold water".

Since cooling issues are closer to us, refrigeration specialists, let us delve a little deeper into the essence of this problem and give two opinions about the nature of such a mysterious phenomenon.

1. A University of Washington scientist has offered an explanation for a mysterious phenomenon known since the time of Aristotle: why hot water freezes faster than cold water.

The phenomenon, called the Mpemba effect, is widely used in practice. For example, experts advise motorists to pour cold rather than hot water into the washer reservoir in winter. But what underlies this phenomenon remained unknown for a long time.

Dr. Jonathan Katz of the University of Washington investigated this phenomenon and concluded that substances dissolved in water play an important role in it, which precipitate when heated, reports EurekAlert.

By solutes, Dr. Katz means the calcium and magnesium bicarbonates found in hard water. When the water is heated, these substances precipitate, forming scale on the walls of the kettle. Water that has never been heated contains these impurities. As it freezes and ice crystals form, the concentration of impurities in water increases 50 times. This lowers the freezing point of water. "And now the water has to cool down in order to freeze," explains Dr. Katz.

There is a second reason that prevents freezing of unheated water. Lowering the freezing point of water reduces the temperature difference between the solid and liquid phases. "Because the rate at which water loses heat depends on this temperature difference, water that has not been heated cools down worse," comments Dr. Katz.

According to the scientist, his theory can be tested experimentally, because. the Mpemba effect becomes more pronounced for harder water.

2. Oxygen plus hydrogen plus cold creates ice. At first glance, this transparent substance seems very simple. In fact, the ice is fraught with many mysteries. The ice created by the African Erasto Mpemba did not think about glory. The days were hot. He wanted popsicles. He took a carton of juice and put it in the freezer. He did this more than once and therefore noticed that the juice freezes especially quickly, if you hold it in the sun before that - just heat it up! This is strange, thought the Tanzanian schoolboy, who acted contrary to worldly wisdom. Is it possible that in order for the liquid to turn into ice faster, it must first ... be heated? The young man was so surprised that he shared his guess with the teacher. He reported this curiosity in the press.

This story happened back in the 1960s. Now the "Mpemba effect" is well known to scientists. But for a long time this seemingly simple phenomenon remained a mystery. Why does hot water freeze faster than cold water?

It wasn't until 1996 that physicist David Auerbach found a solution. To answer this question, he conducted an experiment for a whole year: he heated water in a glass and cooled it again. So what did he find out? When heated, air bubbles dissolved in water evaporate. Water devoid of gases freezes more easily on the walls of the vessel. "Of course, water with a high air content will also freeze," says Auerbach, "but not at zero degrees Celsius, but only at minus four to six degrees." Of course, you will have to wait longer. So, hot water freezes before cold water, this is a scientific fact.

There is hardly a substance that would appear before our eyes with the same ease as ice. It consists only of water molecules - that is, elementary molecules containing two hydrogen atoms and one oxygen. However, ice is perhaps the most mysterious substance in the universe. Scientists have not been able to explain some of its properties so far.

2. Supercooling and "flash" freezing

Everyone knows that water always turns to ice when it cools down to 0 °C... except in some cases! Such a case is, for example, "supercooling", which is the property of very pure water to remain liquid even when cooled below freezing. This phenomenon becomes possible due to the fact that the environment does not contain crystallization centers or nuclei that could provoke the formation of ice crystals. And so water remains in liquid form, even when cooled to temperatures below zero degrees Celsius. The crystallization process can be triggered, for example, by gas bubbles, impurities (pollution), uneven surface of the container. Without them, water will remain in a liquid state. When the crystallization process starts, you can watch how the super-cooled water instantly turns into ice.

Watch the video (2 901 Kb, 60 c) by Phil Medina (www.mrsciguy.com) and see for yourself >>

Comment. Superheated water also remains liquid even when heated above its boiling point.

3. "Glass" water

Quickly and without hesitation, name how many different states water has?

If you answered three (solid, liquid, gas), then you are wrong. Scientists distinguish at least 5 different states of water in liquid form and 14 states of ice.

Remember the conversation about super-chilled water? So, no matter what you do, at -38 ° C, even the purest super-cooled water suddenly turns into ice. What happens with a further decrease

temperature? At -120 °C, something strange begins to happen to water: it becomes super-viscous or viscous, like molasses, and at temperatures below -135 °C it turns into "glassy" or "glassy" water - a solid substance in which there is no crystalline structure.

4. Quantum properties of water

At the molecular level, water is even more surprising. In 1995, scientists conducted an experiment on neutron scattering gave an unexpected result: physicists found that neutrons aimed at water molecules "see" 25% less hydrogen protons than expected.

It turned out that at the speed of one attosecond (10 -18 seconds) an unusual quantum effect takes place, and the chemical formula of water instead of the usual one - H 2 O, becomes H 1.5 O!

5. Does water have a memory?

Homeopathy, an alternative to conventional medicine, claims that a dilute solution of a medicinal product can have a healing effect on the body, even if the dilution factor is so great that there is nothing left in the solution but water molecules. Proponents of homeopathy explain this paradox by a concept called "memory of water", according to which water at the molecular level has a "memory" of the substance once dissolved in it and retains the properties of the solution of the original concentration after not a single molecule of the ingredient remains in it.

An international team of scientists led by Professor Madeleine Ennis from Queen's University of Belfast, who criticized the principles of homeopathy, conducted an experiment in 2002 to refute this concept once and for all. The result was the opposite. After what, the scientists said that they were able to prove the reality of the effect of "memory of water. However, experiments conducted under the supervision of independent experts, did not bring results. Disputes about the existence of the phenomenon of "memory of water" continue.

Water has many other unusual properties that we have not covered in this article.

Literature.

1. 5 Really Weird Things About Water / http://www.neatorama.com.
2. The mystery of water: the theory of the Aristotle-Mpemba effect was created / http://www.o8ode.ru.
3. Nepomniachtchi N.N. Secrets of inanimate nature. The most mysterious substance in the universe / http://www.bibliotekar.ru.

In 1963, a schoolboy from Tanzania named Erasto Mpemba asked his teacher a stupid question - why does warm ice cream freeze faster than cold ice cream in his freezer?

Erasto Mpemba was a student at Magambin High School in Tanzania doing practical cooking work. He had to make homemade ice cream - boil milk, dissolve sugar in it, cool it to room temperature, and then put it in the refrigerator to freeze. Apparently, Mpemba was not a particularly diligent student and procrastinated on the first part of the assignment. Fearing that he would not be in time by the end of the lesson, he put still hot milk in the refrigerator. To his surprise, it froze even earlier than the milk of his comrades, prepared according to a given technology.

He turned to the physics teacher for clarification, but he only laughed at the student, saying the following: "This is not world physics, but the physics of Mpemba." After that, Mpemba experimented not only with milk, but also with ordinary water.

In any case, already being a student at the Mkwawa High School, he asked Professor Dennis Osborne from the University College in Dar es Salaam (invited by the director of the school to give a lecture on physics to the students) about water: “If you take two identical containers with equal volumes of water so that in one of them the water has a temperature of 35 ° C, and in the other - 100 ° C, and put them in the freezer, then in the second the water will freeze faster. Why?" Osborne became interested in this issue and soon in 1969, together with Mpemba, they published the results of their experiments in the journal Physics Education. Since then, the effect they discovered is called the Mpemba effect.

Are you curious to know why this happens? Just a few years ago, scientists managed to explain this phenomenon ...

The Mpemba effect (Mpemba Paradox) is a paradox that states that hot water under certain conditions freezes faster than cold water, although it must pass the temperature of cold water in the process of freezing. This paradox is an experimental fact that contradicts the usual ideas, according to which, under the same conditions, a hotter body needs more time to cool down to a certain temperature than a cooler body to cool down to the same temperature.

This phenomenon was noticed at the time by Aristotle, Francis Bacon and Rene Descartes. Until now, no one knows exactly how to explain this strange effect. Scientists do not have a single version, although there are many. It's all about the difference in the properties of hot and cold water, but it is not yet clear which properties play a role in this case: the difference in supercooling, evaporation, ice formation, convection, or the effect of liquefied gases on water at different temperatures. The paradox of the Mpemba effect is that the time during which the body cools down to the ambient temperature must be proportional to the temperature difference between this body and the environment. This law was established by Newton and since then has been confirmed many times in practice. In the same effect, water at 100°C cools down to 0°C faster than the same amount of water at 35°C.

As you know, water molecules consist of one oxygen atom and two hydrogen atoms held together by covalent bonds, which at the particle level looks like an exchange of electrons. Another well-known fact is that hydrogen atoms are attracted to oxygen atoms from neighboring molecules - hydrogen bonds are formed.

Here are the hypotheses put forward by scientists:

Evaporation

Hot water evaporates faster from the container, thereby reducing its volume, and a smaller volume of water with the same temperature freezes faster. Water heated to 100°C loses 16% of its mass when cooled to 0°C. The evaporation effect is a double effect. First, the mass of water required for cooling is reduced. And secondly, due to evaporation, its temperature decreases.

temperature difference

Due to the fact that the temperature difference between hot water and cold air is greater - therefore, heat transfer in this case is more intense and hot water cools faster.

hypothermia
When water is cooled below 0°C, it does not always freeze. Under certain conditions, it can undergo supercooling while continuing to remain liquid at temperatures below the freezing point. In some cases, water can remain liquid even at -20°C. The reason for this effect is that in order for the first ice crystals to begin to form, centers of crystal formation are needed. If they are not in liquid water, then supercooling will continue until the temperature drops enough that crystals begin to form spontaneously. When they start to form in the supercooled liquid, they will start to grow faster, forming an ice slush that will freeze to form ice. Hot water is most susceptible to hypothermia because heating it eliminates dissolved gases and bubbles, which in turn can serve as centers for the formation of ice crystals. Why does hypothermia cause hot water to freeze faster? In the case of cold water that is not supercooled, what happens is that a thin layer of ice forms on its surface, which acts as an insulator between the water and the cold air, and thus prevents further evaporation. The rate of formation of ice crystals in this case will be less. In the case of hot water undergoing subcooling, the subcooled water does not have a protective surface layer of ice. Therefore, it loses heat much faster through the open top. When the supercooling process ends and the water freezes, much more heat is lost and therefore more ice is formed. Many researchers of this effect consider hypothermia to be the main factor in the case of the Mpemba effect.
Convection

Cold water begins to freeze from above, thereby worsening the processes of heat radiation and convection, and hence the loss of heat, while hot water begins to freeze from below. This effect is explained by an anomaly in the density of water. Water has a maximum density at 4°C. If water is cooled to 4°C and placed in an environment with a lower temperature, the surface layer of water will freeze faster. Because this water is less dense than water at 4°C, it will stay on the surface, forming a thin cold layer. Under these conditions, a thin layer of ice will form on the surface of the water for a short time, but this layer of ice will serve as an insulator protecting the lower layers of water, which will remain at 4°C. Therefore, the further cooling process will be slower. In the case of hot water, the situation is completely different. The surface layer of water will cool more rapidly due to evaporation and greater temperature differences. Also, cold water layers are denser than hot water layers, so the cold water layer will sink down, lifting the warm water layer to the surface. This circulation of water ensures a rapid drop in temperature. But why does this process not reach the equilibrium point? To explain the Mpemba effect from the point of view of convection, one should assume that the cold and hot layers of water are separated and the convection process itself continues after the average water temperature drops below 4°C. However, there is no experimental evidence to support this hypothesis that cold and hot water layers are separated by convection.

gases dissolved in water

Thermal conductivity

This mechanism can play a significant role when water is placed in a refrigerator freezer in small containers. Under these conditions, it has been observed that the container with hot water melts the ice of the freezer under itself, thereby improving thermal contact with the wall of the freezer and thermal conductivity. As a result, heat is removed from the hot water container faster than from the cold one. In turn, the container with cold water does not melt snow under it. All these (as well as other) conditions have been studied in many experiments, but an unambiguous answer to the question - which of them provide a 100% reproduction of the Mpemba effect - has not been obtained. So, for example, in 1995, the German physicist David Auerbach studied the influence of water supercooling on this effect. He discovered that hot water, reaching a supercooled state, freezes at a higher temperature than cold water, and therefore faster than the latter. But cold water reaches a supercooled state faster than hot water, thereby compensating for the previous lag. In addition, Auerbach's results contradicted earlier data that hot water is able to achieve more supercooling due to fewer crystallization centers. When water is heated, gases dissolved in it are removed from it, and when it is boiled, some salts dissolved in it precipitate. So far, only one thing can be asserted - the reproduction of this effect significantly depends on the conditions under which the experiment is carried out. Precisely because it is not always reproduced.

And here's the most likely reason.

Portal for the student. Self-training

History of observations and research

Mpemba effect in real life

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