The British Royal Society of Chemistry is offering a £1,000 reward to anyone who can scientifically explain why hot water freezes faster than cold water in some cases.

“Modern science still cannot answer this seemingly simple question. Ice cream makers and bartenders use this effect in their daily work, but no one really knows why it works. This problem has been known for millennia, with philosophers such as Aristotle and Descartes thinking about it,” said Professor David Phillips, president of the British Royal Society of Chemistry, as quoted in a Society press release.

How a cook from Africa defeated a British physics professor

This is not an April Fool's joke, but a harsh physical reality. Modern science, which easily operates with galaxies and black holes, and builds giant accelerators to search for quarks and bosons, cannot explain how elementary water “works.” The school textbook clearly states that it takes more time to cool a hotter body than to cool a cold body. But for water, this law is not always observed. Aristotle drew attention to this paradox in the 4th century BC. e. Here is what the ancient Greek wrote in his book Meteorologica I: “The fact that water is preheated causes it to freeze. Therefore, many people, when they want to cool hot water faster, first put it in the sun...” In the Middle Ages, Francis Bacon and Rene Descartes tried to explain this phenomenon. Alas, neither the great philosophers nor the numerous scientists who developed classical thermophysics succeeded in this, and therefore such an inconvenient fact was “forgotten” for a long time.

And only in 1968 they “remembered” thanks to the schoolboy Erasto Mpembe from Tanzania, far from any science. While studying at culinary arts school in 1963, 13-year-old Mpembe was given the task of making ice cream. According to the technology, it was necessary to 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 diligent student and hesitated. Fearing that he would not make it 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 all the rules.

When Mpemba shared his discovery with his physics teacher, he laughed at him in front of the whole class. Mpemba remembered the insult. Five years later, already a student at the university in Dar es Salaam, he attended a lecture by the famous physicist Denis G. Osborne. After the lecture, he asked the scientist a question: “If you take two identical containers with equal amounts of water, one at 35 °C (95 °F) and the other at 100 °C (212 °F), and place them in the freezer, then Water in a hot container will freeze faster. Why?" You can imagine the reaction of a British professor to a question from a young man from Godforsaken Tanzania. He made fun of the student. However, Mpemba was ready for such an answer and challenged the scientist to a bet. Their dispute ended with an experimental test that confirmed Mpemba was right and Osborne defeated. Thus, the apprentice cook wrote his name in the history of science, and from now on this phenomenon is called the “Mpemba effect.” It is impossible to discard it, to declare it as “non-existent”. The phenomenon exists, and, as the poet wrote, “it doesn’t hurt.”

Are dust particles and solutes to blame?

Over the years, many have tried to unravel the mystery of freezing water. A whole bunch of explanations for this phenomenon have been proposed: evaporation, convection, the influence of dissolved substances - but none of these factors can be considered definitive. A number of scientists have devoted their entire lives to the Mpemba effect. James Brownridge, a member of the Department of Radiation Safety at the State University of New York, has been studying the paradox in his spare time for a decade. After conducting hundreds of experiments, the scientist claims to have evidence of the “guilt” of hypothermia. Brownridge explains that at 0°C, water only becomes supercooled, and begins to freeze when the temperature drops below. The freezing point is regulated by impurities in the water - they change the rate of formation of ice crystals. Impurities, such as dust particles, bacteria and dissolved salts, have a characteristic nucleation temperature when ice crystals form around crystallization centers. When several elements are present in water at once, the freezing point is determined by the one that has the highest nucleation temperature.

For the experiment, Brownridge took two water samples of the same temperature and placed them in the freezer. He discovered that one of the specimens always froze before the other, presumably due to a different combination of impurities.

Brownridge says hot water cools faster because there is a greater difference between the temperature of the water and the freezer - this helps it reach its freezing point before cold water reaches its natural freezing point, which is at least 5°C lower.

However, Brownridge's reasoning raises many questions. Therefore, those who can explain the Mpemba effect in their own way have a chance to compete for a thousand pounds sterling from the British Royal Society of Chemistry.

It would seem that the good old formula H 2 O contains no secrets. But in fact, water - the source of life and the most famous liquid in the world - is fraught with many mysteries that even scientists are sometimes unable to solve.

Here are the 5 most interesting facts about water:

1. Hot water freezes faster than cold water

Let's take two containers with 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 according to the logic of things, cold water should have turned into ice first: after all, hot water must first cool to the cold temperature, and then turn into ice, while cold water does not need to cool. Why is this happening?

In 1963, Erasto B. Mpemba, a high school student in Tanzania, was freezing an ice cream mixture and noticed that the hot mixture solidified faster in the freezer than the cold one. When the young man shared his discovery with his 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 actually 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 still do not fully understand the nature of this phenomenon, explaining it either by the difference in supercooling, evaporation, ice formation, convection, or by the effect of liquefied gases on hot and cold water.

Note from X.RU on the topic “Hot water freezes faster than cold water.”

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

1. A scientist from the University of Washington has proposed 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, not hot, water into the washer reservoir in winter. But what underlies this phenomenon remained unknown for a long time.

Dr. Jonathan Katz from the University of Washington studied this phenomenon and came to the conclusion that substances dissolved in water, which precipitate when heated, play an important role, reports EurekAlert.

By solutes, Dr. Katz means calcium and magnesium bicarbonates, which are found in hard water. When 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 the water increases 50 times. Because of this, the freezing point of water decreases. “And now the water has to cool further to freeze,” explains Dr. Katz.

There is a second reason that prevents unheated water from freezing. 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 less well,” comments Dr. Katz.

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

2. Oxygen plus hydrogen plus cold creates ice. At first glance, this transparent substance seems very simple. In reality, ice is fraught with many mysteries. Ice, created by the African Erasto Mpemba, did not think about fame. The days were hot. He wanted popsicles. He took the juice box and put it in the freezer. He did this more than once and therefore noticed that the juice freezes especially quickly if you first hold it in the sun - it really heats it up! This is strange, thought the Tanzanian schoolboy, who acted contrary to worldly wisdom. Is it really necessary to pre-heat the liquid in order to turn into ice faster? 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 sixties of the last century. 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 onto 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 appears 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 atom. However, ice is perhaps the most mysterious substance in the Universe. Scientists have not yet been able to explain some of its properties.

2. Supercooling and "instant" freezing

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

Watch the video (2,901 KB, 60 sec) from 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 thinking, name how many different states does water have?

If you answered three (solid, liquid, gas), then you were wrong. Scientists identify at least 5 different states of liquid water 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-chilled water suddenly turns into ice. What happens with further decline?

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 “vitreous” water - a solid substance that lacks crystalline structure.

4. Quantum properties of water

At the molecular level, water is even more surprising. In 1995, a neutron scattering experiment conducted by scientists yielded an unexpected result: physicists discovered that neutrons aimed at water molecules “see” 25% fewer hydrogen protons than expected.

It turned out that at a 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 memory?

Homeopathy, an alternative to conventional medicine, states that a diluted solution of a drug can have a healing effect on the body, even if the dilution factor is so great that there is nothing left in the solution except water molecules. Proponents of homeopathy explain this paradox with a concept called “water memory,” 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 group 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, scientists said that they were able to prove the reality of the “water memory” effect. However, experiments conducted under the supervision of independent experts did not bring any results. The debate about the existence of the “water memory” phenomenon continues.

Water has many other unusual properties that we did not talk about 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. Nepomnyashchy N.N. Secrets of inanimate nature. The most mysterious substance in the universe / http://www.bibliotekar.ru.

Mpemba effect(Mpemba's Paradox) is a paradox that states that hot water under some conditions freezes faster than cold water, although it must pass the temperature of cold water during the freezing process. This paradox is an experimental fact that contradicts the usual ideas, according to which, under the same conditions, a more heated body takes more time to cool to a certain temperature than a less heated body to cool to the same temperature.

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

As a student at Magambi High School in Tanzania, Erasto Mpemba did practical work as a cook. He needed 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 delayed completing the first part of the task. Fearing that he would not make it 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 the given technology.

After this, Mpemba experimented not only with milk, but also with ordinary water. In any case, already as a student at Mkwava Secondary School, he asked Professor Dennis Osborne from the University College in Dar Es Salaam (invited by the school director to give a lecture on physics to the students) specifically 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, he and Mpemba published the results of their experiments in the journal Physics Education. Since then, the effect they discovered has been 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 a body cools down to the ambient temperature should be proportional to the temperature difference between this body and the environment. This law was established by Newton and has since been confirmed many times in practice. In this effect, water with a temperature of 100°C cools to a temperature of 0°C faster than the same amount of water with a temperature of 35°C.

However, this does not yet imply a paradox, since the Mpemba effect can be explained within the framework of 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 at 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. Firstly, the mass of water required for cooling decreases. And secondly, the temperature decreases due to the fact that the heat of evaporation of the transition from the water phase to the steam phase decreases.

Temperature difference

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

Hypothermia

When water cools below 0 C, it does not always freeze. Under some conditions, it can undergo supercooling, continuing to remain liquid at temperatures below freezing. In some cases, water can remain liquid even at a temperature of –20 C.

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

Hot water is most susceptible to hypothermia because heating it removes 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, the following happens. 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 the cold air and will prevent further evaporation. The rate of formation of ice crystals in this case will be lower. In the case of hot water subjected to supercooling, the supercooled 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 heat loss, while hot water begins to freeze from below.

This effect is explained by an anomaly in water density. 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 a temperature of 4 C, it will remain on the surface, forming a thin cold layer. Under these conditions, a thin layer of ice will form on the surface of the water within 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 process 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. In addition, cold water layers are denser than hot water layers, so the cold water layer will sink down, raising the warm water layer to the surface. This circulation of water ensures a rapid drop in temperature.

But why does this process not reach an 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 the process of convection.

Gases dissolved in water

Water always contains gases dissolved in it - oxygen and carbon dioxide. These gases have the ability to reduce the freezing point of water. When water is heated, these gases are released from the water because their solubility in water is lower at high temperatures. Therefore, when hot water cools, it always contains less dissolved gases 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 is no experimental data confirming this fact.

Thermal conductivity

All these (as well as other) conditions were studied in many experiments, but an unambiguous answer to the question - which of them provide one hundred percent reproduction of the Mpemba effect - was never obtained.

For example, in 1995, German physicist David Auerbach studied the effect of supercooling water 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 previous data that hot water was able to achieve greater 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.

For now, only one thing can be stated - the reproduction of this effect significantly depends on the conditions under which the experiment is carried out. Precisely because it is not always reproduced.

In 1963, a Tanzanian schoolboy named Erasto Mpemba asked his teacher a stupid question - why did the warm ice cream in his freezer freeze faster than the cold one?

He turned to the physics teacher for clarification, but he only laughed at the student, saying the following: “This is not universal physics, but Mpemba physics.” After this, Mpemba experimented not only with milk, but also with ordinary water.

In any case, already as a student at Mkwava Secondary School, he asked Professor Dennis Osborne from the University College in Dar Es Salaam (invited by the school director to give a lecture on physics to the students) specifically 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, he and Mpemba published the results of their experiments in the journal Physics Education. Since then, the effect they discovered has been called the Mpemba effect.

Are you interested in knowing 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 some conditions freezes faster than cold water, although it must pass the temperature of cold water during the freezing process. This paradox is an experimental fact that contradicts the usual ideas, according to which, under the same conditions, a more heated body takes more time to cool to a certain temperature than a less heated body to cool to the same temperature.

This phenomenon was noticed in their 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 a body cools down to the ambient temperature should be proportional to the temperature difference between this body and the environment. This law was established by Newton and has since been confirmed many times in practice. In this effect, water with a temperature of 100°C cools to a temperature of 0°C faster than the same amount of water with a temperature of 35°C.

Since then, different versions have been expressed, one of which was as follows: part of the hot water first simply evaporates, and then, when less of it remains, the water freezes faster. This version, due to its simplicity, became the most popular, but did not completely satisfy scientists.

Now a team of researchers from 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 have found out, the secret lies in the amount of energy stored in hydrogen bonds between water molecules.

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 known fact is that hydrogen atoms are attracted to oxygen atoms from neighboring molecules - hydrogen bonds are formed.

At the same time, water molecules generally repel each other. Scientists from Singapore noticed: the warmer the water, the greater the distance between the molecules of the liquid due to an 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 move closer to each other. And the release of energy, as is known, means cooling.

Here are the assumptions put forward by scientists:

Evaporation

Hot water evaporates faster from the container, thereby reducing its volume, and a smaller volume of water at 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. Firstly, the mass of water required for cooling decreases. 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, the heat exchange in this case is more intense and the hot water cools faster.

Hypothermia
When water cools below 0°C it does not always freeze. Under some conditions, it can undergo supercooling, continuing to remain liquid at temperatures below freezing. In some cases, water can remain liquid even at a temperature of -20°C. The reason for this effect is that in order for the first ice crystals to begin to form, crystal formation centers are needed. If they are not present in liquid water, then supercooling will continue until the temperature drops enough for crystals to form spontaneously. When they begin to form in the supercooled liquid, they will begin to grow faster, forming slush ice, which will freeze to form ice. Hot water is most susceptible to hypothermia because heating it removes 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, the following happens: a thin layer of ice forms on its surface, which acts as an insulator between the water and the cold air, and thereby prevents further evaporation. The rate of formation of ice crystals in this case will be lower. In the case of hot water subjected to supercooling, the supercooled 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 heat loss, while hot water begins to freeze from below. This effect is explained by an anomaly in water density. Water has its maximum density at 4°C. If you cool water to 4°C and place it 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 remain on the surface, forming a thin cold layer. Under these conditions, a thin layer of ice will form on the surface of the water within a short time, but this layer of ice will act as an insulator, protecting the lower layers of water, which will remain at a temperature of 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 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, bringing the warm water layer to the surface. This circulation of water ensures a rapid drop in temperature. But why does this process not reach an equilibrium point? To explain the Mpemba effect from the 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 the process of convection.

Gases dissolved in water

Thermal conductivity

This mechanism can play a significant role when water is placed in the refrigerator compartment freezer in small containers. Under these conditions, it has been observed that a container of hot water melts the ice in the freezer underneath, thereby improving thermal contact with the freezer wall and thermal conductivity. As a result, heat is removed from a hot water container faster than from a cold one. In turn, a container with cold water does not melt the snow underneath. All these (as well as other) conditions were studied in many experiments, but an unambiguous answer to the question - which of them ensure 100% reproduction of the Mpemba effect - was never obtained. For example, in 1995, German physicist David Auerbach studied the effect of supercooling water 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 previous data that hot water was able to achieve greater 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. For now, only one thing can be stated: the reproduction of this effect significantly depends on the conditions under which the experiment is carried out. Precisely because it is not always reproduced.

But as they say, the most likely reason.

As the chemists write in their article, which can be found on the preprint website arXiv.org, hydrogen bonds are stronger 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, hardening occurs faster.

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

This is true, although it sounds incredible, because during the freezing process, preheated water must pass the temperature of cold water. Meanwhile, this effect is widely used. For example, skating rinks and slides are filled with hot rather than cold water in winter. Experts advise motorists to pour cold, not hot, water into the washer reservoir in winter. The paradox is known in the world as the “Mpemba Effect”.

This phenomenon was mentioned at one time by Aristotle, Francis Bacon and Rene Descartes, but only in 1963 did physics professors pay attention to it and try to study it. It all started when Tanzanian schoolboy Erasto Mpemba noticed that the sweetened milk he used to make ice cream froze faster if it was preheated and hypothesized that hot water froze faster than cold water. He turned to the physics teacher for clarification, but he only laughed at the student, saying the following: “This is not universal physics, but Mpemba physics.”

Luckily, Dennis Osborne, a physics professor from the University of Dar es Salaam, visited the school one day. And Mpemba turned to him with the same question. The professor was less skeptical, said that he could not judge something that he had never seen, and upon returning home he asked his staff to conduct appropriate experiments. They seemed to confirm the boy's words. In any case, in 1969, Osborne spoke about working with Mpemba in the English magazine. PhysicsEducation" That same year, George Kell of Canada's National Research Council published an article describing the phenomenon in English. AmericanJournalofPhysics».

There are several possible explanations for this paradox:

Hot water evaporates faster, thereby reducing its volume, and a smaller volume of water at the same temperature freezes faster. Cold water should freeze faster in airtight containers.
Availability of snow lining. A container with hot water melts the snow underneath, thereby improving thermal contact with the cooling surface. Cold water does not melt the snow underneath. If there is no snow liner, the cold water container should freeze faster.
Cold water begins to freeze from above, thereby worsening the processes of heat radiation and convection, and hence heat loss, while hot water begins to freeze from below. With additional mechanical mixing of water in containers, cold water should freeze faster.
The presence of crystallization centers in cooled water - substances dissolved in it. With a small number of such centers in cold water, the transformation of water into ice is difficult and even supercooling is possible, when it remains in a liquid state, having a subzero temperature.

Another explanation was recently published. Dr. Jonathan Katz from the University of Washington studied this phenomenon and concluded that substances dissolved in water, which precipitate when heated, play an important role in it.
By solutes, Dr. Katz means calcium and magnesium bicarbonates, which are found in hard water. When water is heated, these substances precipitate and the water becomes “soft.” Water that has never been heated contains these impurities and is “hard.” As it freezes and ice crystals form, the concentration of impurities in the water increases 50 times. Because of this, the freezing point of water decreases.

This explanation does not seem convincing to me, because... We must not forget that the effect was discovered in experiments with ice cream, and not with hard water. Most likely, the causes of the phenomenon are thermophysical, not chemical.

So far, no unambiguous explanation for Mpemba's paradox has been obtained. It must be said that some scientists do not consider this paradox worthy of attention. However, it is very interesting that a simple schoolboy achieved recognition of the physical effect and gained popularity due to his curiosity and perseverance.

Added February 2014

The note was written in 2011. Since then, new studies of the Mpemba effect and new attempts to explain it have appeared. So, in 2012, the Royal Society of Chemistry of Great Britain announced an international competition to solve the scientific mystery “Mpemba Effect” with a prize fund of 1000 pounds. The deadline was set on July 30, 2012. The winner was Nikola Bregovic from the laboratory of the University of Zagreb. He published his work in which he analyzed previous attempts to explain this phenomenon and came to the conclusion that they were not convincing. The model he proposed is based on the fundamental properties of water. Those interested can find a job at http://www.rsc.org/mpemba-competition/mpemba-winner.asp

The research did not end there. In 2013, physicists from Singapore theoretically proved the cause of the Mepemba effect. The work can be found at http://arxiv.org/abs/1310.6514.

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Comments:

Alexey Mishnev. , 06.10.2012 04:14

Why does hot water evaporate faster? Scientists have practically proven that a glass of hot water freezes faster than cold water. Scientists cannot explain this phenomenon for the reason that they do not understand the essence of the phenomena: heat and cold! Heat and cold are a physical sensation that causes the interaction of particles of Matter, in the form of counter compression of magnetic waves that move from space and from the center of the earth. Therefore, the greater the potential difference, this magnetic voltage, the faster the energy exchange occurs by the method of counter penetration of one wave into another. That is, by the diffusion method! In response to my article, one opponent writes: 1) “..Hot water evaporates FASTER, resulting in less of it, so it freezes faster” Question! What energy causes water to evaporate faster? 2) My article is about a glass, and not about a wooden trough, which the opponent cites as a counterargument. Which is not correct! I answer the question: “WHY DOES WATER EVAPORATE IN NATURE?” Magnetic waves, which always move from the center of the earth into space, overcoming the counter pressure of magnetic compression waves (which always move from space to the center of the earth), at the same time, spray water particles, since moving into space, they increase in volume. That is, they are expanding! If the magnetic compression waves are overcome, these water vapors are compressed (condensed) and under the influence of these magnetic compression forces, the water returns to the earth in the form of precipitation! Sincerely! Alexey Mishnev. October 6, 2012.

Alexey Mishnev. , 06.10.2012 04:19

What is temperature? Temperature is the degree of electromagnetic tension of magnetic waves with compression and expansion energy. In the case of an equilibrium state of these energies, the temperature of the body or substance is in a stable state. When the equilibrium state of these energies is disturbed, towards the energy of expansion, the body or substance increases in the volume of space. If the energy of magnetic waves exceeds in the direction of compression, the body or substance decreases in the volume of space. The degree of electromagnetic voltage is determined by the degree of expansion or compression of the reference body. Alexey Mishnev.

Moiseeva Natalia, 23.10.2012 11:36 | VNIIM

Alexey, you are talking about some article that sets out your thoughts on the concept of temperature. But no one read it. Please give me a link. In general, your views on physics are very unique. I've never heard of "electromagnetic expansion of a reference body."

Yuri Kuznetsov, 04.12.2012 12:32

A hypothesis is proposed that this is due to intermolecular resonance and the ponderomotive attraction between molecules it generates. In cold water, molecules move and vibrate chaotically, at different frequencies. When water is heated, with an increase in the frequency of vibrations, their range narrows (the difference in frequencies from liquid hot water to the point of vaporization decreases), the vibration frequencies of the molecules approach each other, as a result of which resonance occurs between the molecules. During cooling, this resonance is partially preserved and does not fade away immediately. Try pressing one of the two guitar strings that are in resonance. Now let go - the string will begin to vibrate again, the resonance will restore its vibrations. Likewise, in frozen water, the external cooled molecules try to lose the amplitude and frequency of vibrations, but the “warm” molecules inside the vessel “pull out” the vibrations back, acting as vibrators, and the external ones as resonators. Ponderomotive attraction* arises between vibrators and resonators. When the ponderomotive force becomes greater than the force caused by the kinetic energy of the molecules (which not only vibrate, but also move linearly), accelerated crystallization occurs - the "Mpemba Effect". The ponderomotive connection is very unstable, the Mpemba effect strongly depends on all the accompanying factors: the volume of water to be frozen, the nature of its heating, freezing conditions, temperature, convection, heat exchange conditions, gas saturation, vibration of the refrigeration unit, ventilation, impurities, evaporation, etc. Possibly even from lighting... Therefore, the effect has a lot of explanations and is sometimes difficult to reproduce. For the same “resonance” reason, boiled water boils faster than unboiled water - resonance retains the intensity of vibrations of water molecules for some time after boiling (the loss of energy during cooling is mainly due to the loss of kinetic energy of the linear movement of molecules). During intense heating, vibrator molecules change roles with resonator molecules in comparison with freezing - the frequency of the vibrators is less than the frequency of the resonators, which means that not attraction, but repulsion occurs between the molecules, which accelerates the transition to another state of aggregation (pair).

Vlad, 12/11/2012 03:42

Broke my brain...

Anton, 02/04/2013 02:02

1. Is this ponderomotive attraction really so great that it affects the heat transfer process? 2. Does this mean that when all bodies are heated to a certain temperature, their structural particles enter into resonance? 3. Why does this resonance disappear when cooled? 4. Is this your guess? If there is a source, please indicate. 5. According to this theory, the shape of the vessel will play an important role, and if it is thin and flat, then the difference in freezing time will not be large, i.e. you can check this.

Gudrat, 03/11/2013 10:12 | METAK

In cold water there are already nitrogen atoms and the distances between water molecules are closer than in hot water. That is, the conclusion: Hot water absorbs nitrogen atoms faster and at the same time it freezes quickly than cold water - this is comparable to the hardening of iron, since hot water turns into ice and hot iron hardens with rapid cooling!

Vladimir, 03/13/2013 06:50

or maybe this: the density of hot water and ice is less than the density of cold water, and therefore the water does not need to change its density, losing some time and it freezes.

Alexey Mishnev, 03/21/2013 11:50

Before talking about resonances, attractions and vibrations of particles, we need to understand and answer the question: What forces cause particles to vibrate? Since, without kinetic energy, there can be no compression. Without compression, there can be no expansion. Without expansion, there can be no kinetic energy! When you start talking about the resonance of strings, you first make an effort so that one of these strings begins to vibrate! When talking about attraction, you must first of all indicate the force that makes these bodies attract! I assert that all bodies are compressed by the electromagnetic energy of the atmosphere and which compresses all bodies, substances and elementary particles with a force of 1.33 kg. not per cm2, but per elementary particle. Since atmospheric pressure cannot be selective! Not to be confused with the amount of force!

Dodik, 05/31/2013 02:59

It seems to me that you have forgotten one truth - “Science begins where measurements begin.” What is the temperature of the "hot" water? What is the temperature of the “cold” water? The article doesn't say a word about this. From this we can conclude - the whole article is bullshit!

Grigory, 06/04/2013 12:17

Dodik, before calling an article nonsense, you need to think about learning, at least a little. And not just measure.

Dmitry, 12/24/2013 10:57

Hot water molecules move faster than in cold water, because of this there is closer contact with the environment, they seem to absorb all the cold, quickly slowing down.

Ivan, 01/10/2014 05:53

It is surprising that such an anonymous article appears on this site. The article is completely unscientific. Both the author and commentators vying with each other in search of an explanation for the phenomenon, without bothering to find out whether the phenomenon is observed at all and, if observed, under what conditions. Moreover, there is not even an agreement on what we are actually observing! Thus, the author insists on the need to explain the effect of rapid freezing of hot ice cream, although from the entire text (and the words “the effect was discovered in experiments with ice cream”) it follows that he himself did not conduct such experiments. From the options for “explanation” of the phenomenon listed in the article, it is clear that completely different experiments are being described, carried out under different conditions with different aqueous solutions. Both the essence of the explanations and the subjunctive mood in them suggest that even a basic check of the ideas expressed was not carried out. Someone accidentally heard a funny story and casually expressed his speculative conclusion. Sorry, but this is not a physical scientific study, but a conversation in a smoking room.

Ivan, 01/10/2014 06:10

Regarding the comments in the article about filling the rollers with hot water and the windshield washer reservoirs with cold water. Everything is simple here from the point of view of elementary physics. The skating rink is filled with hot water precisely because it freezes more slowly. The skating rink must be level and smooth. Try to fill it with cold water - you will get bumps and “swells”, because... The water will freeze _quickly_ without having time to spread out in an even layer. And the hot one will have time to spread in an even layer, and will melt the existing ice and snow tubercles. The washer is also not difficult: there is no point in pouring clean water in cold weather - it freezes on the glass (even hot); and a hot non-freezing liquid can lead to cracking of cold glass, plus the glass will have an increased freezing point due to the accelerated evaporation of alcohols on the way to the glass (is everyone familiar with the principle of operation of a moonshine still? - the alcohol evaporates, the water remains).

Ivan, 01/10/2014 06:34

But in essence of the phenomenon, it is stupid to ask why two different experiments under different conditions proceed differently. If the experiment is carried out purely, then you need to take hot and cold water of the same chemical composition - we take pre-cooled boiling water from the same kettle. Pour into identical vessels (for example, thin-walled glasses). We do not place it on the snow, but on an equally flat, dry base, for example, a wooden table. And not in a micro-freezer, but in a fairly voluminous thermostat - I conducted an experiment a couple of years ago at the dacha, when the weather outside was stable and frosty, about -25C. Water crystallizes at a certain temperature after releasing the heat of crystallization. The hypothesis boils down to the statement that hot water cools faster (this is true, in accordance with classical physics, the rate of heat transfer is proportional to the temperature difference), but maintains an increased cooling rate even when its temperature becomes equal to the temperature of cold water. The question is, how does water that has cooled to a temperature of +20C outside differ from exactly the same water that has cooled to a temperature of +20C an hour before, but in a room? Classical physics (by the way, based not on chatter in the smoking room, but on hundreds of thousands and millions of experiments) says: nothing, the further dynamics of cooling will be the same (only the boiling water will reach the +20 point later). And the experiment shows the same thing: when a glass of initially cold water already had a strong crust of ice, the hot water didn’t even think about freezing. P.S. To the comments of Yuri Kuznetsov. The presence of a certain effect can be considered established when the conditions for its occurrence are described and it is consistently reproduced. And when we have unknown experiments with unknown conditions, it is premature to build theories to explain them and this does not give anything from a scientific point of view. P.P.S. Well, it’s impossible to read Alexei Mishnev’s comments without tears of tenderness - a person lives in some kind of fictional world that has nothing to do with physics and real experiments.

Gregory, 01/13/2014 10:58

Ivan, I understand that you are refuting the Mpemba effect? It doesn't exist, as your experiments show? Why is it so famous in physics, and why are many trying to explain it?

Ivan, 02/14/2014 01:51

Good afternoon, Gregory! The effect of an impure experiment exists. But, as you understand, this is not a reason to look for new laws in physics, but a reason to improve the skill of an experimenter. As I already noted in the comments, in all the mentioned attempts to explain the “Mpemba effect,” researchers cannot even clearly formulate what exactly and under what conditions they measure. And you want to say that these are experimental physicists? Don't be ridiculous. The effect is known not in physics, but in pseudo-scientific discussions on various forums and blogs, of which there are now a lot. It is perceived as a real physical effect (in the sense as a consequence of some new physical laws, and not as a consequence of an incorrect interpretation or just a myth) by people far from physics. So there is no reason to speak of the results of different experiments conducted under completely different conditions as a single physical effect.

Pavel, 02/18/2014 09:59

hmm, guys... article for "Speed Info"... No offense... ;) Ivan is right about everything...

Grigory, 02/19/2014 12:50

Ivan, I agree that there are now a lot of pseudo-scientific sites publishing unverified sensational material.? After all, the Mpemba effect is still being studied. Moreover, scientists from universities are researching. For example, in 2013, this effect was studied by a group from the University of Technology in Singapore. Look at the link http://arxiv.org/abs/1310.6514. They believe they have found an explanation for this effect. I will not write in detail about the essence of the discovery, but in their opinion, the effect is associated with the difference in energies stored in hydrogen bonds.

Moiseeva N.P. , 02/19/2014 03:04

For everyone interested in research into the Mpemba effect, I have slightly supplemented the material in the article and provided links where you can familiarize yourself with the latest results (see text). Thanks for your comments.

Ildar, 02/24/2014 04:12 | there's no point in listing everything

If this Mpemba effect really takes place, then the explanation must be sought, I think, in the molecular structure of water. Water (as I learned from popular science literature) exists not as individual H2O molecules, but as clusters of several molecules (even dozens). As the water temperature increases, the speed of movement of molecules increases, clusters break up against each other and the valence bonds of molecules do not have time to assemble large clusters. It takes a little more time to form clusters than to reduce the speed of molecular movement. And since the clusters are smaller, the formation of the crystal lattice occurs faster. In cold water, apparently, large, fairly stable clusters prevent the formation of a lattice; it takes some time to destroy them. I myself saw on TV a curious effect when cold water standing quietly in a jar remained liquid for several hours in the cold. But as soon as the jar was picked up, that is, slightly moved from its place, the water in the jar immediately crystallized, became opaque, and the jar burst. Well, the priest who showed this effect explained it by the fact that the water was blessed. By the way, it turns out that water greatly changes its viscosity depending on temperature. This is imperceptible to us, as large creatures, but at the level of small (mm or smaller) crustaceans, and even more so bacteria, the viscosity of water is a very significant factor. This viscosity, I think, is also determined by the size of the water clusters.

GRAY, 03/15/2014 05:30

everything around us that we see are superficial characteristics (properties), so we accept as energy only what we can measure or prove its existence in any way, otherwise it’s a dead end. This phenomenon, the Mpemba effect, can only be explained by a simple volumetric theory that will unite all physical models into a single interaction structure. it's actually simple

Nikita, 06/06/2014 04:27 | car

But how can you make sure that the water stays cold rather than warm when you’re driving in the car?

Alexey, 03.10.2014 01:09

Here's another "discovery" on the way. Water in a plastic bottle freezes much faster with the cap open. For fun, I performed the experiment many times in severe frost. The effect is obvious. Hello theorists!

Evgeniy, 12/27/2014 08:40

The principle of an evaporative cooler. We take two hermetically sealed bottles with cold and hot water. We put it in the cold. Cold water freezes faster. Now we take the same bottles with cold and hot water, open them and put them in the cold. Hot water will freeze faster than cold water. If we take two basins with cold and hot water, then the hot water will freeze much faster. This is due to the fact that we are increasing contact with the atmosphere. The more intense the evaporation, the faster the temperature drops. Here we must mention the humidity factor. The lower the humidity, the stronger the evaporation and the stronger the cooling.

gray TOMSK, 03/01/2015 10:55

GRAY, 03/15/2014 05:30 - continued What you know about temperature is not everything. There's something else there. If you correctly construct a physical model of temperature, it will become the key to describing energy processes from diffusion, melting and crystallization to such scales as an increase in temperature with an increase in pressure, an increase in pressure with an increase in temperature. Even the physical model of the Sun's energy will become clear from the above. I'm in winter. . in the early spring of 20013, looking at temperature models, I compiled a general temperature model. A couple of months later, I remembered the temperature paradox and then I realized... that my temperature model also describes the Mpemba paradox. This was in May - June 2013. I'm a year late, but it's for the best. My physical model is a freeze frame and it can be rewound both forward and backward and it contains motor activity, the same activity in which everything moves. I have 8 years of school and 2 years of college with a repetition of the topic. 20 years have passed. So I cannot attribute any kind of physical models to famous scientists, nor can I attribute formulas. So sorry.

Andrey, 08.11.2015 08:52

In general, I have an idea about why hot water freezes faster than cold water. And in my explanations everything is very simple, if you are interested, write to me by email: [email protected]

Andrey, 08.11.2015 08:58

I'm sorry, I gave the wrong email address, here's the correct email: [email protected]

Victor, 12/23/2015 10:37

It seems to me that everything is simpler, snow falls here, it is evaporated gas, cooled, so maybe in cold weather the hot one cools down faster because it evaporates and immediately crystallizes without rising far, and water in the gaseous state cools down faster than in the liquid)

Bekzhan, 01/28/2016 09:18

Even if someone had revealed these laws of the world that are associated with this effect, he would not have written here. From my point of view, it would not be logical to reveal its secrets to Internet users when he can publish it in famous scientific journals and prove it himself personally before the people. So, what will be written here about this effect, most of it is not logical.)))

Alex, 02/22/2016 12:48

Hello Experimenters You are right when you say that Science begins where... not Measurements, but Calculations. “Experiment” is an eternal and indispensable argument for those deprived of Imagination and Linear thinking. It offended everyone, now in the case of E= mc2 - does everyone remember? The speed of molecules flying out of cold water into the atmosphere determines the amount of energy they carry away from the water (cooling is a loss of energy). The speed of molecules from hot water is much higher and the energy carried away is squared (the rate of cooling of the remaining mass of water) That's all, if you get away from " experimentation" and remember the Basic Fundamentals of Science

Vladimir, 04/25/2016 10:53 | Meteo

In those days when antifreeze was rare, water was drained from the car cooling system in an unheated garage after a working day so as not to defrost the cylinder block or radiator - sometimes both together. In the morning hot water was poured. In severe frost, the engines started without problems. Somehow, due to the lack of hot water, water was poured from the tap. The water immediately froze. The experiment was expensive - exactly as much as it costs to buy and replace the cylinder block and radiator of a ZIL-131 car. Whoever doesn’t believe it, let him check it. and Mpemba experimented with ice cream. In ice cream, crystallization occurs differently than in water. Try biting off a piece of ice cream and a piece of ice with your teeth. Most likely it did not freeze, but thickened as a result of cooling. And fresh water, whether it is hot or cold, freezes at 0*C. Cold water is quick, but hot water takes time to cool down.

Wanderer, 05/06/2016 12:54 | to Alex

"c" - speed of light in vacuum E=mc^2 - formula expressing the equivalence of mass and energy

Albert, 07/27/2016 08:22

First, an analogy with solids (there is no evaporation process). I recently soldered copper water pipes. The process occurs by heating a gas burner to the melting temperature of the solder. The heating time for one joint with a coupling is approximately one minute. I soldered one joint to the coupling and after a couple of minutes I realized that I had soldered it incorrectly. It was necessary to rotate the pipe a little in the coupling. I started heating the joint again with a burner and, to my surprise, it took 3-4 minutes to heat the joint to the melting temperature. How so!? After all, the pipe is still hot and it would seem that much less energy is needed to heat it to the melting temperature, but everything turned out to be the opposite. It's all about thermal conductivity, which is significantly higher in an already heated pipe and the boundary between the heated and cold pipe has managed to move far from the joint in two minutes. Now about the water. We will operate with the concepts of a hot and semi-heated vessel. In a hot vessel, a narrow temperature boundary is formed between hot, highly mobile particles and slow-moving, cold particles, which moves relatively quickly from the periphery to the center, because at this boundary fast particles quickly give up their energy (cooled) by particles on the other side of the boundary. Since the volume of external cold particles is larger, fast particles, giving up their thermal energy, cannot significantly warm up the external cold particles. Therefore, the process of cooling hot water occurs relatively quickly. Semi-heated water has much lower thermal conductivity and the width of the boundary between semi-heated and cold particles is much wider. The shift to the center of such a wide boundary occurs much more slowly than in the case of a hot vessel. As a result, the hot vessel cools faster than the warm one. I think we need to follow the dynamics of the cooling process of water of different temperatures by placing several temperature sensors from the middle to the edge of the vessel.

Max, 11/19/2016 05:07

It has been verified: in Yamal, when it’s cold, the pipe with hot water freezes and you have to warm it up, but the cold one doesn’t!

Artem, 09.12.2016 01:25

It’s difficult, but I think that cold water is denser than hot water, even better than boiled water, and here there is an acceleration in cooling, etc. hot water reaches the cold temperature and overtakes it, and if you take into account the fact that hot water freezes from below and not from the top, as written above, this speeds up the process a lot!

Alexander Sergeev, 21.08.2017 10:52

There is no such effect. Alas. In 2016, a detailed article on the topic was published in Nature: https://en.wikipedia.org/wiki/Mpemba_effect From it it is clear that with careful experiments (if the samples of warm and cold water are the same in everything except temperature), the effect is not observed .

Zavlab, 08/22/2017 05:31

Victor , 10/27/2017 03:52

"It really is." - if at school you didn’t understand what heat capacity and the law of conservation of energy are. It’s easy to check - for this you need: desire, head, hands, water, refrigerator and alarm clock. And the skating rinks, as experts write, are frozen (filled) with cold water, and the cut ice is leveled with warm water. And in winter you need to pour antifreeze liquid into the washer reservoir, not water. The water will freeze in any case, and cold water will freeze faster.

Irina, 01/23/2018 10:58

Scientists all over the world have been struggling with this paradox since the time of Aristotle, and Victor, Zavlab and Sergeev turned out to be the smartest.

Denis, 02/01/2018 08:51

Everything is written correctly in the article. But the reason is somewhat different. During the boiling process, the air dissolved in it is evaporated from water; therefore, as the boiling water cools, its density will ultimately be less than that of raw water at the same temperature. There are no other reasons for different thermal conductivity other than different densities.

Zavlab, 03/01/2018 08:58 | Head of Lab

Irina:), “scientists around the world” do not struggle with this “paradox”; for real scientists this “paradox” simply does not exist - it is easily verified under well-reproducible conditions. The “paradox” appeared due to the irreproducible experiments of the African boy Mpemba and was inflated by similar “scientists” :)

Why does warm water freeze faster than cold water? How does modern physics explain this? Mpemba effect in real life

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