Hello, dear lovers of interesting facts. Today we will talk about. But I think that the question posed in the title may seem simply absurd - but is it always necessary to completely trust the notorious "common sense", and not strictly set testing experience. Let's try to figure out why hot water freezes faster than cold water?

Historical reference

That in the issue of freezing cold and hot water “not everything is pure” was mentioned in the works of Aristotle, then similar notes were made by F. Bacon, R. Descartes and J. Black. In recent history, the name “Mpemba paradox” has been attached to this effect - after the name of a schoolboy from Tanganyika, Erasto Mpemba, who asked the same question to a visiting professor of physics.

The boy's question arose not from scratch, but from purely personal observations of the process of cooling ice cream mixtures in the kitchen. Of course, the classmates who were present there, together with the school teacher, laughed at Mpemba - however, after an experimental check personally by Professor D. Osborne, the desire to make fun of Erasto "evaporated" from them. Moreover, Mpemba, together with the professor, published a detailed description of this effect in 1969 in Physics Education - and since then the above name has been fixed in the scientific literature.

What is the essence of the phenomenon?

The setup of the experiment is quite simple: other things being equal, identical thin-walled vessels are tested, in which there are strictly equal amounts of water, differing only in temperature. The vessels are loaded into the refrigerator, after which the time is recorded before the formation of ice in each of them. The paradox is that in a vessel with an initially hotter liquid, this happens faster.


How does modern physics explain this?

The paradox has no universal explanation, since several parallel processes proceed together, the contribution of which may differ from specific initial conditions - but with a uniform result:

  • the ability of a liquid to supercool - initially cold water is more prone to hypothermia, i.e. remains liquid when its temperature is already below the freezing point
  • accelerated cooling - steam from hot water is transformed into ice microcrystals, which, when falling back, accelerate the process, working as an additional "external heat exchanger"
  • isolation effect - unlike hot water, cold water freezes from above, which leads to a decrease in heat transfer by convection and radiation

There are a number of other explanations (the last time the competition for the best hypothesis was held by the British Royal Society of Chemistry recently, in 2012) - but there is still no unambiguous theory for all cases of combinations of input conditions ...

Have you ever thought about the question why water heated to 82 degrees C freezes faster than cold water? Most likely not, I’m even more than sure that the question never occurred to you - which water freezes faster hot or cold?

However, this amazing discovery was made by an ordinary African schoolboy Erasto Mpemba back in 1963. It was the usual experience of a curious boy, of course, he could not correctly interpret the meaning of his own, and moreover, scientists from all over the world until 1966 could not give a clear and reasonable the answer to the question - why hot water freezes faster than cold.

Why does hot water freeze at 4 degrees Celsius and cold water at 0.

Cold water contains a lot of dissolved oxygen, it is he who maintains the freezing point of water at 0 degrees. If oxygen is removed from the water, and this is exactly what happens when the water is heated, the air bubbles dissolved in the water, as it is fashionable to say now, collapse, the water does not turn into ice at zero degrees as usual, and already at 4 °C. It is oxygen dissolved in water that breaks the bonds between water molecules, preventing water from moving from a liquid state to a solid state, simply turning into

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, 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, 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, one would have 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

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 underneath, 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 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 supercooling of 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 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 greater supercooling due to fewer crystallization centers. When water is heated, the 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.

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, 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, 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, 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, one would have 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

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 underneath, 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 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 supercooling of 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 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 greater supercooling due to fewer crystallization centers. When water is heated, the 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.

Many researchers have put forward and are putting forward their own versions as to why hot water freezes faster than cold water. It would seem a paradox - after all, in order to freeze, hot water first needs to cool down. However, the fact remains, and scientists explain it in different ways.

At the moment, there are several versions that explain this fact:

  1. Since evaporation in hot water is faster, its volume decreases. A smaller amount of water of the same temperature freezes faster.
  2. The freezer compartment of the refrigerator has a snow lining. A container containing hot water melts the snow underneath. This improves thermal contact with the freezer.
  3. Freezing of cold water, unlike hot, begins from above. In this case, convection and heat radiation, and, consequently, heat loss worsen.
  4. In cold water there are centers of crystallization - substances dissolved in it. With a small content of them in water, icing is difficult, although at the same time, its hypothermia is possible - when it has a liquid state at sub-zero temperatures.

Although in fairness it can be said that this effect is not always observed. Cold water often freezes faster than hot water.

At what temperature does water freeze

Why does water freeze at all? It contains a certain amount of mineral or organic particles. These, for example, can be very fine particles of sand, dust or clay. As the air temperature drops, these particles become centers around which ice crystals form.

The role of crystallization nuclei can also be performed by air bubbles and cracks in a container containing water. The rate of the process of turning water into ice is largely influenced by the number of such centers - if there are many of them, the liquid freezes faster. Under normal conditions, with normal atmospheric pressure, water passes into a solid state from liquid at a temperature of 0 degrees.

The essence of the Mpemba effect

The Mpemba effect is understood as a paradox, the essence of which is that, under certain circumstances, hot water freezes faster than cold water. This phenomenon was noticed by Aristotle and Descartes. However, it was not until 1963 that Erasto Mpemba, a schoolboy from Tanzania, determined that hot ice cream freezes in a shorter time than cold ice cream. He made such a conclusion while performing the task of cooking.

He had to dissolve sugar in boiled milk and, after cooling it, place it in the refrigerator to freeze. Apparently, Mpemba did not differ in special diligence and began to carry out the first part of the task late. Therefore, he did not wait for the milk to cool, and put it in the refrigerator hot. He was very surprised when it froze even faster than that of his classmates, who did the work in accordance with the given technology.

This fact interested the young man very much, and he began experiments with plain water. In 1969, the journal Physics Education published the results of research by Mpemba and Professor Dennis Osborn of the University of Dar es Salaam. The effect they described was given the name Mpemba. However, even today there is no clear explanation for the phenomenon. All scientists agree that the main role in this belongs to the differences in the properties of chilled and hot water, but what exactly is unknown.

Singapore version

Physicists from one of Singapore's universities were also interested in the question, which water freezes faster - hot or cold? A team of researchers led by Xi Zhang explained this paradox precisely by the properties of water. Everyone still knows the composition of water from school - an oxygen atom and two hydrogen atoms. Oxygen to some extent draws electrons from hydrogen, so the molecule is a certain kind of "magnet".

As a result, certain molecules in water are slightly attracted to each other and are united by a hydrogen bond. Its strength is many times lower than the covalent bond. Singaporean researchers believe that the explanation of the Mpemba paradox lies precisely in hydrogen bonds. If water molecules are placed very closely together, then such a strong interaction between molecules can deform the covalent bond in the middle of the molecule itself.

But when water is heated, the bound molecules move slightly away from each other. As a result, relaxation of covalent bonds occurs in the middle of the molecules with the return of excess energy and the transition to the lowest energy level. This leads to the fact that hot water begins to cool rapidly. At least, this is what the theoretical calculations carried out by Singaporean scientists show.

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