Why does hot water freeze faster than cold water? Why does hot water freeze faster than cold water? Why does boiling water freeze faster than cold water?

Mpemba effect(Mpemba's Paradox) - a paradox that states that hot water under some 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 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

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 a clear 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.

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.

In this article we will look at the question of why hot water freezes faster than cold water.

Heated water freezes much faster than cold water! This amazing property of water, for which scientists still cannot find an exact explanation, has been known since ancient times. For example, even in Aristotle there is a description of winter fishing: fishermen inserted fishing rods into holes in the ice, and so that they would freeze faster, they poured warm water on the ice. This phenomenon was named after Erasto Mpemba in the 60s of the 20th century. Mnemba noticed a strange effect while making ice cream and turned to his physics teacher, Dr. Denis Osborne, for an explanation. Mpemba and Dr. Osborne experimented with water at different temperatures and concluded that almost boiling water begins to freeze much faster than water at room temperature. Other scientists conducted their own experiments and each time obtained similar results.

Explanation of a physical phenomenon

There is no generally accepted explanation for why this happens. Many researchers suggest that the whole point is in the supercooling of the liquid, which occurs when its temperature drops below the freezing point. In other words, if water freezes at a temperature below 0°C, then supercooled water can have a temperature of, for example, -2°C and still remain liquid without turning into ice. When we try to freeze cold water, there is a chance that it will first become supercooled and only harden after some time. Other processes occur in heated water. Its faster transformation into ice is associated with convection.

Convection- this is a physical phenomenon in which the warm lower layers of a liquid rise, and the upper, cooled ones, fall.

The short answer to why hot water freezes faster

It turns out that the liquid seems to mix and cool on its own. Due to the fact that the convection process occurs actively in heated water, ice crystals from the surface will fall faster and cool the warm water at the bottom.

The Mpemba effect or why does hot water freeze faster than cold water? 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 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 the 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 a 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 effect of evaporation 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 data that would confirm 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 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 a clear 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. O. V. Mosin

Hello, dear lovers of interesting facts. Today we will talk to you about. But I think that the question posed in the title may seem simply absurd - but should one always undividedly trust the notorious “common sense” and not a strictly established test experiment. 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, this effect has been given the name “Mpemba’s Paradox” - named after a schoolboy from Tanganyika, Erasto Mpemba, who asked the same question to a visiting physics professor.

The boy’s question did not arise out of nowhere, 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, made Mpemba laugh - however, after an experimental test personally by Professor D. Osborne, the desire to make fun of Erasto “evaporated” from them. Moreover, Mpemba, together with a professor, published a detailed description of this effect in Physics Education in 1969 - and since then the above-mentioned name has been fixed in the scientific literature.

What is the essence of the phenomenon?

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

How does modern physics explain this?

The paradox does not have a universal explanation, since several parallel processes occur together, the contribution of which may vary depending on the specific initial conditions - but with a uniform result:

• the ability of a liquid to supercool - initially cold water is more prone to supercooling, i.e. remains liquid when its temperature is already below 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”
• insulation 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 British Royal Society of Chemistry held a competition for the best hypothesis was recently, in 2012) - but there is still no unambiguous theory for all cases of combinations of input conditions...

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 like a paradox - after all, in order to freeze, hot water first needs to cool. However, the fact remains a fact, and scientists explain it in different ways.

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

1. Because hot water evaporates faster, its volume decreases. And freezing of a smaller amount of water at the same temperature occurs faster.
2. The freezer compartment of the refrigerator has a snow liner. A container containing hot water melts the snow underneath. This improves thermal contact with the freezer.
3. Freezing of cold water, unlike hot water, begins at the top. At the same time, convection and heat radiation, and, consequently, heat loss worsen.
4. Cold water contains crystallization centers - substances dissolved in it. If their content in water is small, icing is difficult, although at the same time, supercooling is possible - when at sub-zero temperatures it has a liquid state.

Although in fairness we can say that this effect is not always observed. Very often, cold water 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 could be, for example, very small particles of sand, dust or clay. As the air temperature decreases, these particles are the centers around which ice crystals form.

The role of crystallization nuclei can also be played by air bubbles and cracks in the container containing water. The speed 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 turns into a solid state from liquid at a temperature of 0 degrees.

The essence of the Mpemba effect

The Mpemba effect is 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 Tanzanian schoolboy Erasto Mpemba determined that hot ice cream freezes in a shorter time than cold ice cream. He made this conclusion while completing a cooking assignment.

He had to dissolve sugar in boiled milk and, having cooled it, place it in the refrigerator to freeze. Apparently, Mpemba was not particularly diligent and began completing the first part of the task late. Therefore, he did not wait for the milk to cool down, and put it in the refrigerator hot. He was very surprised when it froze even faster than that of his classmates, who were doing 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 Osborne 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 the Singapore universities were also interested in the question of 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 knows the composition of water from school - an oxygen atom and two hydrogen atoms. Oxygen to some extent pulls electrons away 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 a covalent bond. Singaporean researchers believe that the explanation for Mpemba's paradox lies precisely in hydrogen bonds. If water molecules are placed very tightly together, then such a strong interaction between the 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 release of excess energy and a transition to a lower energy level. This leads to the fact that hot water begins to cool quickly. At least, this is what theoretical calculations carried out by Singaporean scientists show.

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