Mpemba effect(Mpemba paradox) - a paradox that states that hot water under some conditions it freezes faster than cold water, although at the same time it must pass the temperature of cold water during the freezing process. This paradox is an experimental fact that contradicts the usual concepts, according to which, under the same conditions, a more heated body to cool to a certain temperature takes longer than a less heated body to cool to the same temperature.
This phenomenon was noticed at the time by Aristotle, Francis Bacon and Rene Descartes, but it was not until 1963 that a Tanzanian schoolboy Erasto Mpemba found that a hot ice cream mixture freezes faster than a cold one.
As a student of Magambinskaya high school in Tanzania, Erasto Mpemba did practical work on the cookery business. 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 he delayed completing the first part of the assignment. Fearing that he would not be in time by the end of the lesson, he put the 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 plain water... In any case, already being a student of the Mkvavskaya high school, he asked Professor Dennis Osborne from the University College in Dar es Salaam (invited by the headmaster to give the students a lecture on physics) specifically about water: “If we 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 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 a body cools down to ambient temperature should be proportional to the difference in temperatures between this body and the environment. This law was established by Newton and since then has been confirmed many times in practice. In this effect, water with a temperature of 100 ° C cools down to a temperature of 0 ° C faster than the same amount of water with a temperature of 35 ° C.
However, this does not yet suggest a paradox, since the Mpemba effect can be explained within the framework of well-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.
Evaporation effect - double effect. First, the amount of water required for cooling is reduced. And secondly, the temperature decreases due to the fact that the heat of vaporization 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 more cold air - therefore, 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 some conditions, it can undergo hypothermia, continuing to remain liquid at temperatures below the freezing point. 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, centers of crystal formation are needed. If they are not present in liquid water, then hypothermia will continue until the temperature drops so much that crystals begin to form spontaneously. When they begin to form in a supercooled liquid, they will begin to grow faster, forming an ice slush, which, freezing, will 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 case of cold water that is not supercooled, the following occurs. In this case thin layer 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 slower. In the case of hot water subject to supercooling, supercooled water does not have a protective surface layer of ice. Therefore, it loses heat much faster through the open top.
When the hypothermia process ends and the water freezes, much is lost more warmth and therefore more ice forms.
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 the water density anomaly. 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 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 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 faster 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 fail to reach an equilibrium point? To explain the Mpemba effect from this point of view of convection, it should be assumed that 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 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 reduce the freezing point of water. When the water is heated, these gases are released from the water, since their solubility in water at high temperature below. Therefore, when hot water is cooled, there is always less 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 the freezer. refrigerating chamber in small containers. Under these conditions, it was noticed that the container with hot water melts the ice of the freezer under it, thereby improving thermal contact with the freezer wall and thermal conductivity. As a result, heat is removed from a container with hot water faster than from cold water. In turn, the container with cold water does not thaw snow under it.
All these (and 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 - has not been obtained.
For example, in 1995 the German physicist David Auerbach studied the effect of supercooling of water on this effect. He found that hot water, reaching a supercooled state, freezes at a higher temperature than cold water, which means faster than the latter. But cold water reaches a supercooled state faster than hot water, thereby compensating for the previous lag.
In addition, Auerbach's results contradicted earlier findings that hot water can achieve greater hypothermia due to fewer crystallization centers. When water is heated, gases dissolved in it are removed from it, and when it is boiled, some salts dissolved in it precipitate.
So far, only one thing can be asserted - the reproduction of this effect essentially depends on the conditions in which the experiment is carried out. Precisely because it is not always reproduced.
The good old formula H 2 O, it would seem, does not contain any secrets. But in fact, water - the source of life and the most famous liquid in the world - is fraught with many mysteries that sometimes even scientists cannot solve.
Here are the 5 most interesting facts about water:
1. Hot water freezes faster than cold water
Take two containers of water: pour hot water into one and cold water into the other, and place them in freezer... Hot water will freeze faster than cold water, although logically, cold water should have been the first to turn into ice: after all, hot water must first cool down to cold temperature, and then turn into ice, while cold water does not need to cool down. Why is this happening?
In 1963, Erasto B. Mpemba, a senior high school student in Tanzania, while freezing a prepared ice cream concoction, noticed that the hot concoction would freeze faster in the freezer than the cold concoction. When the young man shared his discovery with the physics teacher, he only laughed at him. Fortunately, the student was persistent and convinced the teacher to conduct an experiment, which confirmed his discovery: under certain conditions, hot water really freezes faster than cold water.
Now this phenomenon of hot water freezing faster than cold water is called the "Mpemba effect". True, long before him this unique property 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 hypothermia, evaporation, ice formation, convection, or by the effect of liquefied gases on hot and cold water.
Note from X.RU to the topic "Hot water freezes faster than cold water".
Since the issues of refrigeration are closer to us, refrigerators, we will allow ourselves to delve a little into the essence of this problem and give two opinions about the nature of such mysterious phenomenon.
1. A scientist from the University of Washington offered an explanation for a mysterious phenomenon known since the time of Aristotle: why hot water freezes faster than cold water.
The phenomenon called the Mpemba effect is widely used in practice. For example, experts advise motorists to pour cold, not hot, water into the washer reservoir in winter. But what lies at the heart of this phenomenon, for a long time remained unknown.
Dr. Jonathan Katz of the University of Washington has investigated this phenomenon and concluded that substances dissolved in water, which precipitate when heated, play an important role, according to EurekAlert.
Under the dissolved substances dr Katz refers to the calcium and magnesium bicarbonates found in hard water. When the water is heated, these substances are deposited, forming scale on the walls of the teapot. 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. This lowers the freezing point of water. "And now the water must still be cooling 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 is dependent on this temperature difference, water that has not been heated cools less quickly," says Dr. Katz.
According to the scientist, his theory can be verified experimentally, since the Mpemba effect becomes more pronounced for harder water.
2. Oxygen plus hydrogen plus cold makes ice. At first glance, this transparent substance seems very simple. In reality, the ice is fraught with many mysteries. The ice created by the African Erasto Mpemba did not dream of fame. It was hot days. He wanted fruit ice... He would take a pack of juice and put it in the freezer. He did this more than once and therefore noticed that the juice freezes especially quickly, if you hold it in the sun beforehand - it’s really hot! This is strange, thought the Tanzanian schoolboy, who was acting in defiance. worldly wisdom... Really, in order for the liquid to turn into ice faster, it must first be ... heated? The young man was so surprised that he shared his guess with the teacher. He reported this curiosity in the press.
This story happened back in the 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 on the walls of the vessel. "Of course, water with a high air content will freeze too," says Auerbach, "but not at zero degrees Celsius, but only at minus four or six degrees." Obviously, the wait will take longer. So, hot water freezes before cold water, this is a scientific fact.
There is hardly a substance that would appear before our eyes with the same ease as ice. It consists only of water molecules - that is, elementary molecules containing two hydrogen atoms and one oxygen. However, ice is arguably 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 to ice when cooled down to 0 ° C ... except in some cases! Such a case, for example, is "overcooling", which is a property of very pure water remain liquid even when cooled to below freezing point. This phenomenon becomes possible due to the fact that the environment does not contain centers or nuclei of crystallization, which could provoke the formation of ice crystals. And therefore, water remains in liquid form, even when cooled to temperatures below zero degrees Celsius. The crystallization process can be triggered, for example, by gas bubbles, impurities (impurities), uneven surface capacity. Without them, water will remain liquid. When the crystallization process starts, you can observe how supercooled water instantly turns 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 to a temperature above the boiling point.
3. "Glass" water
Quickly and without hesitation, name how much different conditions does the water have?
If you answered three (solid, liquid, gaseous), then you are wrong. Scientists distinguish at least 5 different states of liquid water and 14 states of ice.
Remember the conversation about supercooled water? So, no matter what you do, at a temperature of -38 ° C, even the purest supercooled water suddenly turns into ice. What happens with a further decrease
temperature? At -120 ° C, something strange begins to happen to the water: it becomes super-viscous or viscous, like molasses, and at temperatures below -135 ° C, it turns into "glass" or "glassy" water - solid in which there is no crystal structure.
4. Quantum properties of water
At the molecular level, water is even more surprising. In 1995, a neutron scattering experiment conducted by scientists gave an unexpected result: physicists found 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, and chemical formula water instead of the usual - H 2 O, becomes H 1.5 O!
5. Does water have a memory?
Homeopathy, alternative official medicine, claims that the diluted solution medicinal product can provide healing effect on the body, even if the dilution factor is so large that nothing is left in the solution except water molecules. Proponents of homeopathy explain this paradox by a concept called "memory of water", according to which water at the molecular level has a "memory" of a substance that was once dissolved in it and retains the properties of a solution of its original concentration after not a single molecule of an ingredient remains in it.
An international group of scientists led by Professor Madeleine Ennis of Queen's University of Belfast, who criticized the principles of homeopathy, conducted an experiment in 2002 to refute this concept once and for all. which, the scientists said that they were able to prove the reality of the effect of "memory of water." However, experiments conducted under the supervision of independent experts, have not yielded results.Disputes about the existence of the phenomenon of "memory of water" continues.
Water has many other unusual properties that we have not covered in this article.
Literature.
1.5 Really Weird Things About Water / http://www.neatorama.com.
2. The mystery of water: the theory of the Aristotle-Mpemba effect has been created / http://www.o8ode.ru.
3. Nepomnyashchy N.N. Mysteries inanimate nature... The most mysterious substance in the universe / http://www.bibliotekar.ru.
It seems obvious that cold water freezes faster than hot water, since under equal conditions hot water takes longer to cool down and subsequently freeze. However, millennial observations, as well as modern experiments have shown that the opposite is also true: under certain conditions, hot water freezes faster than cold water. Sciencium explains this phenomenon:
As explained in the video above, the phenomenon of hot water freezing faster than cold water is known as the Mpemba effect, named for Erasto Mpemba, a Tanzanian student who made ice cream in 1963 as part of a school project. Students had to bring a mixture of cream and sugar to a boil, let it cool, and then put it in the freezer.
Instead, Erasto put his mixture on immediately, hot, without waiting for it to cool. As a result, after 1.5 hours, his mixture was already frozen, but the mixtures of other students were not. Intrigued by the phenomenon, Mpemba began to study the matter with physics professor Denis Osborne, and in 1969 they published an article stating that warm water freezes faster than cold water. This was the first peer-reviewed such study, but the phenomenon itself is mentioned in the papers of Aristotle, dating back to the 4th century BC. e. Francis Bacon and Descartes also noted this phenomenon in their studies.
The video lists several options for explaining what is happening:
- Frost is a dielectric, and therefore frosty cold water stores heat better than a warm glass, which melts ice in contact with it
- There are more dissolved gases in cold water than in warm water, and the researchers speculate that this may play a role in the cooling rate, although it is not yet clear how
- Hot water loses more water molecules due to evaporation, so less is left for freezing
- Warm water can cool faster due to increased convective currents. These currents arise because, first of all, the water in the glass is cooled on the surface and on the sides, causing the cold water to sink and the hot one to rise. In a warm glass, convective currents are more active, which can affect the cooling rate.
However, a carefully controlled study was conducted in 2016, which showed the opposite: hot water froze much slower than cold water. At the same time, scientists noticed that changing the location of the thermocouple - a device that determines temperature drops - by just a centimeter leads to the appearance of the Mpemba effect. The study of other similar works showed that in all cases when this effect was observed, there was a displacement of the thermocouple within a centimeter.
Water is one of the most amazing liquids in the world with unusual properties. For example, ice is a solid state of a liquid, has specific gravity lower than the water itself, which made the emergence and development of life on Earth in many ways possible. In addition, in the pseudo-scientific, and even in the scientific world, there are discussions about which water freezes faster - hot or cold. Anyone who proves faster freezing of hot liquids under certain conditions and scientifically substantiates their decision will receive an award of £ 1000 from the British Royal Society of Chemists.
History of the issue
The fact that when a number of conditions are fulfilled, hot water is faster than cold water in terms of freezing rate, was noticed back in the Middle Ages. Francis Bacon and René Descartes have gone to great lengths to explain this phenomenon. However, from the point of view of classical heating engineering, this paradox cannot be explained, and they tried to shyly hush up about it. The impetus for the continuation of the controversy was a somewhat curious story that happened to the Tanzanian schoolboy Erasto Mpemba in 1963. Once, during a lesson on making desserts at a school of cooks, the boy, distracted by extraneous matters, did not have time to cool the ice cream mixture in time and put a hot solution of sugar in milk into the freezer. To his surprise, the product cooled somewhat faster than that of his fellow practitioners observing temperature regime making ice cream.
Trying to understand the essence of the phenomenon, the boy turned to his physics teacher, who, without going into details, ridiculed his culinary experiments. However, Erasto was distinguished by enviable persistence and continued his experiments no longer with milk, but with water. He was convinced that in some cases hot water freezes faster than cold water.
After entering the University of Dar es Salaam, Erasto Mpembe attended a lecture by Professor Dennis G. Osborne. After graduation, the student puzzled the scientist with the problem of the rate of water freezing depending on its temperature. D.G. Osborne ridiculed the very statement of the question, stating with aplomb that any failing student knows that cold water will freeze faster. However, the young man's natural stubbornness made itself felt. He made a bet with the professor, suggesting here, in the laboratory, to conduct an experimental test. Erasto placed two containers of water in the freezer, one at 95 ° F (35 ° C) and the other 212 ° F (100 ° C). Imagine the surprise of the professor and the surrounding "fans" when the water in the second container froze faster. Since then, this phenomenon has been called the "Mpemba Paradox".
However, to date, there is no coherent theoretical hypothesis explaining the "Mpemba Paradox". It is not clear which external factors, chemical composition water, the presence of dissolved gases and minerals in it affect the freezing rate of liquids at different temperatures. The paradox of the "Mpemba Effect" is that it contradicts one of the laws discovered by I. Newton, which says that the cooling time of water is directly proportional to the temperature difference between the liquid and the environment. And if all other liquids completely obey this law, then water in some cases is an exception.
Why hot water freezes fasterT
There are several versions why hot water freezes faster than cold water. The main ones are:
- hot water evaporates faster, while its volume decreases, and a smaller volume of liquid cools faster - when water is cooled from + 100 ° C to 0 ° C, volumetric losses at atmospheric pressure reach 15%;
- the intensity of heat transfer between the liquid and environment the higher, the greater the temperature difference, therefore, the heat losses of boiling water pass faster;
- when hot water cools down, a crust of ice forms on its surface, which prevents the liquid from completely freezing and evaporating;
- at a high temperature of water, its convection mixing occurs, which reduces the freezing time;
- gases dissolved in water lower the freezing point, taking away energy for crystallization - there are no dissolved gases in hot water.
All these conditions have been repeatedly tested experimentally. In particular, the German scientist David Auerbach discovered that the crystallization temperature of hot water is slightly higher than that of cold water, which makes it possible for the former to freeze faster. However, later his experiments were criticized and many scientists are convinced that the "Mpemba effect" about which water freezes faster - hot or cold, can be reproduced only under certain conditions, the search and specification of which until now no one has been engaged.
In this article we will look at the question of why hot water freezes faster than cold water.
Hot 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 Aristotle has a description winter fishing: fishermen inserted fishing rods into holes in the ice, and to make them freeze faster, they poured the ice warm water... The name of this phenomenon was given by the name of Erasto Mpemba in the 60s of the XX century. Mnemba noticed a strange effect when he was preparing ice cream, and turned to his physics teacher, Dr. Denis Osborne, for an explanation. Mpemba and Dr. Osborne experimented with water of 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 got similar results each time.
Explanation of the physical phenomenon
There is no generally accepted explanation of why this is happening. Many researchers suggest that it is all about the hypothermia of the liquid, which occurs when its temperature drops below freezing point. In other words, if water freezes at temperatures below 0 ° C, then supercooled water can have a temperature of, for example, -2 ° C and at the same time remain liquid without turning into ice. When we try to freeze cold water, there is a chance that it will first be supercooled and harden only after a while. Other processes take place in heated water. Its faster transformation into ice is associated with convection.
Convection- it physical phenomenon, in which the warm lower layers of the liquid rise, and the upper, cooled down, fall.
