Video: which water freezes faster - hot or cold. Why does hot water freeze faster than cold water?

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

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.

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.

One of my favorite subjects at school was chemistry. Once a chemistry teacher gave us a very strange and difficult task. He gave us a list of questions that we had to answer in terms of chemistry. We were given several days for this task and were allowed to use libraries and other available sources of information. One of these questions concerned the freezing point of water. I don’t remember exactly how the question sounded, but it was about the fact that if you take two wooden buckets of the same size, one with hot water, the other with cold (with a precisely indicated temperature), and place them in an environment with a certain temperature, which one will Will they freeze faster? Of course, the answer immediately suggested itself - a bucket of cold water, but we thought it was too simple. But this was not enough to give a complete answer; we needed to prove it from a chemical point of view. Despite all my thinking and research, I could not come to a logical conclusion. I even decided to skip this lesson that day, so I never learned the solution to this riddle.

Years passed, and I learned many everyday myths about the boiling point and freezing point of water, and one myth said: “hot water freezes faster.” I looked at many websites, but the information was too conflicting. And these were just opinions, unfounded from a scientific point of view. And I decided to conduct my own experiment. Since I couldn't find wooden buckets, I used the freezer, stove, some water and a digital thermometer. I will tell you about the results of my experience a little later. First, I will share with you some interesting arguments about water:

Hot water freezes faster than cold water. Most experts say that cold water will freeze faster than hot water. But one funny phenomenon (the so-called Memba effect), for unknown reasons, proves the opposite: Hot water freezes faster than cold water. One of several explanations is the process of evaporation: if very hot water is placed in a cold environment, the water will begin to evaporate (the remaining amount of water will freeze faster). And according to the laws of chemistry, this is not a myth at all, and most likely this is what the teacher wanted to hear from us.

Boiled water freezes faster than tap water. Despite the previous explanation, some experts argue that boiled water that has cooled to room temperature should freeze faster because boiling reduces the amount of oxygen.

Cold water boils faster than hot water. If hot water freezes faster, then maybe cold water boils faster! This is contrary to common sense and scientists say that this simply cannot be. Hot tap water should actually boil faster than cold water. But using hot water to boil does not save energy. You may use less gas or light, but the water heater will use the same amount of energy needed to heat cold water. (With solar energy the situation is a little different). As a result of heating the water by the water heater, sediment may appear, so the water will take longer to heat up.

If you add salt to water, it will boil faster. Salt increases the boiling point (and accordingly lowers the freezing point - which is why some housewives add a little rock salt to their ice cream). But in this case we are interested in another question: how long will the water boil and whether the boiling point in this case can rise above 100°C). Despite what cookbooks say, scientists say that the amount of salt we add to boiling water is not enough to affect the boiling time or temperature.

But here's what I got:

Cold water: I used three 100 ml glass glasses of purified water: one glass with room temperature (72°F/22°C), one with hot water (115°F/46°C), and one with boiled water (212 °F/100°C). I placed all three glasses in the freezer at -18°C. And since I knew that water would not immediately turn into ice, I determined the degree of freezing using a “wooden float”. When the stick placed in the center of the glass no longer touched the base, I considered the water to be frozen. I checked the glasses every five minutes. And what are my results? The water in the first glass froze after 50 minutes. Hot water froze after 80 minutes. Boiled - after 95 minutes. My findings: Given the conditions in the freezer and the water I used, I was unable to reproduce the Memba effect.

I also tried this experiment with previously boiled water that had cooled to room temperature. It froze within 60 minutes - still took longer than cold water to freeze.

Boiled water: I took a liter of water at room temperature and put it on the fire. It boiled in 6 minutes. I then cooled it back down to room temperature and added it to it while it was hot. With the same fire, hot water boiled in 4 hours and 30 minutes. Conclusion: As expected, hot water boils much faster.

Boiled water (with salt): I added 2 large tablespoons of table salt per 1 liter of water. It boiled in 6 minutes 33 seconds, and as the thermometer showed, it reached a temperature of 102°C. Undoubtedly, salt affects the boiling point, but not much. Conclusion: salt in water does not greatly affect the temperature and boiling time. I honestly admit that my kitchen can hardly be called a laboratory, and perhaps my conclusions contradict reality. My freezer may not freeze food evenly. My glass glasses may have been irregularly shaped, Etc. But no matter what happens in the laboratory, when it comes to freezing or boiling water in the kitchen, the most important thing is common sense.

link with interesting facts about waterall about water
as suggested on the forum forum.ixbt.com, this effect (the effect of hot water freezing faster than cold water) is called the “Aristotle-Mpemba effect”

Those. Boiled water (chilled) freezes faster than “raw” water

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 logically, 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 true that in order for the liquid to turn into ice faster, it must first be... heated? The young man was so surprised that he shared his guess with the teacher. He reported this curiosity in the press.

This story happened back in the 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 results. Disputes about the existence of the “water memory” phenomenon continue.

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.