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 assignment and allowed to use libraries and other available sources of information. One of these questions was about 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 water (with exactly the specified temperature), and place them in an environment with a certain temperature, which of will freeze them faster? Of course, the answer immediately suggested itself - a bucket of cold water, but it seemed to us 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 was unable to draw a logical conclusion. On this day, I even decided to skip this tutorial, so I never found out the solution to this riddle.

Years passed, and I learned a lot of household 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 the point of view of science. And I decided to conduct my own experience. Since I couldn't find wooden buckets, I used a freezer, stovetop, some water, and a digital thermometer. I will talk about the results of my experience a little later. First, I'll share with you some interesting arguments about water:

Hot water freezes faster than cold water. Most experts say cold water will freeze faster than hot water. But one funny phenomenon (the so-called Membe effect), for unknown reasons, proves the opposite: Hot water freezes faster than cold water. One of several explanations is the evaporation process: 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 down 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 the cold water may boil faster! This is contrary to common sense and scientists argue that this simply cannot be. Hot tap water should actually boil faster than cold water. But by using hot water for boiling, you are not saving energy. You may use less gas or light, but the water heater will use the same amount of energy that is needed to heat cold water. (This is a little different with solar power.) As a result of the heating of the water by the water heater, sediment may appear, so the water will take longer to heat up.

If you add salt to the 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 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 they write in cookbooks, scientists argue 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 beakers of purified water: one at 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 by the "wooden float". When the stick, placed in the center of the glass, no longer touched the base, I assumed that the water had frozen. I checked the glasses every five minutes. And what are my results? The water in the first glass froze after 50 minutes. The 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 Memb effect.

I also tried this experiment with previously boiled water cooled to room temperature. She froze after 60 minutes - it still took longer than freezing cold water.

Boiled water: I took a liter of water at room temperature and put it on fire. It boiled in 6 minutes. Then I cooled it down to room temperature again and added it to the hot one. With the same heat, 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 to 1 liter of water. It boiled after 6 minutes 33 seconds, and 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 strongly affect the temperature and boiling time. I honestly admit that my kitchen can hardly be called a laboratory, and perhaps my conclusions are contrary to reality. My freezer compartment may freeze food unevenly. My glasses might have been irregular, etc. But whatever happens in the laboratory, when it comes to freezing or boiling water in the kitchen, the most important thing is common sense.

link with fun facts about vaudews about water
as suggested on the forum forum.ixbt.com this effect (the effect of freezing hot water faster than cold water) is called the "Aristotle-Mpemba effect"

Those. boiled water (chilled) freezes faster than "raw"

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 freezes faster than cold water under certain conditions, although 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 at Magamba High School in Tanzania, Erasto Mpemba did practical cooking work. 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 ordinary water. In any case, already being a student of the Mkvavskaya high school, he asked a question to 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 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 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 a container, thus decreasing 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. 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 cold air is greater - 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 the case of cold water that is not supercooled, the following occurs. In this case, a thin layer of ice will form on the surface of the vessel. This layer of ice will act as an insulator between the water and cold air and will prevent further evaporation. The rate of formation of ice crystals in this case will be 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 more heat is lost 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 would be necessary to accept 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 confirm 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 because their solubility in water at high temperatures is lower. Therefore, when hot water is cooled, there are 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 a refrigerator compartment in small containers. Under these conditions, it has been observed that a 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 the container with hot water faster than from cold water. In turn, a container with cold water does not thaw snow under it. All these (as well as others) conditions have been 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 influence 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 previously obtained data that hot water can achieve more 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. 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. O. V. Mosin

Water- a fairly simple substance from a chemical point of view, but at the same time it has a number of unusual properties that never cease to amaze scientists. Below are some facts that few people know about.

1. Which water freezes faster - cold or hot?

Take two containers of 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 down to cold temperature, and then turn into ice, while cold water does not need to cool down. Why is this happening?

In 1963, a Tanzanian student named Erasto B. Mpemba, while freezing a prepared ice cream concoction, noticed that the hot concoction hardened 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 does freeze faster than cold water.

Now this phenomenon of hot water freezing faster than cold water is called “ 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 hypothermia, evaporation, ice formation, convection, or by the effect of liquefied gases on hot and cold water.

2. She is able to freeze instantly

Everybody knows that water always turns to ice when cooled to 0 ° C ... except in some cases! Such a case, for example, is supercooling, which is the property of very pure water to 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.

Crystallization process can be triggered, for example, by gas bubbles, impurities (dirt), an uneven surface of the container. Without them, water will remain liquid. When the crystallization process starts, you can observe how supercooled water instantly turns into ice.

Note that "superheated" water also remains liquid, even when heated to temperatures above its boiling point.

3.19 states of water

Without hesitation, name how many different states water has? If you answered three: solid, liquid, gaseous, then you are mistaken. Scientists distinguish at least 5 different states of water in liquid form and 14 states in frozen form.

Remember the conversation about supercooled water? So, no matter what you do, at a temperature of -38 ° C, even the purest supercooled water will suddenly turn into ice. What happens if the temperature drops further? 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 "glass" or "glassy" water - a solid that lacks crystalline structure.

4. Water surprises physicists

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 takes place, and the chemical formula of water instead of H2O, becomes H1.5O!

5. Memory of water

Alternative to mainstream medicine homeopathy states that a diluted solution of a drug can have a healing effect on the body, even if the dilution factor is so high that nothing but water molecules is left in the solution. Homeopathic proponents attribute this paradox to 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. The result was the opposite. After that, the scientists said that they were able to prove the reality of the effect " memory of water". However, experiments carried out under the supervision of independent experts did not yield any results. Disputes about the existence of the phenomenon " memory of water"Continue.

Water has many other unusual properties that we have not covered in this article. For example, the density of water changes with temperature (the density of ice is less than that of water); water has a fairly high surface tension; in a liquid state, water is a complex and dynamically changing network of water clusters, and it is the behavior of the clusters that affects the structure of water, etc.

About these and many other unexpected features water can be read in the article “ Abnormal properties of water"By Martin Chaplin, professor at the University of London.

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

Major versions

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

1. As hot water evaporates faster, its volume decreases. Less water of the same temperature freezes faster.
2. The freezer compartment of the refrigerator has a snow pad. The hot water container melts the snow underneath. This improves the thermal contact with the freezer.
3. The freezing of cold water, unlike hot water, starts from the top. In this case, convection and heat radiation, and, consequently, heat loss deteriorate.
4. In cold water there are crystallization centers - substances dissolved in it. With a small content of them in water, icing is difficult, although at the same time, it may be overcooled - 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, for example, can be very fine particles of sand, dust or clay. When the air temperature drops, these particles are centers around which ice crystals form.

The role of crystallization nuclei can also be played by air bubbles and cracks in a 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 a liquid at a temperature of 0 degrees.

The essence of the Mpemba effect

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

He had to dissolve sugar in boiled milk and, after cooling it, place it in the refrigerator to freeze. Apparently, Mpemba did not differ in special zeal and began to perform the first part of the assignment with a delay. Therefore, he did not wait for the milk to cool, and put it in the refrigerator hot. He was very much surprised when it froze even faster than his classmates, who performed the work in accordance with the given technology.

The young man was very interested in this fact, and he began experiments with plain water. In 1969, Physics Education published the results of research by Mpemba and Professor Dennis Osborne from the University of Dar es Salaam. The effect they described was named Mpemba. However, even today there is no clear explanation for the phenomenon. All scientists agree that the main role in this difference between the properties of chilled and hot water belongs to, but it is not known exactly which one.

Singapore version

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

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

But when the water is heated, the bound molecules move slightly away from each other. As a result, in the middle of the molecules there is a relaxation of covalent bonds 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 down at an accelerated rate. At least, this is what the theoretical calculations carried out by Singaporean scientists show.

Instant freezing of water - 5 incredible tricks: Video

The British Royal Society of Chemistry is offering a £ 1,000 award 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 day to day work, but no one really knows why it works. This problem has been known for millennia, philosophers such as Aristotle and Descartes have pondered about it, ”said President of the British Royal Society of Chemistry, Professor David Philips, quoted in a press release from the Society.

How a cook from Africa defeated a British physics professor

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

And only in 1968 they "remembered" thanks to the schoolboy Erasto Mpemba from Tanzania, far from any science. While studying at the school of cookery, in 1963, 13-year-old Mpembe was tasked with 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 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 all the rules.

When Mpemba shared his discovery with a physics teacher, he made fun of him in front of the whole class. Mpemba remembered the hurt. Five years later, already a student at the university in Dar es Salaam, he was at 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 put them in the freezer, then water in a hot container will freeze faster. Why?" You can imagine the reaction of a British professor to the question of a young man from God-forsaken 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 the correctness of Mpemba and the defeat of Osborne. So the pupil-cook inscribed his name in the history of science, and henceforth this phenomenon is called the "Mpemba effect". To discard it, to declare it as if "non-existent" does not work. The phenomenon exists, and, as the poet wrote, "not to the teeth."

Are dust 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 solutes - but none of these factors can be considered final. A number of scientists have devoted their entire lives to the Mpemba effect. James Brownridge, a staff member of the Department of Radiation Safety at the State University of New York, has been studying the paradox in his spare time for over a decade. After conducting hundreds of experiments, the scientist claims to have evidence of the "guilt" of hypothermia. Brownridge explains that at 0 ° C, the water is only supercooled, and begins to freeze when the temperature drops below. The freezing point is controlled by impurities in the water - they change the rate of formation of ice crystals. Impurities, and these are dust grains, bacteria and dissolved salts, have a characteristic nucleation temperature for them, when ice crystals are formed around the centers of crystallization. When there are several elements in water at once, the freezing point is determined by the one with the highest nucleation temperature.

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

Brownridge claims that hot water cools faster due to the greater temperature difference between 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 from the British Royal Chemical Society.