Which water will turn to ice faster. Why does hot water freeze faster than cold water? The Mpemba effect. How modern physics explains it

In 1963, a student from Tanzania named Erasto Mpemba asked his teacher a stupid question - why does warm ice cream freeze faster in his freezer than cold ice cream?

As a student of Magambinskaya high school in Tanzania, Erasto Mpemba did practical work on cooking. 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 still hot milk in the refrigerator. To his surprise, it froze even earlier than the milk of his comrades, prepared according to a given technology.

He turned to the physics teacher for clarification, but he only laughed at the student, saying the following: "This is not world physics, but the physics of Mpemba." After that, Mpemba experimented not only with milk, but also with ordinary water.

In any case, already being a student of the Mkvava 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 the Mpemba effect.

Are you curious to know why this is happening? Just a few years ago, scientists managed to explain this phenomenon ...

The Mpemba effect (Mpemba paradox) is a paradox that says 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. Until now, no one knows exactly how to explain this strange effect. Scientists do not have a single version, although there are many. It's all about the difference in the properties of hot and cold water, but it is not yet clear which properties play a role in this case: the difference in supercooling, evaporation, ice formation, convection, or the effect of liquefied gases on water at different temperatures. The paradox of the Mpemba effect is that the time during which the body cools down to a temperature the environment, should be proportional to the temperature difference between this body and the environment. This law was established by Newton and since then has been confirmed many times in practice. In 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.

Since then, various versions have been expressed, one of which sounded as follows: part of the hot water at first simply evaporates, and then, when there is less of it, the water freezes faster. This version, due to its simplicity, became the most popular, but scientists did not fully satisfy.

Now a team of researchers from Technological University Nanyang Technological University in Singapore, led by chemist Xi Zhang, said they have solved the age-old mystery of why warm water freezes faster than cold water. As the Chinese experts have found out, the secret lies in the amount of energy stored in hydrogen bonds between water molecules.

As you know, water molecules are made up of one oxygen atom and two hydrogen atoms held together covalent bonds, which at the particle level looks like an exchange of electrons. Another known fact consists in the fact that hydrogen atoms are attracted to oxygen atoms from neighboring molecules - hydrogen bonds are formed in this case.

At the same time, water molecules are generally repelled from each other. Scientists from Singapore noticed that the warmer the water, the greater the distance between the liquid molecules due to the increase in repulsive forces. As a result, hydrogen bonds are stretched, and therefore store more energy. This energy is released when the water cools - the molecules move closer to each other. And the release of energy, as you know, means cooling.

Here are the assumptions made by scientists:

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, due to evaporation, its temperature 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 –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, when frozen, 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, which is not supercooled, the following happens: a thin layer of ice forms on its surface, which acts as an insulator between water and cold air, and thus prevents 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 forms more ice... 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 place it in a lower temperature environment, 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 4 ° C. Therefore, the further cooling process will be slower. In the case of hot water, the situation is completely different. The surface layer of water will cool more quickly due to evaporation and a greater temperature difference. 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 the 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 because their solubility in water at high temperatures is lower. 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 a refrigerator compartment 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 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 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 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 under which the experiment is carried out. Precisely because it is not always reproduced.

But as they say, the most likely reason.

As chemists write in their article, which can be found on the arXiv.org preprint site, hydrogen bonds are taut in hot water more than in cold water. Thus, it turns out that more energy is stored in the hydrogen bonds of hot water, which means that more of it is released when cooled to sub-zero temperatures. For this reason, solidification is faster.

To date, scientists have solved this riddle only theoretically. When they present convincing evidence of their version, the question of why hot water freezes faster than cold water can be considered closed.

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 any 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 Memb 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 as shown by the thermometer 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.

reference with entertaining 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"

Have you ever wondered why water heated to 82 degrees C freezes faster than cold? Most likely not, I am even more than sure that the question never crossed your mind - which water freezes faster, hot or cold?

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

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

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

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 unequivocally 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. Employee of the Department of Radiation Safety State University New York City - James Brownridge has spent a decade studying the paradox in his spare time. 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 the water at once, the freezing point is determined by the one that has the most high fever nucleation.

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.

Many factors influence which water freezes faster, hot or cold, but the question itself seems a little odd. It is implied, and this is known from physics, that hot water still needs time to cool down to the temperature of comparable cold water to turn into ice. this stage can be skipped, and, accordingly, it wins in time.

But the answer to the question of which water freezes faster - cold or hot - outside in frost, knows any inhabitant of northern latitudes. In fact, scientifically, it turns out that in any case, cold water simply must freeze faster.

The physics teacher, who was approached by schoolboy Erasto Mpemba in 1963 with a request to explain why the cold mixture of future ice cream freezes longer than a similar but hot one, thought the same way.

"This is not world physics, but some kind of Mpemba physics"

At that time, the teacher only laughed at this, but Deniss Osborne, a physics professor who at one time drove to the same school where Erasto studied, experimentally confirmed the presence of such an effect, although there was no explanation for this then. In 1969, a popular scientific journal published a joint article by these two people who described this peculiar effect.

Since then, by the way, the question of which water freezes faster - hot or cold - has its own name - the effect, or paradox, of Mpemba.

The question arose for a long time

Naturally, such a phenomenon took place before, and it was mentioned in the works of other scientists. Not only the schoolboy was interested in this issue, but Rene Descartes and even Aristotle thought about it in their time.

Here are just approaches to solving this paradox began to look only at the end of the twentieth century.

Conditions for a paradox to occur

As with ice cream, it's not just ordinary water that freezes during the experiment. Must be present certain conditions in order to start arguing which water freezes faster - cold or hot. What influences the course of this process?

Now, in the 21st century, several options have been put forward that can explain this paradox. Which water freezes faster, hot or cold, may depend on the fact that it has a faster evaporation rate than cold water. Thus, its volume decreases, and with a decrease in volume, the freezing time becomes shorter than if we take a similar initial volume of cold water.

Defrost the freezer for a long time

Which water freezes faster, and why it happens, can be influenced by the snow lining that can be found in the freezer of the refrigerator used for the experiment. If you take two containers that are identical in volume, but one of them contains hot water, and the other contains cold water, the container with hot water will melt the snow under it, thereby improving the contact of the thermal level with the wall of the refrigerator. A cold water container cannot do that. If there is no such lining with snow in the refrigerator compartment, cold water should freeze faster.

Top - Bottom

Also, the phenomenon of which water freezes faster - hot or cold, is explained as follows. Following certain laws, cold water begins to freeze from the upper layers, when hot water does it the other way around - it begins to freeze from the bottom up. At the same time, it turns out that cold water, having a cold layer on top with ice already formed in places, thus worsens the processes of convection and thermal radiation, thereby explaining which water freezes faster - cold or hot. Photo from amateur experiments is attached, and it is clearly visible here.

The heat goes out, tending upward, and there it meets a very cooled layer. There is no free path for heat radiation, so the cooling process becomes difficult. Hot water has no such obstacles on its way. Which one freezes faster - cold or hot, on which the probable outcome depends, you can expand the answer by the fact that any water has certain substances dissolved in it.

Impurities in water as a factor affecting outcome

If you do not cheat and use water with the same composition, where the concentrations of certain substances are identical, then cold water should freeze faster. But if a situation occurs when dissolved chemical elements available only in hot water, and cold water does not have them, then there is an opportunity for hot water to freeze earlier. This is explained by the fact that solutes in water create crystallization centers, and with a small number of these centers, the transformation of water into solid state difficult. It is even possible overcooling of water, in the sense that at sub-zero temperatures it will be in a liquid state.

But all these versions, apparently, did not completely suit the scientists and they continued to work on this issue. In 2013, a team of researchers in Singapore said they had solved an age-old mystery.

A group of Chinese scientists argue that the secret of this effect lies in the amount of energy that is stored between water molecules in its bonds, called hydrogen bonds.

Clue from Chinese Scientists

This is followed by information, for understanding which it is necessary to have some knowledge in chemistry in order to figure out which water freezes faster - hot or cold. As you know, it consists of two H atoms (hydrogen) and one O (oxygen) atom, held together by covalent bonds.

But also hydrogen atoms of one molecule are attracted to neighboring molecules, to their oxygen component. It is these bonds that are called hydrogen bonds.

It is worth remembering that at the same time, water molecules are repulsive to each other. Scientists noted that when water is heated, the distance between its molecules increases, and this is facilitated by just repulsive forces. It turns out that occupying one distance between the molecules in the cold state, one might say, they stretch, and they have a greater supply of energy. It is this store of energy that is released when water molecules begin to approach each other, that is, cooling occurs. It turns out that a greater supply of energy in hot water, and its greater release when cooled to subzero temperatures, occurs faster than in cold water, which has less such energy. So which water freezes faster - cold or hot? On the street and in the laboratory, the Mpemba paradox should occur, and hot water should turn into ice faster.

But the question is still open

There is only a theoretical confirmation of this clue - all this is written in beautiful formulas and seems plausible. But when the experimental data, which water freezes faster - hot or cold, are put in a practical sense, and their results are presented, then the question of the Mpemba paradox can be considered closed.