Fast cold. Why hot water freezes faster than cold water

There are many factors that influence which water freezes faster, hot or cold, but the question itself seems a little odd. It is implied, and it is known from physics, that hot water still needs time to cool down to the temperature of comparable cold water to turn into ice. In cold water, 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, in a scientific way, it turns out that in any case cold water just have to 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 then there was no explanation for this. 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. Certain conditions must be present 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 will contain hot water, and in the other - cold, container with hot water melts the snow underneath, 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 this 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 a solid state is 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 argues 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 beautiful formulas and it 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.

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 related to 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 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 lesson, so I never found out the solution to this riddle.

Years passed, and I learned a lot of 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 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 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 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 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 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. At 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 glass glasses could be irregular shape, 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"

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 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 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 the body cools down to a temperature 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. Nevertheless, 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 cools 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. In this case, the rate of formation of ice crystals 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 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 melt 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 effect of supercooling of water on this effect. He found that hot water, reaching a supercooled state, freezes at a higher temperature than cold water, which means faster than the latter. But cold water reaches a supercooled state faster than hot water, thereby compensating for the previous lag. In addition, Auerbach's results contradicted 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. O. V. Mosin

It seems obvious that cold water freezes faster than hot water, since under equal conditions hot water takes longer to cool down and subsequently freeze. However, thousands of years of observations, as well as modern experiments have shown that the opposite is also true: under certain conditions, hot water freezes faster than cold water. Science channel Sciencium explains this phenomenon:

As explained in the video above, the phenomenon where hot water freezes faster than cold water is known as the Mpemba effect, named for Erasto Mpemba, a Tanzanian student who made ice cream as part of school project... Students had to bring a mixture of cream and sugar to a boil, let it cool, and then put it in the freezer.

Instead, Erasto put his mixture on immediately, hot, without waiting for it to cool. As a result, after 1.5 hours, his mixture was already frozen, but the mixtures of other students were not. Intrigued by the phenomenon, Mpemba began studying the issue with physics professor Denis Osborne, and in 1969 they published an article stating that warm water freezes faster than cold water. This was the first peer-reviewed such study, but the phenomenon itself is mentioned in the papers of Aristotle, dating back to the 4th century BC. NS. Francis Bacon and Descartes also noted this phenomenon in their studies.

The video lists several options for explaining what is happening:

1. Frost is a dielectric, and therefore frosty cold water stores heat better than a warm glass, which melts ice in contact with it
2. There are more dissolved gases in cold water than in warm water, and the researchers speculate that this may play a role in the cooling rate, although it is not yet clear how
3. Hot water loses more water molecules due to evaporation, so less is left for freezing
4. Warm water can be cooled faster by increasing convective currents. These currents arise because, first of all, the water in the glass cools on the surface and on the sides, forcing the cold water to sink, and the hot one to rise. In a warm glass, convective currents are more active, which can affect the cooling rate.

However, a carefully controlled study was conducted in 2016, which showed the opposite: hot water froze much slower than cold water. At the same time, scientists noticed that changing the location of the thermocouple - a device that determines temperature drops - by just a centimeter leads to the appearance of the Mpemba effect. The study of other similar works showed that in all cases when this effect was observed, there was a displacement of the thermocouple within a centimeter.

21.11.2017 11.10.2018 Alexander Firtsev

« Which water freezes faster cold or hot?"- try to ask a question to your friends, most likely most of them will answer that cold water freezes faster - and make a mistake.

In fact, if at the same time you put two vessels of the same shape and volume in the freezer, one of which will have cold water and the other hot, then the hot water will freeze faster.

Such a statement may seem absurd and unreasonable. If you follow the logic, then hot water must first cool down to a cold temperature, and cold at this time should have already turned into ice.

So why does hot water overtake cold water on its way to freezing? Let's try to figure it out.

History of observations and research

People have observed the paradoxical effect since ancient times, but no one attached much importance to it. This is how Arrestotel, as well as René Descartes and Francis Bacon, noted in their notes that there is no coincidence in the freezing rate of cold and hot water. An unusual phenomenon often manifested itself in everyday life.

For a long time, the phenomenon was not studied in any way and did not arouse much interest among scientists.

The study of the unusual effect began in 1963 when an inquisitive student from Tanzania, Erasto Mpemba, noticed that hot milk for ice cream freezes faster than cold milk. Hoping to get an explanation of the reasons for the unusual effect, the young man asked his physics teacher at school. However, the teacher only laughed at him.

Later, Mpemba repeated the experiment, but in his experiment he no longer used milk, but water, and the paradoxical effect was repeated again.

6 years later, in 1969, Mpemba asked this question to professor of physics Dennis Osborne who came to his school. The professor was interested in the observation of the young man, as a result, an experiment was carried out, which confirmed the presence of the effect, but the reasons for this phenomenon were not established.

Since then, the phenomenon has been called Mpemba effect.

Throughout the history of scientific observation, many hypotheses have been put forward about the causes of the phenomenon.

So in 2012, the British Royal Chemical Society would have announced a competition for hypotheses explaining the Mpemba effect. Scientists from all over the world took part in the competition, in total 22,000 were registered scientific works... Despite such an impressive number of articles, none of them clarified the Mpemba paradox.

The most common version was that hot water freezes faster, because it simply evaporates faster, its volume becomes smaller, and as the volume decreases, its cooling rate increases. The most widespread version was eventually disproved as an experiment was carried out in which evaporation was excluded, but the effect was nevertheless confirmed.

Other scientists believed that the cause of the Mpemba effect is the evaporation of gases dissolved in water. In their opinion, in the process of heating, gases dissolved in water evaporate, due to which it gains more high density than cold. As you know, an increase in density leads to a change physical properties water (increase in thermal conductivity), and therefore an increase in the cooling rate.

In addition, a number of hypotheses have been put forward describing the rate of water circulation as a function of temperature. In many studies, an attempt was made to establish the relationship between the material of the containers in which the liquid was located. Many theories seemed very plausible, but it was not possible to scientifically confirm them due to the lack of initial data, contradictions in other experiments, or due to the fact that the revealed factors were simply not comparable to the rate of water cooling. Some scientists in their works questioned the existence of the effect.

In 2013, researchers at Nanyang University of Technology in Singapore said they had solved the mystery of the Mpemba effect. According to their research, the reason for the phenomenon lies in the fact that the amount of energy stored in hydrogen bonds between the molecules of cold and hot water is significantly different.

Computer simulation methods have shown the following results: the higher the water temperature, the greater the distance between the molecules due to the fact that the repulsive forces increase. Therefore, the hydrogen bonds of molecules are stretched, storing large quantity energy. When cooled, the molecules begin to approach each other, releasing energy from hydrogen bonds. In this case, the release of energy is accompanied by a decrease in temperature.

In October 2017, Spanish physicists in the course of another study found that it is the removal of matter from equilibrium (strong heating before strong cooling) plays a large role in the formation of the effect. They identified the conditions under which the likelihood of the effect being greatest. In addition, scientists from Spain have confirmed the existence of the opposite Mpemba effect. They found that when heated, a colder sample can reach high temperature faster than warm.

Despite exhaustive information and numerous experiments, scientists intend to continue studying the effect.

The Mpemba effect in real life

Have you ever wondered why in winter the ice rink is filled with hot water, and not cold? As you already understood, they do this because an ice rink filled with hot water will freeze faster than if it were filled with cold water. For the same reason, slides in winter ice towns are flooded with hot water.

Thus, knowledge of the existence of the phenomenon allows people to save time when preparing sites for winter species sports.

In addition, the Mpemba effect is sometimes used in industry - to reduce the freezing time of products, substances and materials containing water.