It seems obvious that cold water freezes faster than hot water, since under equal conditions hot water takes longer to cool 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. The Sciencium Science Channel explains this phenomenon:
As explained in the video above, the phenomenon of hot water freezing faster than cold water is known as the Mpemba effect, named after Erasto Mpemba, a Tanzanian student who made ice cream as part of a school project in 1963. 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 in immediately, hot, without waiting for it to cool. As a result, after 1.5 hours his mixture was already frozen, but the other students’ mixtures were not. Interested in the phenomenon, Mpemba began studying the issue with physics professor Denis Osborne, and in 1969 they published a paper stating that warm water freezes faster than cold water. This was the first peer-reviewed study of its kind, but the phenomenon itself is mentioned in the papers of Aristotle, dating back to the 4th century BC. e. Francis Bacon and Descartes also noted this phenomenon in their studies.
The video lists several options for explaining what is happening:
However, in 2016, a carefully controlled study was conducted that showed the opposite: hot water froze much more slowly than cold water. At the same time, scientists noticed that changing the location of the thermocouple - a device that determines temperature changes - by just a centimeter leads to the appearance of the Mpemba effect. A study of other similar studies showed that in all cases where this effect was observed, there was a displacement of the thermocouple within a centimeter.
In this article we will look at the question of why hot water freezes faster than cold water.
Heated water freezes much faster than cold water! This amazing property water, an exact explanation for which scientists still cannot find, has been known since ancient times. For example, even in Aristotle there is a description of winter fishing: fishermen inserted fishing rods into holes in the ice, and so that they would freeze faster, they poured warm water on the ice. This phenomenon was named after Erasto Mpemba in the 60s of the 20th century. Mnemba noticed strange effect while making ice cream, and turned to his physics teacher, Dr. Denis Osborne, for an explanation. Mpemba and Dr. Osborne experimented with water different temperatures and concluded: almost boiling water begins to freeze much faster than water at room temperature. Other scientists conducted their own experiments and each time obtained similar results.
There is no generally accepted explanation for why this happens. Many researchers suggest that the whole point is in the supercooling of the liquid, which occurs when its temperature drops below the freezing point. In other words, if water freezes at a temperature below 0°C, then supercooled water can have a temperature of, for example, -2°C and still remain liquid without turning into ice. When we try to freeze cold water, there is a chance that it will first become supercooled and only harden after some time. Other processes occur in heated water. Its faster transformation into ice is associated with convection.
Convection- this is a physical phenomenon in which the warm lower layers of a liquid rise, and the upper, cooled ones, fall.
There are many factors that influence which water freezes faster, hot or cold, but the question itself seems a little strange. The implication, and this is known from physics, is that hot water still needs time to cool to the temperature of the cold water being compared in order to turn into ice. Cold water can skip this stage, and, accordingly, it gains time.
But the answer to the question of which water freezes faster - cold or hot - outside in the cold, any resident of northern latitudes knows. In fact, scientifically, it turns out that in any case, cold water is simply bound to freeze faster.
The physics teacher, to whom schoolboy Erasto Mpemba approached in 1963, thought the same thing with a request to explain why the cold mixture of future ice cream takes longer to freeze than a similar, but hot one.
At that time, the teacher only laughed at this, but Deniss Osborne, a professor of physics, who at one time visited the same school where Erasto studied, experimentally confirmed the presence of such an effect, although there was no explanation for it then. In 1969, a joint article by these two people was published in a popular scientific journal, 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 Mpemba effect, or paradox.
Naturally, such a phenomenon took place before, and it was mentioned in the works of other scientists. Not only the schoolchild was interested in this issue, but Rene Descartes and even Aristotle also thought about it at one time.
But they began to look for approaches to solving this paradox only at the end of the twentieth century.
As with ice cream, it's not easy plain water 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 higher evaporation rate than cold water. Thus, its volume decreases, and as the volume decreases, the freezing time becomes shorter than if we take the same initial volume of cold water.
Which water freezes faster and why this happens can be influenced by the snow lining that may be present 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 underneath, thereby improving the contact of the thermal level with the wall of the refrigerator. A container of cold water cannot do this. If there is no such lining with snow in the refrigerator compartment, cold water should freeze faster.
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 the opposite - it begins to freeze from the bottom up. 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. Photos from amateur experiments are attached, and this is clearly visible here.
The heat goes out, rushing upward, and there it meets a very cool layer. There is no free path for heat radiation, so the cooling process becomes difficult. Hot water has absolutely no such obstacles in its path. Which one freezes faster - cold or hot, what determines the likely outcome? You can expand the answer by saying that any water has certain substances dissolved in it.
If you don't 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 are available only in hot water, and cold water does not have them, then there is a possibility for hot water to freeze earlier. This is explained by the fact that dissolved substances 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 that the water will be supercooled, 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 claim that the secret this effect consists of the amount of energy that is stored between water molecules in its bonds, called hydrogen bonds.
What follows is information, to understand which you need to have some knowledge of chemistry in order to understand which water freezes faster - hot or cold. As is known, it consists of two H (hydrogen) atoms and one O (oxygen) atom, held together by covalent bonds.
But also the hydrogen atoms of one molecule are attracted to neighboring molecules, to their oxygen component. These bonds are called hydrogen bonds.
It is worth remembering that at the same time, water molecules have a repulsive effect on each other. Scientists noted that when water is heated, the distance between its molecules increases, and this is facilitated by repulsive forces. It turns out that by occupying the same distance between the molecules in a cold state, they can be said to stretch, and they have a greater supply of energy. It is this energy reserve that is released when water molecules begin to move closer to each other, that is, cooling occurs. It turns out that a greater reserve of energy in hot water, and its greater release when cooling to sub-zero temperatures, occurs faster than in cold water, which has a smaller reserve of such energy. So which water freezes faster - cold or hot? On the street and in the laboratory, Mpemba's paradox should occur, and hot water should turn into ice faster.
There is only theoretical confirmation of this solution - it’s all written beautiful formulas and it seems plausible. But when the experimental data on which water freezes faster - hot or cold - are put into practical use, and their results are presented, then the question of Mpemba’s paradox can be considered closed.
The British Royal Society of Chemistry is offering a £1,000 reward to anyone who can explain scientific point understanding why in some cases hot water freezes faster than cold water.
“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. This is what I wrote ancient Greek in the book Meteorologica I: “The fact that water is preheated contributes to its freezing. 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. Employee of the Department of Radiation Safety State University New York - James Brownridge - in free time has been studying the paradox for over a decade now. 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 claims that hot water cools faster because there is a greater difference between the temperatures of the water and the freezer - this helps it reach its freezing point before cold water reaches its freezing point. natural point 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.
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.
Being a student of Magambinskaya high school in Tanzania Erasto Mpemba did practical work in 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 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 the body cools down to temperature environment, must 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 that crystals begin 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 thin layer 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 layer warm water 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 high temperature below. 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.
