In this article, we will look at why hot water freezes faster than cold water.

Heated water freezes much faster than cold water! This amazing property of water, the 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. The name of this phenomenon was named after Erasto Mpemba in the 60s of the XX century. Mnemba noticed the strange effect while making ice cream and turned to his physics teacher, Dr. Denis Osborn, for an explanation. Mpemba and Dr. Osborne experimented with water at different temperatures and concluded that almost boiling water begins to freeze much faster than water at room temperature. Other scientists have carried out their own experiments and each time they have obtained similar results.

Explanation of a physical phenomenon

There is no generally accepted explanation as to why this is happening. Many researchers suggest that it's all about the supercooling of a liquid, which occurs when its temperature drops below freezing. 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 become supercooled at first, and will only harden after some time. In heated water, other processes take place. Its faster transformation into ice is associated with convection.

Convection- This is a physical phenomenon in which the warm lower layers of the liquid rise, and the upper, cooled ones, fall.

Have you ever thought about the question why water heated to 82 degrees C freezes faster than cold water? Most likely not, I’m even more than sure that the question never occurred to you - which water freezes faster hot or cold?

However, this amazing 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.

Cold water contains a lot of dissolved oxygen, it is he who maintains the freezing point of water at 0 degrees. If oxygen is removed from the water, and this is exactly what happens when the water is heated, the air bubbles dissolved in the water, as it is fashionable to say now, collapse, the water does not turn into ice 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 moving from a liquid state to a solid state, simply turning into

Hello, dear lovers of interesting facts. Today we will talk about. But I think that the question posed in the title may seem simply absurd - but is it always necessary to completely trust the notorious "common sense", and not strictly set testing experience. Let's try to figure out why hot water freezes faster than cold water?

Historical reference

That in the issue of freezing cold and hot water “not everything is pure” was mentioned in the works of Aristotle, then similar notes were made by F. Bacon, R. Descartes and J. Black. In recent history, the name “Mpemba paradox” has been attached to this effect - after the name of a schoolboy from Tanganyika, Erasto Mpemba, who asked the same question to a visiting professor of physics.

The boy's question arose not from scratch, but from purely personal observations of the process of cooling ice cream mixtures in the kitchen. Of course, the classmates who were present there, together with the school teacher, laughed at Mpemba - however, after an experimental check personally by Professor D. Osborne, the desire to make fun of Erasto "evaporated" from them. Moreover, Mpemba, together with the professor, published a detailed description of this effect in 1969 in Physics Education - and since then the above name has been fixed in the scientific literature.

What is the essence of the phenomenon?

The setup of the experiment is quite simple: other conditions being equal, identical thin-walled vessels are tested, in which there are strictly equal amounts of water, differing only in temperature. The vessels are loaded into the refrigerator, after which the time is recorded before the formation of ice in each of them. The paradox is that in a vessel with an initially hotter liquid, this happens faster.

How does modern physics explain this?

The paradox has no universal explanation, since several parallel processes proceed together, the contribution of which may differ from specific initial conditions - but with a uniform result:

• the ability of a liquid to supercool - initially cold water is more prone to hypothermia, i.e. remains liquid when its temperature is already below the freezing point
• accelerated cooling - steam from hot water is transformed into ice microcrystals, which, when falling back, accelerate the process, working as an additional "external heat exchanger"
• isolation effect - unlike hot water, cold water freezes from above, which leads to a decrease in heat transfer by convection and radiation

There are a number of other explanations (the last time the competition for the best hypothesis was held by the British Royal Society of Chemistry recently, in 2012) - but there is still no unambiguous theory for all cases of combinations of input conditions ...

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

Erasto Mpemba was a student at Magambin High School in Tanzania doing practical cooking work. He had 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 procrastinated on 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 at Mkwawa High School, he asked Professor Dennis Osborne from the University College in Dar es Salaam (invited by the director of the school to give a lecture on physics to students) 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?" Osborn became interested in this issue and soon in 1969, together with Mpemba, they 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 happens? Just a few years ago, scientists managed to explain this phenomenon ...

The Mpemba effect (Mpemba Paradox) is a paradox that states that hot water under certain 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 hotter body needs more time to cool down to a certain temperature than a cooler body to cool down 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 the ambient temperature must 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 the same effect, water at 100°C cools down to 0°C faster than the same amount of water at 35°C.

Since then, different versions have been expressed, one of which was as follows: part of the hot water simply evaporates at first, and then, when a smaller amount remains, the water solidifies faster. This version, due to its simplicity, became the most popular, but scientists were not completely satisfied.

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

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

At the same time, water molecules as a whole repel each other. Scientists from Singapore noticed that the warmer the water, the greater the distance between the molecules of the liquid 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 approach each other. And the return of energy, as you know, means cooling.

Here are the hypotheses put forward 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. The evaporation effect is a double effect. First, the mass 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 cold air is greater - therefore, heat transfer 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 certain conditions, it can undergo supercooling while 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 in liquid water, then supercooling will continue until the temperature drops enough that crystals begin to form spontaneously. When they start to form in the supercooled liquid, they will start to grow faster, forming an ice slush that will freeze to form ice. Hot water is most susceptible to hypothermia because heating it eliminates 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, what happens is that a thin layer of ice forms on its surface, which acts as an insulator between the water and the cold air, and thus prevents further evaporation. The rate of formation of ice crystals in this case will be less. In the case of hot water undergoing subcooling, the subcooled 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 the loss of heat, while hot water begins to freeze from below. This effect is explained by an anomaly in the density of water. Water has a maximum density at 4°C. If you cool water to 4°C and place it in an environment with a lower temperature, the surface layer of water will freeze faster. Because this water is less dense than water at 4°C, it will stay 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 rapidly due to evaporation and greater temperature differences. Also, cold water layers are denser than hot water layers, so the cold water layer will sink down, lifting the warm water layer to the surface. This circulation of water ensures a rapid drop in temperature. But why does this process not reach the equilibrium point? To explain the Mpemba effect from the point of view of convection, it would be assumed 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 water layers 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 lower the freezing point of water. When the water is heated, these gases are released from the water because their solubility in water at high temperature is lower. Therefore, when hot water is cooled, there are always fewer 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 freezer in small containers. Under these conditions, it has been observed that the container with hot water melts the ice of the freezer underneath, thereby improving thermal contact with the wall of the freezer and thermal conductivity. As a result, heat is removed from the hot water container faster than from the cold one. In turn, the container with cold water does not melt snow under it. All these (as well as other) conditions have been studied in many experiments, but an unambiguous answer to the question - which of them provide a 100% reproduction of the Mpemba effect - has not been obtained. So, for example, in 1995, the German physicist David Auerbach studied the influence of supercooling of 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 earlier data that hot water is 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. So far, only one thing can be asserted - the reproduction of this effect significantly depends on the conditions under which the experiment is carried out. Precisely because it is not always reproduced.

And here's the most likely reason.

As the chemists write in their article, which can be found on the arXiv.org preprint site, hydrogen bonds are stretched more strongly in hot water 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, freezing is faster.

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

The British Royal Society of Chemistry is offering a £1,000 reward to anyone who can scientifically explain 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, philosophers such as Aristotle and Descartes have thought about it,” said Professor David Philips, president of the British Royal Society of Chemistry, quoted in a press release from the Society.

How an African chef beat a British physics professor

This is not an April Fool's joke, but a harsh physical reality. Today's 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 unambiguously states that it takes more time to cool a hot body than to cool a cold body. But for water, this law is not always observed. Aristotle drew attention to this paradox in the 4th century BC. e. Here is what the ancient Greek wrote in the book "Meteorologica I": "The fact that the water is preheated contributes to its freezing. Therefore, many people, when they want to quickly cool hot water, 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 thermal physics 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 Mpemba from Tanzania, far from any science. While studying at a cooking 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 be in time by the end of the lesson, he put the 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 a physics teacher, he made fun of him in front of the whole class. Mpemba remembered the insult. Five years later, already a student at the University of 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 the same amount 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 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 wager. Their argument culminated in an experimental test that proved Mpemba right and Osborne defeated. So the student-cooker 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 in the tooth with a foot."

Are dust particles and dissolved substances 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 definitive. A number of scientists devoted their entire lives to the Mpemba effect. James Brownridge, a member of the Department of Radiation Safety at the State University of New York, has been studying the paradox in his spare time for over a decade. After conducting hundreds of experiments, the scientist claims that he has evidence of the "guilt" of hypothermia. Brownridge explains that at 0°C, water only supercools, 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, and these are dust particles, bacteria and dissolved salts, have their characteristic nucleation temperature, when ice crystals form around the crystallization centers. When several elements are present in water at once, the freezing point is determined by the one with the highest nucleation temperature.

For the experiment, Brownridge took two samples of water at 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 sterling from the British Royal Society of Chemistry.