Why does hot water freeze faster than cold water? Hot and cold water: the secrets of freezing Which water freezes faster - hot or cold

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 under some conditions freezes faster than cold water, although it must pass the temperature of cold water during the freezing process. 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. As a student at Magambi High School in Tanzania, Erasto Mpemba did practical work as a cook. 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 the Mpemba effect. Until now, no one knows exactly how to explain this strange effect. Scientists do not have a single version, although there are many. It's all about the difference in the properties of hot and cold water, but it is not yet clear which properties play a role in this case: the difference in supercooling, evaporation, ice formation, convection, or the effect of liquefied gases on water at different temperatures. The paradox of the Mpemba effect is that the time during which a body cools down to the ambient temperature should 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 a 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 effect of evaporation 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 for crystals 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, 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 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 warm water layer 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 data that would confirm 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 lower at high temperatures. 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

Water is one of the most amazing liquids in the world, which has unusual properties. For example, ice, a solid state of liquid, has a specific gravity lower than water itself, which made the emergence and development of life on Earth largely possible. In addition, in the pseudo-scientific and scientific world there are discussions about which water freezes faster - hot or cold. Anyone who can prove that hot liquid freezes faster under certain conditions and scientifically substantiates their solution will receive a £1,000 reward from the British Royal Society of Chemists.

Background

The fact that under a number of conditions, hot water freezes faster than cold water was noticed back in the Middle Ages. Francis Bacon and René Descartes spent a lot of effort explaining this phenomenon. However, from the point of view of classical heat engineering, this paradox cannot be explained, and they tried to bashfully hush up about it. The impetus for the continuation of the debate was a somewhat curious story that happened to Tanzanian schoolboy Erasto Mpemba in 1963. One day, during a lesson on making desserts at a chef school, the boy, distracted by other things, did not have time to cool the ice cream mixture in time and put a hot solution of sugar in milk into the freezer. To his surprise, the product cooled somewhat faster than that of his fellow students who observed the temperature regime for preparing ice cream.

Trying to understand the essence of the phenomenon, the boy turned to a physics teacher, who, without going into details, ridiculed his culinary experiments. However, Erasto was distinguished by enviable tenacity and continued his experiments not on milk, but on water. He became convinced that in some cases hot water freezes faster than cold water.

Having entered the University of Dar es Salaam, Erasto Mpembe attended a lecture by Professor Dennis G. Osborne. After its completion, the student puzzled the scientist with a problem about the rate of freezing of water depending on its temperature. D.G. Osborne ridiculed the very posing of the question, declaring with aplomb that any poor student knows that cold water will freeze faster. However, the young man’s natural tenacity made itself felt. He made a bet with the professor, proposing to conduct an experimental test right here in the laboratory. Erasto placed two containers of water in the freezer, one at 95°F (35°C) and the other at 212°F (100°C). Imagine the surprise of the professor and the surrounding “fans” when the water in the second container froze faster. Since then, this phenomenon has been called the “Mpemba Paradox”.

However, to date there is no coherent theoretical hypothesis explaining the “Mpemba Paradox”. It is not clear what external factors, the chemical composition of water, the presence of dissolved gases and minerals in it, influence the rate of freezing of liquids at different temperatures. The paradox of the “Mpemba Effect” is that it contradicts one of the laws discovered by I. Newton, which states that the cooling time of water is directly proportional to the temperature difference between the liquid and the environment. And if all other liquids completely obey this law, then water in some cases is an exception.

Why does hot water freeze faster?T

There are several versions of why hot water freezes faster than cold water. The main ones are:

• hot water evaporates faster, while its volume decreases, and a smaller volume of liquid cools faster - when cooling water from + 100°C to 0°C, volumetric losses at atmospheric pressure reach 15%;
• the greater the temperature difference, the greater the temperature difference, the higher the intensity of heat exchange between the liquid and the environment, so the heat loss of boiling water occurs faster;
• when hot water cools, a crust of ice forms on its surface, preventing the liquid from completely freezing and evaporating;
• at high water temperatures, convection mixing occurs, reducing the freezing time;
• Gases dissolved in water lower the freezing point, removing energy for crystal formation - there are no dissolved gases in hot water.

All these conditions have been repeatedly tested experimentally. In particular, the German scientist David Auerbach discovered that the crystallization temperature of hot water is slightly higher than that of cold water, which makes it possible for the former to freeze more quickly. However, later his experiments were criticized and many scientists are convinced that the “Mpemba Effect”, which determines which water freezes faster - hot or cold, can only be reproduced under certain conditions, which no one has been searching for and specifying until now.

Have you ever wondered why water heated to 82 degrees C freezes faster than cold water? Most likely not, I’m even more than sure that the question has 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. This was the usual experience of a curious boy, of course he could not correctly interpret the meaning of his and, moreover, scientists from all over the world until 1966 could not give a clear and substantiated 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 has a lot of dissolved oxygen, it is he who maintains the freezing temperature of water at 0 degrees. If oxygen is removed from the water, and this is what happens when water is heated, the air bubbles dissolved in the 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 going from a liquid to a solid state, and will simply turn into

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

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

1. Because hot water evaporates faster, its volume decreases. And freezing of a smaller amount of water at the same temperature occurs faster.
2. The freezer compartment of the refrigerator has a snow liner. A container containing hot water melts the snow underneath. This improves thermal contact with the freezer.
3. Freezing of cold water, unlike hot water, begins at the top. At the same time, convection and heat radiation, and, consequently, heat loss worsen.
4. Cold water contains crystallization centers - substances dissolved in it. If their content in water is small, icing is difficult, although at the same time, supercooling is possible - when at sub-zero temperatures it has a liquid state.

Although in fairness we can say that this effect is not always observed. Very often, cold water freezes faster than hot water.

At what temperature does water freeze

Why does water freeze at all? It contains a certain amount of mineral or organic particles. These could be, for example, very small particles of sand, dust or clay. As the air temperature decreases, these particles are the centers around which ice crystals form.

The role of crystallization nuclei can also be played by air bubbles and cracks in the container containing water. The speed of the process of turning water into ice is largely influenced by the number of such centers - if there are many of them, the liquid freezes faster. Under normal conditions, with normal atmospheric pressure, water turns into a solid state from liquid at a temperature of 0 degrees.

The essence of the Mpemba effect

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

He had to dissolve sugar in boiled milk and, having cooled it, place it in the refrigerator to freeze. Apparently, Mpemba was not particularly diligent and began completing the first part of the task late. Therefore, he did not wait for the milk to cool down, and put it in the refrigerator hot. He was very surprised when it froze even faster than that of his classmates, who were doing the work in accordance with the given technology.

This fact interested the young man very much, and he began experiments with plain water. In 1969, the journal Physics Education published the results of research by Mpemba and Professor Dennis Osborne of the University of Dar Es Salaam. The effect they described was given the name Mpemba. However, even today there is no clear explanation for the phenomenon. All scientists agree that the main role in this belongs to the differences in the properties of chilled and hot water, but what exactly is unknown.

Singapore version

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

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

But when water is heated, the bound molecules move slightly away from each other. As a result, relaxation of covalent bonds occurs in the middle of the molecules with the release of excess energy and a transition to a lower energy level. This leads to the fact that hot water begins to cool rapidly. At least, this is what theoretical calculations carried out by Singaporean scientists show.

Instantly freezing water - 5 incredible tricks: Video

The phenomenon of hot water freezing at a faster rate than cold water is known in science as the Mpemba effect. Great minds such as Aristotle, Francis Bacon and Rene Descartes pondered this paradoxical phenomenon, but for thousands of years no one has yet been able to offer a reasonable explanation for this phenomenon.

Only in 1963, a schoolboy from the Republic of Tanganyika, Erasto Mpemba, noticed this effect using the example of ice cream, but no adult gave him an explanation. Nevertheless, physicists and chemists have seriously thought about such a simple, but so incomprehensible phenomenon.

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

Now a team of researchers from 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 have 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 known fact is that hydrogen atoms are attracted to oxygen atoms from neighboring molecules - hydrogen bonds are formed.

At the same time, water molecules generally repel each other. Scientists from Singapore noticed: the warmer the water, the greater the distance between the molecules of the liquid due to an 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 is known, means cooling.

As chemists write in their article, which can be found on the preprint website arXiv.org, in hot water hydrogen bonds are stronger 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, hardening occurs faster.

To date, scientists have solved this mystery 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.