The mpemba effect, or why hot water freezes faster than cold water. Why does hot water freeze faster than cold water?

It would seem that the good old formula H 2 O contains no secrets. But in fact, water - the source of life and the most famous liquid in the world - is fraught with many mysteries that even scientists are sometimes unable to solve.

Here are the 5 most interesting facts about water:

1. Hot water freezes faster than cold water

Let's take two containers with water: pour hot water into one, and cold water into the other, and place them in the freezer. Hot water will freeze faster than cold water, although logically, cold water should have turned into ice first: after all, hot water must first cool to the cold temperature, and then turn into ice, while cold water does not need to cool. Why is this happening?

In 1963, Erasto B. Mpemba, a high school student in Tanzania, was freezing an ice cream mixture and noticed that the hot mixture solidified faster in the freezer than the cold one. When the young man shared his discovery with his physics teacher, he only laughed at him. Fortunately, the student was persistent and convinced the teacher to conduct an experiment, which confirmed his discovery: under certain conditions, hot water actually freezes faster than cold water.

Now this phenomenon of hot water freezing faster than cold water is called the “Mpemba effect.” True, long before him this unique property of water was noted by Aristotle, Francis Bacon and Rene Descartes.

Scientists still do not fully understand the nature of this phenomenon, explaining it either by the difference in supercooling, evaporation, ice formation, convection, or by the effect of liquefied gases on hot and cold water.

Note from X.RU on the topic “Hot water freezes faster than cold water.”

Since the issues of cooling are closer to us, refrigeration specialists, we will allow ourselves to delve a little deeper into the essence of this problem and give two opinions about the nature of such a mysterious phenomenon.

1. A scientist from the University of Washington has proposed an explanation for a mysterious phenomenon known since the time of Aristotle: why hot water freezes faster than cold water.

The phenomenon, called the Mpemba effect, is widely used in practice. For example, experts advise motorists to pour cold, not hot, water into the washer reservoir in winter. But what underlies this phenomenon remained unknown for a long time.

Dr. Jonathan Katz from the University of Washington studied this phenomenon and came to the conclusion that substances dissolved in water, which precipitate when heated, play an important role, reports EurekAlert.

By solutes, Dr. Katz means calcium and magnesium bicarbonates, which are found in hard water. When water is heated, these substances precipitate, forming scale on the walls of the kettle. Water that has never been heated contains these impurities. As it freezes and ice crystals form, the concentration of impurities in the water increases 50 times. Because of this, the freezing point of water decreases. “And now the water has to cool further to freeze,” explains Dr. Katz.

There is a second reason that prevents unheated water from freezing. Lowering the freezing point of water reduces the temperature difference between the solid and liquid phases. “Because the rate at which water loses heat depends on this temperature difference, water that has not been heated cools down less well,” comments Dr. Katz.

According to the scientist, his theory can be tested experimentally, because The Mpemba effect becomes more noticeable for harder water.

2. Oxygen plus hydrogen plus cold creates ice. At first glance, this transparent substance seems very simple. In reality, ice is fraught with many mysteries. Ice, created by the African Erasto Mpemba, did not think about fame. The days were hot. He wanted popsicles. He took the juice box and put it in the freezer. He did this more than once and therefore noticed that the juice freezes especially quickly if you first hold it in the sun - it really heats it up! This is strange, thought the Tanzanian schoolboy, who acted contrary to worldly wisdom. Is it really true that in order for the liquid to turn into ice faster, it must first be... heated? The young man was so surprised that he shared his guess with the teacher. He reported this curiosity in the press.

This story happened back in the sixties of the last century. Now the "Mpemba effect" is well known to scientists. But for a long time this seemingly simple phenomenon remained a mystery. Why does hot water freeze faster than cold water?

It wasn't until 1996 that physicist David Auerbach found a solution. To answer this question, he conducted an experiment for a whole year: he heated water in a glass and cooled it again. So what did he find out? When heated, air bubbles dissolved in water evaporate. Water devoid of gases freezes more easily onto the walls of the vessel. “Of course, water with a high air content will also freeze,” says Auerbach, “but not at zero degrees Celsius, but only at minus four to six degrees.” Of course, you will have to wait longer. So, hot water freezes before cold water, this is a scientific fact.

There is hardly a substance that appears before our eyes with the same ease as ice. It consists only of water molecules - that is, elementary molecules containing two hydrogen atoms and one oxygen atom. However, ice is perhaps the most mysterious substance in the Universe. Scientists have not yet been able to explain some of its properties.

2. Supercooling and "instant" freezing

Everyone knows that water always turns into ice when cooled to 0°C... except in some cases! An example of this is “supercooling,” which is the property of very pure water to remain liquid even when cooled to below freezing. This phenomenon is made possible due to the fact that the environment does not contain centers or nuclei of crystallization that could trigger the formation of ice crystals. And so water remains in liquid form even when cooled to below zero degrees Celsius. The crystallization process can be triggered, for example, by gas bubbles, impurities (contaminants), or an uneven surface of the container. Without them, water will remain in a liquid state. When the crystallization process starts, you can watch the super-cooled water instantly turn into ice.

Watch the video (2,901 KB, 60 sec) from Phil Medina (www.mrsciguy.com) and see for yourself >>

Comment. Superheated water also remains liquid even when heated above its boiling point.

3. "Glass" water

Quickly and without thinking, name how many different states does water have?

If you answered three (solid, liquid, gas), then you were wrong. Scientists identify at least 5 different states of liquid water and 14 states of ice.

Remember the conversation about super-chilled water? So, no matter what you do, at -38 °C even the purest super-chilled water suddenly turns into ice. What happens with further decline?

temperature? At -120 °C something strange begins to happen to water: it becomes super viscous or viscous, like molasses, and at temperatures below -135 °C it turns into “glassy” or “vitreous” water - a solid substance that lacks crystalline structure.

4. Quantum properties of water

At the molecular level, water is even more surprising. In 1995, a neutron scattering experiment conducted by scientists yielded an unexpected result: physicists discovered that neutrons aimed at water molecules “see” 25% fewer hydrogen protons than expected.

It turned out that at a speed of one attosecond (10 -18 seconds) an unusual quantum effect takes place, and the chemical formula of water, instead of the usual one - H 2 O, becomes H 1.5 O!

5. Does water have memory?

Homeopathy, an alternative to conventional medicine, states that a diluted solution of a drug can have a healing effect on the body, even if the dilution factor is so great that there is nothing left in the solution except water molecules. Proponents of homeopathy explain this paradox with a concept called “water memory,” according to which water at the molecular level has a “memory” of the substance once dissolved in it and retains the properties of the solution of the original concentration after not a single molecule of the ingredient remains in it.

An international group of scientists led by Professor Madeleine Ennis from Queen's University of Belfast, who criticized the principles of homeopathy, conducted an experiment in 2002 to refute this concept once and for all. The result was the opposite. After What, scientists said that they were able to prove the reality of the “water memory” effect. However, experiments conducted under the supervision of independent experts did not bring results. Disputes about the existence of the “water memory” phenomenon continue.

Water has many other unusual properties that we did not talk about in this article.

Literature.

1. 5 Really Weird Things About Water / http://www.neatorama.com.
2. The mystery of water: the theory of the Aristotle-Mpemba effect was created / http://www.o8ode.ru.
3. Nepomnyashchy N.N. Secrets of inanimate nature. The most mysterious substance in the universe / http://www.bibliotekar.ru.

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.

Major versions

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

Hello, dear lovers of interesting facts. Today we will talk to you about. But I think that the question posed in the title may seem simply absurd - but should one always undividedly trust the notorious “common sense” and not a strictly established test experiment. 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, this effect has been given the name “Mpemba’s Paradox” - named after a schoolboy from Tanganyika, Erasto Mpemba, who asked the same question to a visiting physics professor.

The boy’s question did not arise out of nowhere, 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, made Mpemba laugh - however, after an experimental test personally by Professor D. Osborne, the desire to make fun of Erasto “evaporated” from them. Moreover, Mpemba, together with a professor, published a detailed description of this effect in Physics Education in 1969 - and since then the above-mentioned name has been fixed in the scientific literature.

What is the essence of the phenomenon?

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

How does modern physics explain this?

The paradox does not have a universal explanation, since several parallel processes occur together, the contribution of which may vary depending on the specific initial conditions - but with a uniform result:

• the ability of a liquid to supercool - initially cold water is more prone to supercooling, i.e. remains liquid when its temperature is already below 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”
• insulation 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 British Royal Society of Chemistry held a competition for the best hypothesis was recently, in 2012) - but there is still no unambiguous theory for all cases of combinations of input conditions...

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.

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 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 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 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 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 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

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.

21.11.2017 11.10.2018 Alexander Firtsev

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

In fact, if you simultaneously place two vessels of the same shape and volume in the freezer, one of which contains cold water and the other hot, then it is the hot water that will freeze faster.

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

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

History of observations and research

People have been observing this paradoxical effect since ancient times, but no one attached much importance to it. Thus, Arestotle, as well as Rene Descartes and Francis Bacon, noted in their notes the inconsistencies in the rate of freezing of cold and hot water. An unusual phenomenon often appeared 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 this unusual effect began in 1963, when an inquisitive schoolboy from Tanzania, Erasto Mpemba, noticed that hot milk for ice cream froze faster than cold milk. Hoping to get an explanation for 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 physics professor Dennis Osborn, who came to his school. The professor was interested in the young man’s observation, and as a result, an experiment was conducted that 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 observations, many hypotheses have been put forward about the causes of the phenomenon.

So in 2012, the British Royal Society of Chemistry would announce a competition of hypotheses explaining the Mpemba effect. Scientists from all over the world participated in the competition; a total of 22,000 scientific papers were registered. Despite such an impressive number of articles, none of them brought clarity to the Mpemba paradox.

The most common version was according to which hot water freezes faster, since it simply evaporates faster, its volume becomes smaller, and as the volume decreases, its cooling rate increases. The most common version was eventually refuted because an experiment was conducted in which evaporation was excluded, but the effect was nevertheless confirmed.

Other scientists believed that the cause of the Mpemba effect was the evaporation of gases dissolved in water. In their opinion, during the heating process, gases dissolved in water evaporate, due to which it acquires a higher density than cold water. As is known, an increase in density leads to a change in the physical properties of water (an 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 depending on temperature. Many studies have attempted to establish the relationship between the material of the containers in which the liquid was located. Many theories seemed very plausible, but they could not be scientifically confirmed due to a lack of initial data, contradictions in other experiments, or because the identified factors were simply not comparable with the rate of cooling of water. Some scientists in their works questioned the existence of the effect.

In 2013, researchers at Nanyang Technological University in Singapore claimed to have 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 cold and hot water molecules is significantly different.

Computer modeling methods showed the following results: the higher the water temperature, the greater the distance between the molecules due to the fact that the repulsive forces increase. Consequently, the hydrogen bonds of molecules stretch, storing more energy. When cooled, the molecules begin to move closer to 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 a major role in the formation of the effect is played by the removal of a substance from equilibrium (strong heating before strong cooling). They determined the conditions under which the likelihood of the effect occurring is maximum. In addition, scientists from Spain confirmed the existence of the reverse Mpemba effect. They found that when heated, a colder sample can reach a high temperature faster than a warmer one.

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

Mpemba effect in real life

Have you ever wondered why in winter the skating rink is filled with hot water and not cold? As you already understand, they do this because a skating rink filled with hot water will freeze faster than if it was filled with cold water. For the same reason, hot water is poured into the slides in winter ice towns.

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

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