Mpemba effect or why does hot water freeze faster than cold water? Revealed the secret of the rapid solidification of hot water

"We have already encountered some interesting properties of water that allow us to live in particular, and living beings in general. Let's continue the topic and bring to your attention one more interesting property (though it is not clear whether it is true or fictional).

Interesting about water - the Mpemba effect: did you know that there are rumors on the Internet that hot water freezes faster than cold water? You may not know, but these rumors are circulating. And very tenacious. So what are we talking about - an experimental error or a new, interesting property of water that has not yet been studied?

Let's figure it out. The legend, repeated from site to site, is this: let's take two containers of 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. Why is this happening?

In 1963, a Tanzanian student named Erasto B. Mpemba, while freezing a prepared ice cream mixture, noticed that the hot mixture solidified faster in the freezer than the cold one. When the young man shared his discovery with a 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 really freezes faster than cold water.

The second version of the legend - Mpemba turned to the great scientist, who, fortunately, was near the African school of Mpemba. And the scientist believed the boy and double-checked what was what. Well, off we go ... 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 do not fully understand the nature of this phenomenon, explaining it either by the difference in hypothermia, evaporation, ice formation, convection, or the effect of liquefied gases on hot and cold water.

So, we have the Mpemba effect (Mpemba Paradox) - a paradox that says that hot water (under certain conditions) can freeze faster than cold water. Although at the same time it must pass the temperature of cold water in the process of freezing.

Accordingly, in order to deal with the paradox, there are two ways. The first is to start explaining this phenomenon, coming up with theories and rejoicing that water is a mysterious liquid. Or you can go the other way - independently conduct this experiment. And draw the appropriate conclusions.

Let's turn to people who have actually had this experience trying to replicate the Mpemba effect. And at the same time, let's look at a small study that determines "where the legs grow from."

In Russian, a message about the Mpemba effect first appeared 42 years ago, as reported by the journal "Chemistry and Life" (1970, No. 1, p. 89). Being conscientious, the employees of "Chemistry and Life" decided to conduct experiments themselves and made sure: "hot milk stubbornly did not want to freeze first." This result was given a natural explanation: “A hot liquid should not freeze earlier. After all, its temperature must first be equal to the temperature of the cold liquid.

One of the readers of "Chemistry and Life" reported the following about his experiments (1970, No. 9, p. 81). He brought the milk to a boil, cooled it to room temperature, and put it in the refrigerator at the same time as the unboiled milk, which was also at room temperature. Boiled milk froze faster. The same effect, but weaker, was achieved by heating milk to 60°C rather than boiling. Boiling could be of fundamental importance: this will evaporate part of the water and evaporate the lighter part of the fat. As a result, the freezing point may change. In addition, when heated, and even more so when boiled, some chemical transformations of the organic part of milk are possible.

But the “broken phone” was already working, and after more than 25 years, this story was described as follows: “A serving of ice cream becomes cold faster if it is put in the refrigerator, after warming it up well, than if it is first left at a cold temperature” (“Knowledge is power “, 1997, No. 10, p. 100). They gradually began to forget about milk, and it was mainly about water.

After 13 years, in the same "Chemistry and Life" the following dialogue appeared: "If you take two cups out into the cold - with cold and hot water, then which water will freeze faster? .. Wait for winter and check: hot water will freeze faster" ( 1993, No. 9, p. 79). A year later, a letter from one conscientious reader followed, who diligently took cups of cold and hot water out into the cold in winter and became convinced that cold water freezes faster (1994, No. 11, p. 62).

A similar experiment was carried out using a refrigerator, in which the freezer is covered with a thick layer of frost. When I put cups of hot and cold water on this freezer, the frost under the cups of hot water thawed, they sank and the water in them froze faster. When I put glasses on the frost, the effect was not observed, since the frost under the glasses did not melt. The effect was not observed even when, after defrosting the refrigerator, I put the cups on a frost-free freezer. This proves that the cause of the effect is the thawing of frost under cups of hot water (Chemistry and Life, 2000, no. 2, p. 55).

The story about the paradox noticed by the Tanzanian boy was repeatedly accompanied by a significant remark - they say, one should not neglect any, even very strange, information. The wish is good, but unrealizable. If we do not pre-screen unreliable information, then we will drown in it. And the wrong information is often wrong. In addition, it often happens (as in the case of the Mpemba effect) that the improbability is a consequence of the distortion of information in the transmission process.

Thus, it is interesting about water in general, and the Mpemba effect in particular, is not always true 🙂

More details - on the page http://wsyachina.narod.ru/physics/mpemba.html

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 the President of the British Royal Society of Chemistry, Professor David Philips, 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 on galaxies and black holes, building giant accelerators to search for quarks and bosons, cannot explain how elemental 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. Osborn. 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.

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 Osborne, 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.

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 in many ways possible. In addition, in the near-scientific, and indeed the scientific world, there are discussions about which water freezes faster - hot or cold. Whoever proves faster freezing of a hot liquid under certain conditions and scientifically substantiates his decision will receive an award of £1,000 from the British Royal Society of Chemists.

Background

The fact that, under a number of conditions, hot water is ahead of cold water in terms of freezing rate, was noticed back in the Middle Ages. Francis Bacon and René Descartes have put a lot of effort into explaining this phenomenon. However, from the point of view of classical heat engineering, this paradox cannot be explained, and they tried to bashfully hush it up. The impetus for the continuation of the dispute was a somewhat curious story that happened to the Tanzanian schoolboy Erasto Mpemba (Erasto Mpemba) in 1963. Once, during a dessert-making lesson at a cooking school, a boy, distracted by other things, did not have time to cool the ice cream mixture in time and put a solution of sugar in hot milk into the freezer. To his surprise, the product cooled somewhat faster than his fellow practitioners who observed the temperature regime for making 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 perseverance and continued his experiments no longer on milk, but on water. He made sure that in some cases hot water freezes faster than cold water.

Entering the University of Dar es Salaam, Erasto Mpembe attended a lecture by Professor Dennis G. Osborne. After graduation, the student puzzled the scientist with the problem of the rate of freezing of water depending on its temperature. D.G. Osborne ridiculed the very posing of the question, stating with aplomb that any loser knows that cold water will freeze faster. However, the natural tenacity of the young man made itself felt. He made a bet with the professor, offering to conduct an experimental test 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). What was 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, affect 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 are completely subject to 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 water is cooled from + 100 ° C to 0 ° C, volume losses at atmospheric pressure reach 15%;
the intensity of heat exchange between the liquid and the environment is the higher, the greater the temperature difference, so the heat loss of boiling water passes faster;
when hot water cools, an ice crust forms on its surface, which prevents the liquid from completely freezing and evaporating;
at a high temperature of water, its convection mixing occurs, reducing the freezing time;
gases dissolved in water lower the freezing point, taking energy for crystal formation - there are no dissolved gases in hot water.

All these conditions have been subjected to repeated experimental verification. 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 to freeze the former more quickly. However, later his experiments were criticized and many scientists are convinced that the “Mpemba Effect” about which water freezes faster - hot or cold, can only be reproduced under certain conditions, which no one has been looking for and concretizing so far.

In the good old formula H 2 O, it would seem that there are no secrets. But in fact, water - the source of life and the most famous liquid in the world - is fraught with many mysteries that sometimes even scientists cannot solve.

Here are the 5 most interesting facts about water:

1. Hot water freezes faster than cold water

Take two containers of 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 down to cold temperature, and then turn into ice, while cold water does not need to cool down. Why is this happening?

In 1963, Erasto B. Mpemba, a senior high school student in Tanzania, while freezing a prepared ice cream mix, noticed that the hot mix solidified faster in the freezer than the cold one. When the young man shared his discovery with a 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 really 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.

Note from Х.RU to the topic "Hot water freezes faster than cold water".

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

1. A University of Washington scientist has offered 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 rather than hot water into the washer reservoir in winter. But what underlies this phenomenon remained unknown for a long time.

Dr. Jonathan Katz of the University of Washington investigated this phenomenon and concluded that substances dissolved in water play an important role in it, which precipitate when heated, reports EurekAlert.

By solutes, Dr. Katz means the calcium and magnesium bicarbonates found in hard water. When the 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 water increases 50 times. This lowers the freezing point of water. "And now the water has to cool down in order to freeze," explains Dr. Katz.

There is a second reason that prevents freezing of unheated water. 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 is less likely to cool down," says Dr. Katz.

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

2. Oxygen plus hydrogen plus cold creates ice. At first glance, this transparent substance seems very simple. In fact, the ice is fraught with many mysteries. The ice created by the African Erasto Mpemba did not think about glory. The days were hot. He wanted popsicles. He took a carton of juice and put it in the freezer. He did this more than once and therefore noticed that the juice freezes especially quickly, if you hold it in the sun before that - just heat it up! This is strange, thought the Tanzanian schoolboy, who acted contrary to worldly wisdom. Is it possible 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 1960s. 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 on 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 would appear 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. However, ice is perhaps the most mysterious substance in the universe. Scientists have not been able to explain some of its properties so far.

2. Supercooling and "flash" freezing

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

Watch the video (2 901 Kb, 60 c) by 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 hesitation, name how many different states water has?

If you answered three (solid, liquid, gas), then you are wrong. Scientists distinguish at least 5 different states of water in liquid form 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-cooled water suddenly turns into ice. What happens with a further decrease

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 "glassy" water - a solid substance in which there is no 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 gave an unexpected result: physicists found that neutrons directed at water molecules “see” 25% fewer hydrogen protons than expected.

It turned out that at the 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 a memory?

Homeopathy, an alternative to conventional medicine, claims that a dilute solution of a medicinal product can have a healing effect on the body, even if the dilution factor is so great that there is nothing left in the solution but water molecules. Proponents of homeopathy explain this paradox by a concept called "memory of water", 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 team 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, the scientists said that they were able to prove the reality of the effect of "memory of water. However, experiments conducted under the supervision of independent experts, did not bring results. Disputes about the existence of the phenomenon of "memory of water" continue.

Water has many other unusual properties that we have not covered 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. Nepomniachtchi N.N. Secrets of inanimate nature. The most mysterious substance in the universe / http://www.bibliotekar.ru.