Why does hot water freeze faster than cold water? The Mpemba effect.

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

History reference

That in the issue of freezing cold and hot water "not everything is clean" was mentioned in the writings 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" - after a schoolchild 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 mixes in the kitchen. Of course, the classmates who were present there, together with the school teacher, made Mpemba laugh - however, after the experimental verification personally by Professor D. Osborne, their desire to make fun of Erasto "evaporated" from them. Moreover, Mpemboy, together with a professor in 1969 in Physics Education, published a detailed description of this effect - and since then the aforementioned name has stuck in the scientific literature.

What is the essence of the phenomenon?

The setting of the experiment is quite simple: all other things being equal, the same 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 until the formation of ice 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 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 hypothermia - initially cold water is more prone to hypothermia, 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 competition for the best hypothesis was held by the British Royal Chemical Society was recently, in 2012) - but there is still no unambiguous theory for all cases of combinations of input conditions ...

The British Royal Society of Chemistry is offering a £ 1,000 award to anyone who can scientifically explain why hot water freezes faster than cold water in some cases.

“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 like Aristotle and Descartes have pondered about it, ”said the president of the British Royal Society of Chemistry, Professor David Philips, quoted in a press release from the Society.

How a cook from Africa defeated a British physics professor

This is not an April Fool's joke, but a harsh physical reality. The current science, which easily operates with galaxies and black holes, builds giant accelerators to search for quarks and bosons, cannot explain how elementary water "works". A school textbook clearly states that a warmer body takes longer to cool than a colder body. But for water, this law is not always observed. Aristotle drew attention to this paradox in the 4th century BC. e. This is what the ancient Greek wrote in the book Meteorologica I: “The fact that the water is preheated makes it freeze. Therefore, many people, when they want to quickly cool hot water, first put it in the sun ... ”In the Middle Ages, Francis Bacon and René 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 this 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 the art school, in 1963, 13-year-old Mpembe was assigned to make 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 hot milk in the refrigerator. To his surprise, it froze even before 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 hurt. Five years later, already a student at the university in 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 equal amounts 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 the question of a young man from God-forsaken Tanzania. He made fun of the student. However, Mpemba was ready for such an answer and challenged the scientist to a bet. Their dispute ended with an experimental test that confirmed the correctness of Mpemba and the defeat of Osborne. So the pupil-cook inscribed his name in the history of science, and henceforth this phenomenon is called the "Mpemba effect". It is impossible to discard it, to declare it as if it does not exist. The phenomenon exists, and, as the poet wrote, "not to the teeth."

Are dust particles and solutes to blame?

Over the years, many have tried to unravel the mystery of freezing water. A whole bunch of explanations for this phenomenon have been proposed: evaporation, convection, the effect of solutes - but none of these factors can be considered final. A number of scientists have devoted their entire lives to the Mpemba effect. James Brownridge, a staff 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 to have 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 grains, bacteria and dissolved salts, have a characteristic nucleation temperature for them, when ice crystals are formed around the centers of crystallization. When there are several elements in water at once, the freezing point is determined by the one with the highest nucleation temperature.

For the experiment, Brownridge took two water samples of 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 the 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 from the British Royal Chemical Society.

In this article we will look at the question of why hot water freezes faster than cold water.

Hot water freezes much faster than cold water! This amazing property of water, for which scientists cannot find an exact explanation until now, 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 rather freeze in, poured warm water on the ice. The name of this phenomenon was given by the name of Erasto Mpemba in the 60s of the XX century. Mnemba noticed a strange effect when he was preparing ice cream, and turned to his physics teacher, Dr. Denis Osborne, for an explanation. Mpemba and Dr. Osborne experimented with water of different temperatures and concluded that almost boiling water begins to freeze much faster than water at room temperature. Other scientists ran their own experiments and got similar results each time.

Explanation of the physical phenomenon

There is no generally accepted explanation of why this is happening. Many researchers suggest that it is all about the hypothermia of the liquid, which occurs when its temperature drops below freezing point. In other words, if water freezes at temperatures below 0 ° C, then supercooled water can have a temperature of, for example, -2 ° C and at the same time remain liquid without turning into ice. When we try to freeze cold water, there is a possibility that it will first be supercooled, and will harden only after a while. Other processes take place in heated water. Its faster transformation into ice is associated with convection.

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

One of my favorite subjects in school was chemistry. Once a chemistry teacher gave us a very strange and difficult task. He gave us a list of questions that we had to answer in terms of chemistry. We were given several days for this assignment and allowed to use libraries and other available sources of information. One of these questions was about the freezing point of water. I don't remember exactly how the question sounded, but it was about the fact that if you take two wooden buckets of the same size, one with hot water, the other with cold (with exactly the specified temperature), and place them in an environment with a certain temperature, which of will freeze them faster? Of course, the answer immediately suggested itself - a bucket of cold water, but it seemed to us too simple. But this was not enough to give a complete answer, we needed to prove it from a chemical point of view. Despite all my thinking and research, I was unable to draw a logical conclusion. On this day, I even decided to skip this tutorial, so I never found out the solution to this puzzle.

Years passed, and I learned a lot of everyday myths about the boiling point and freezing point of water, and one myth said: "hot water freezes faster." I looked at many websites, but the information was too conflicting. And these were just opinions, unfounded from the point of view of science. And I decided to conduct my own experience. Since I couldn't find any wooden buckets, I used a freezer, stovetop, some water, and a digital thermometer. I will talk about the results of my experience a little later. First, I'll share with you some interesting arguments about water:

Hot water freezes faster than cold water. Most experts claim that cold water will freeze faster than hot water. But one funny phenomenon (the so-called Membe effect), for unknown reasons, proves the opposite: Hot water freezes faster than cold water. One of several explanations is the evaporation process: if very hot water is placed in a cold environment, the water will begin to evaporate (the remaining amount of water will freeze faster). And according to the laws of chemistry, this is not a myth at all, and most likely this is what the teacher wanted to hear from us.

Boiled water freezes faster than tap water. Despite the previous explanation, some experts argue that boiled water that has cooled down to room temperature should freeze faster because boiling reduces oxygen.

Cold water boils faster than hot water. If hot water freezes faster, then cold water may boil faster! This is contrary to common sense and scientists argue that this simply cannot be. Hot tap water should actually boil faster than cold water. But by using hot water for boiling, you are not saving energy. You may use less gas or light, but the water heater will use the same amount of energy that is needed to heat cold water. (This is a little different with solar power.) As a result of the heating of the water by the water heater, sediment may appear, so the water will take longer to heat up.

If you add salt to the water, it will boil faster. Salt increases the boiling point (and accordingly lowers the freezing point - that's why some housewives add a little rock salt to ice cream). But in this case, we are interested in another question: how long will the water boil and whether the boiling point in this case can rise above 100 ° C). Despite what they write in cookbooks, scientists argue that the amount of salt we add to boiling water is not enough to affect the boiling time or temperature.

But here's what I got:

Cold water: I used three 100 ml glass beakers of purified water: one at room temperature (72 ° F / 22 ° C), one with hot water (115 ° F / 46 ° C), and one with boiled water (212 ° F / 100 ° C). I placed all three glasses in the freezer at –18 ° C. And since I knew that water would not immediately turn into ice, I determined the degree of freezing by the "wooden float". When the stick, placed in the center of the glass, no longer touched the base, I assumed that the water had frozen. I checked the glasses every five minutes. And what are my results? The water in the first glass froze after 50 minutes. Hot water froze in 80 minutes. Boiled - after 95 minutes. My findings: Given the conditions in the freezer and the water I used, I was unable to reproduce the Memb effect.

I also tried this experiment with previously boiled water cooled to room temperature. She froze in 60 minutes - it still took longer than freezing cold water.

Boiled water: I took a liter of water at room temperature and put it on fire. It boiled in 6 minutes. Then I cooled it down to room temperature again and added it to the hot one. With the same heat, hot water boiled in 4 hours and 30 minutes. Conclusion: as expected, hot water boils much faster.

Boiled water (with salt): I added 2 large tablespoons of table salt to 1 liter of water. It boiled after 6 minutes 33 seconds, and as shown by the thermometer reached a temperature of 102 ° C. Undoubtedly, salt affects the boiling point, but not much. Conclusion: salt in water does not strongly affect the temperature and boiling time. I honestly admit that my kitchen can hardly be called a laboratory, and perhaps my conclusions are contrary to reality. My freezer compartment may freeze food unevenly. My glasses might have been irregular, etc. But whatever happens in the laboratory, when it comes to freezing or boiling water in the kitchen, the most important thing is common sense.

link with fun facts about vaudews about water
as suggested on the forum forum.ixbt.com this effect (the effect of freezing hot water faster than cold water) is called the "Aristotle-Mpemba effect"

Those. boiled water (chilled) freezes faster than "raw"

21.11.2017 11.10.2018 Alexander Firtsev

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

In fact, if you simultaneously put two vessels of the same shape and volume in the freezer, one of which will have cold water and the other hot, then the hot water will freeze faster.

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

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

History of observations and research

People have observed the paradoxical effect since ancient times, but no one attached much importance to it. This is how Arrestotel, as well as Rene Descartes and Francis Bacon, noted in their notes that there is no match in the freezing rate of cold and hot water. An unusual phenomenon often manifested itself 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 the unusual effect began in 1963 when an inquisitive student from Tanzania, Erasto Mpemba, noticed that hot ice cream freezes faster than cold milk. Hoping to get an explanation of 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 professor of physics Dennis Osborne who came to his school. The professor was interested in the observation of the young man, as a result, an experiment was carried out, which 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 observation, many hypotheses have been put forward about the causes of the phenomenon.

So in 2012, the British Royal Chemical Society would have announced a competition for hypotheses explaining the Mpemba effect. Scientists from all over the world took part in the competition, a total of 22,000 scientific papers were registered. Despite such an impressive number of articles, none of them clarified the Mpemba paradox.

The most common version was that hot water freezes faster, because it simply evaporates faster, its volume becomes smaller, and as the volume decreases, its cooling rate increases. The most widespread version was ultimately disproved as an experiment was carried out 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, gases dissolved in water evaporate during heating, due to which it acquires a higher density than cold water. As you know, an increase in density leads to a change in the physical properties of water (an increase in thermal conductivity), and, consequently, an increase in the cooling rate.

In addition, a number of hypotheses have been put forward describing the rate of water circulation as a function of 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 the lack of initial data, contradictions in other experiments, or due to the fact that the revealed factors were simply not comparable with the rate of water cooling. Some scientists in their works questioned the existence of the effect.

In 2013, researchers at Nanyang University of Technology in Singapore said they had 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 molecules of cold and hot water differs significantly.

Computer simulation methods have shown 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 are stretched, storing more energy. When cooled, the molecules begin to approach 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 their next study found that it is the removal of matter from equilibrium (strong heating before strong cooling) plays an important role in the formation of the effect. They identified the conditions under which the likelihood of the effect being greatest. In addition, scientists from Spain have confirmed the existence of the opposite Mpemba effect. They found that when heated, a colder sample can reach a high temperature faster than a warm one.

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

The Mpemba effect in real life

Have you ever wondered why in winter the ice rink is filled with hot water and not cold? As you already understood, they do this because an ice rink filled with hot water will freeze faster than if it were filled with cold water. For the same reason, slides in winter ice towns are flooded with hot water.

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.