Hot and cold water: the secrets of freezing. Why does hot water freeze faster than cold water?

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 make a mistake.

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

Such a statement may seem absurd and unreasonable. Logically, hot water must first cool down to cold temperature, and cold water should already turn into ice at this time.

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. So inconsistencies in the rate of freezing of cold and hot water were noted in their notes by Arestotel, as well as by Rene Descartes and Francis Bacon. 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 milk for 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.

Six years later, in 1969, Mpemba asked this question to physics professor Dennis Osborne, who came to his school. The professor was interested in the observation of the young man, 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 to explain the Mpemba effect. Scientists from all over the world participated in the competition, in total 22,000 scientific papers were registered. Despite such an impressive number of articles, none of them clarified the Mpemba paradox.

The most common was the version 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, since an experiment was conducted in which evaporation was excluded, but the effect was nevertheless confirmed.

Other scientists believed that the reason for the Mpemba effect is 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 hence an increase in the cooling rate.

In addition, a number of hypotheses have been put forward that describe the rate of water circulation as a function of temperature. In many studies, an attempt was made 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 due to the fact that the identified 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 the Nanyang Technological University in Singapore claimed to have solved the mystery of the Mpemba effect. According to their study, the reason for the phenomenon lies in the fact that the amount of energy stored in hydrogen bonds between cold and hot water molecules differs significantly.

Computer simulation methods have shown the following results: the higher the temperature of the water, 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 another study, found out that it is the removal of matter from equilibrium (strong heating before strong cooling) that plays a large role in the formation of the effect. They determined the conditions under which the likelihood of the effect is maximum. In addition, scientists from Spain have confirmed the existence of the reverse 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.

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 a skating 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 poured with hot water.

Thus, knowledge about 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.

Which water freezes faster, hot or cold, is influenced by many factors, but the question itself seems a little strange. It is understood, and it is known from physics, that hot water still needs time to cool down to the temperature of comparable cold water in order to turn into ice. Cold water can skip this stage, and, accordingly, it wins in time.

But the answer to the question of which water freezes faster - cold or hot - on the street in frost, any inhabitant of the northern latitudes knows. In fact, scientifically, it turns out that in any case, cold water simply has to freeze faster.

So did the teacher of physics, who was approached by the schoolboy Erasto Mpemba in 1963 with a request to explain why the cold mixture of future ice cream freezes longer than a similar, but hot one.

"This is not world physics, but some kind of Mpemba physics"

At that time, the teacher only laughed at this, but Deniss Osborne, a professor of physics, who at one time went to the same school where Erasto studied, experimentally confirmed the existence of such an effect, although there was no explanation for this then. In 1969 a popular scientific journal published a joint article by the two men who described this peculiar effect.

Since then, by the way, the question of which water freezes faster - hot or cold, has its own name - the effect, or paradox, Mpemba.

The question has been around for a long time

Naturally, such a phenomenon has taken place before, and it was mentioned in the works of other scientists. Not only the schoolboy was interested in this question, but Rene Descartes and even Aristotle thought about it at one time.

Here are just approaches to solving this paradox began to look only at the end of the twentieth century.

Conditions for a paradox to occur

As with ice cream, it's not just ordinary water that freezes during the experiment. Certain conditions must be present in order to start arguing which water freezes faster - cold or hot. What influences this process?

Now, in the 21st century, several options have been put forward that can explain this paradox. Which water freezes faster, hot or cold, may depend on the fact that it has a higher evaporation rate than cold water. Thus, its volume decreases, and with a decrease in volume, the freezing time becomes shorter than if we take a similar initial volume of cold water.

Freezer has long been defrosted

Which water freezes faster, and why it does so, can be affected by the snow lining that may be present in the freezer of the refrigerator used for the experiment. If you take two containers that are identical in volume, but one of them will have hot water and the other cold water, the container with hot water will melt the snow under it, thereby improving the contact of the thermal level with the refrigerator wall. A cold water container can't do that. If there is no such lining with snow in the refrigerator, cold water should freeze faster.

Top - bottom

Also, the phenomenon of which water freezes faster - hot or cold, is explained as follows. Following certain laws, cold water starts to freeze from the upper layers, when hot water does it the other way around - it starts to freeze from the bottom up. It turns out that cold water, having a cold layer on top with ice already formed in some places, thus worsens the processes of convection and thermal radiation, thereby explaining which water freezes faster - cold or hot. A photo from amateur experiments is attached, and here it is clearly visible.

The heat goes out, tending upwards, and there it meets a very cool layer. There is no free path for heat radiation, so the cooling process becomes difficult. Hot water has absolutely no such barriers in its path. Which freezes faster - cold or hot, on which the probable outcome depends, you can expand the answer by saying that any water has certain substances dissolved in it.

Impurities in the composition of water as a factor influencing the outcome

If you do not cheat and use water with the same composition, where the concentrations of certain substances are identical, then cold water should freeze faster. But if a situation occurs when dissolved chemical elements are present only in hot water, while cold water does not possess them, then hot water has the opportunity to freeze earlier. This is explained by the fact that the dissolved substances in water create centers of crystallization, and with a small number of these centers, the transformation of water into a solid state is difficult. Even supercooling of water is possible, in the sense that at sub-zero temperatures it will be in a liquid state.

But all these versions, apparently, did not suit the scientists to the end, and they continued to work on this issue. In 2013, a team of researchers in Singapore said they had solved the age-old mystery.

A group of Chinese scientists claim that the secret of this effect lies in the amount of energy that is stored between water molecules in its bonds, called hydrogen bonds.

The answer from Chinese scientists

Further information will follow, for the understanding of which it is necessary to have some knowledge in chemistry in order to figure out which water freezes faster - hot or cold. As you know, it consists of two H (hydrogen) atoms and one O (oxygen) atom held together by covalent bonds.

But hydrogen atoms of one molecule are also attracted to neighboring molecules, to their oxygen component. These bonds are called hydrogen bonds.

At the same time, it is worth remembering that at the same time, water molecules act repulsively on each other. Scientists noted that when water is heated, the distance between its molecules increases, and this is facilitated by repulsive forces. It turns out that occupying one distance between molecules in a cold state, one can say that they stretch, and they have a greater supply of energy. It is this energy reserve that is released when water molecules begin to approach each other, that is, cooling occurs. It turns out that a larger supply of energy in hot water, and its greater release when cooled to sub-zero temperatures, occurs faster than in cold water, which has a smaller supply of such energy. So which water freezes faster - cold or hot? On the street and in the laboratory, the Mpemba paradox should occur, and hot water should turn into ice faster.

But the question is still open

There is only theoretical confirmation of this clue - all this is written in beautiful formulas and seems plausible. But when the experimental data, which water freezes faster - hot or cold, will be put in a practical sense, and their results will be presented, then it will be possible to consider the question of the Mpemba paradox closed.

Water- a fairly simple substance from a chemical point of view, however, it has a number of unusual properties that never cease to amaze scientists. Below are some facts that few people know about.

1. Which water freezes faster - cold or hot?

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, 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.

Now this phenomenon of hot water freezing faster than cold water is called " 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.

2. She is able to freeze instantly

Everyone knows that water always turns to ice when cooled 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 to a temperature 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.

crystallization process can be provoked, 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.

Note that "superheated" water also remains liquid even when heated above its boiling point.

3. 19 states of water

Without hesitation, name how many different states water has? If you answered three: solid, liquid, gaseous, then you are mistaken. Scientists distinguish at least 5 different states of water in liquid form and 14 states in frozen form.

Remember the conversation about super-chilled water? So, no matter what you do, at -38 ° C, even the purest super-cooled water will suddenly turn into ice. What happens as the temperature drops further? 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 that lacks crystalline structure.

4. Water surprises physicists

At the molecular level, water is even more surprising. In 1995, scientists conducted an experiment on neutron scattering gave an unexpected result: physicists found that neutrons directed at water molecules "see" 25% less 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 H2O, becomes H1.5O!

5. Water memory

Alternative to official medicine homeopathy argues that a dilute solution of a drug can have a therapeutic effect on the body, even if the dilution factor is so large that there is nothing left in the solution but 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 a substance that was once dissolved in it and retains the properties of a solution of the initial concentration after not a single ingredient molecule remains in it.

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

Water has many other unusual properties that we have not covered in this article. For example, the density of water varies with temperature (the density of ice is less than that of water); water has a fairly large surface tension; in a liquid state, water is a complex and dynamically changing network of water clusters, and it is the behavior of clusters that affects the structure of water, etc.

About these and many other unexpected features water can be read in the article Anomalous properties of water”, the author of which is Martin Chaplin, professor at the University of London.

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, 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.

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.

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 to a temperature 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.

Note that "superheated" water also remains liquid even when heated above its boiling point.

3. "Glass" water

Without hesitation, name how many different states water has?

If you answered three: solid, liquid, gaseous, then you are mistaken. Scientists distinguish at least 5 different states of water in liquid form and 14 states in frozen form.

Remember the conversation about super-chilled water? So, no matter what you do, at -38 ° C, even the purest super-cooled water will suddenly turn into ice.

What happens as the temperature drops further?

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 that lacks crystalline structure.

4. Quantum properties of water

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 H2O becomes H1.5O!

5. Does water have a memory?

Homeopathy, an alternative to official medicine, claims that a dilute solution of a drug can have a healing effect on the body, even if the dilution factor is so large that nothing is 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.

The result was the opposite. After that, the scientists said that they were able to prove the reality of the "memory of water" effect. 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.

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

In 1963, Erasto B. Mpemba, a senior high school student in Tanzania, 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.

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, let us 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 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 cools down worse," comments 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 amazing. In 1995, scientists conducted an experiment on neutron scattering gave an unexpected result: physicists found that neutrons aimed at water molecules "see" 25% less 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.

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