Hot water freezes faster than cold water. Hot and cold water: secrets of freezing

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 it freezes faster cold water- and they will make a mistake.

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

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 observed the paradoxical effect since ancient times, but no one gave it special significance. 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. 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 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 were registered scientific works. Despite such an impressive number of articles, none of them brought clarity to 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 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 physical properties water (increased 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 There is a significant difference between cold and hot water molecules.

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. And therefore the hydrogen bonds of molecules stretch, storing large quantity 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 winter time the skating rink is being flooded 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 species sports

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

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

On this moment There are several versions that explain this fact:

Because hot water evaporates faster, its volume decreases. And freezing of a smaller amount of water at the same temperature occurs faster.
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.
Freezing of cold water, unlike hot water, begins at the top. At the same time, convection and heat radiation, and, consequently, heat loss worsen.
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. This, for example, can be very fine particles 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 took longer to freeze. a short time than cold. 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 occurs in the middle of the molecules covalent bonds 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

Mpemba effect(Mpemba's Paradox) is a paradox that states that hot water under some conditions freezes faster than cold water, although it must pass the temperature of cold water during the freezing process. This paradox is an experimental fact that contradicts the usual ideas, according to which, under the same conditions, a more heated body takes more time to cool to a certain temperature than a less heated body to cool to the same temperature.

This phenomenon was noticed at one time by Aristotle, Francis Bacon and Rene Descartes, but it was only in 1963 that Tanzanian schoolboy Erasto Mpemba discovered that a hot ice cream mixture freezes faster than a cold one.

Being a student of Magambinskaya high school in Tanzania Erasto Mpemba did practical work in cooking. He needed to make homemade ice cream - boil milk, dissolve sugar in it, cool it until room temperature and then place 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 plain water. In any case, already as a student at Mkwawa 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 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 question 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 the body cools down to temperature environment, must 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 process cooling will occur more slowly.

In the case of hot water, the situation is completely different. The surface layer of water will cool more quickly due to evaporation and bigger difference temperatures In addition, cold water layers are denser than hot water layers, so the cold water layer will sink down, raising the layer warm water 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 average temperature water will drop 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 was noticed that a container with hot water melts ice underneath it freezer, 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.

There are many factors that influence which water freezes faster, hot or cold, but the question itself seems a little strange. The implication, and this is known from physics, is that hot water still needs time to cool to the temperature of the cold water being compared in order to turn into ice. Cold water can skip this stage, and, accordingly, it gains time.

But the answer to the question of which water freezes faster - cold or hot - outside in the cold, any resident knows northern latitudes. In fact, scientifically, it turns out that in any case, cold water is simply bound to freeze faster.

The physics teacher, to whom schoolboy Erasto Mpemba approached in 1963, thought the same thing with a request to explain why the cold mixture of future ice cream takes longer to freeze than a similar, but hot one.

“This is not universal 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 visited the same school where Erasto studied, experimentally confirmed the presence of such an effect, although there was no explanation for it then. In 1969, a joint article by these two people was published in a popular scientific journal, 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 Mpemba effect, or paradox.

The question has been around for a long time

Naturally, such a phenomenon took place before, and it was mentioned in the works of other scientists. Not only the schoolchild was interested in this issue, but also Rene Descartes and even Aristotle thought about it at one time or another.

But they began to look for approaches to solving this paradox only at the end of the twentieth century.

Conditions for a paradox to occur

As with ice cream, it's not just plain 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 the course of 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 as the volume decreases, the freezing time becomes shorter than if we take the same initial volume of cold water.

It's been a while since you defrosted the freezer.

Which water freezes faster and why this happens can be influenced 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 contains hot water and the other cold, the container with hot water will melt the snow underneath, thereby improving the contact of the thermal level with the wall of the refrigerator. A container of cold water cannot do this. If there is no such lining with snow in the refrigerator compartment, 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 begins to freeze at upper layers, when hot it does the opposite - it begins to freeze from the bottom up. It turns out that cold water, having a cold layer on top with ice already formed in places, thus worsens the processes of convection and thermal radiation, thereby explaining which water freezes faster - cold or hot. Photos from amateur experiments are attached, and this is clearly visible here.

The heat goes out, rushing upward, 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 obstacles in its path. Which freezes faster - cold or hot, what determines the likely outcome, you can expand the answer by saying that any water has certain substances dissolved in it.

Impurities in water as a factor influencing the outcome

If you don't 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 where dissolved chemical elements are present only in hot water, and cold water does not have them, then the hot water has the opportunity to freeze earlier. This is explained by the fact that dissolved substances in water create crystallization centers, and with a small number of these centers, the transformation of water into solid state difficult. It is even possible that the water will be supercooled, in the sense that at sub-zero temperatures it will be in a liquid state.

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

What follows is information, to understand which you need to have some knowledge of chemistry in order to understand which water freezes faster - hot or cold. As is known, it consists of two H (hydrogen) atoms and one O (oxygen) atom, held together by covalent bonds.

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

It is worth remembering that at the same time, water molecules have a repulsive effect 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 by occupying the same distance between the molecules in a cold state, they can be said to stretch, and they have a greater supply of energy. It is this energy reserve that is released when water molecules begin to move closer to each other, that is, cooling occurs. It turns out that a greater reserve of energy in hot water, and its greater release when cooling to sub-zero temperatures, occurs faster than in cold water, which has a smaller reserve of such energy. So which water freezes faster - cold or hot? On the street and in the laboratory, Mpemba's paradox should occur, and hot water should turn into ice faster.

But the question is still open

There is only theoretical confirmation of this solution - it’s all written beautiful formulas and it seems plausible. But when the experimental data on which water freezes faster - hot or cold - are put into practical use, and their results are presented, then the question of Mpemba’s paradox can be considered closed.

The British Royal Society of Chemistry is offering a £1,000 reward 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, with philosophers such as Aristotle and Descartes thinking about it,” said Professor David Phillips, president of the British Royal Society of Chemistry, as quoted in a Society press release.

How a cook from Africa defeated a British physics professor

This is not an April Fool's joke, but a harsh physical reality. Modern science, which easily operates with galaxies and black holes, and builds giant accelerators to search for quarks and bosons, cannot explain how elementary water “works.” The school textbook clearly states that it takes more time to cool a hotter 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 his book Meteorologica I: “The fact that water is preheated causes it to freeze. Therefore, many people, when they want to cool hot water faster, 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 thermophysics 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 Mpembe from Tanzania, far from any science. While studying at culinary arts 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 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 all the rules.

When Mpemba shared his discovery with his physics teacher, he laughed at him in front of the whole class. Mpemba remembered the insult. Five years later, already a student at the university in Dar es Salaam, he attended 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 place 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 bet. Their dispute ended with an experimental test that confirmed Mpemba was right and Osborne defeated. Thus, the apprentice cook wrote his name in the history of science, and from now on this phenomenon is called the “Mpemba effect.” It is impossible to discard it, to declare it as “non-existent”. The phenomenon exists, and, as the poet wrote, “it doesn’t hurt.”

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 influence of dissolved substances - but none of these factors can be considered definitive. A number of scientists have devoted their entire lives to the Mpemba effect. Employee of the Department of Radiation Safety State University New York - James Brownridge - in free time has been studying the paradox for over a decade now. After conducting hundreds of experiments, the scientist claims to have evidence of the “guilt” of hypothermia. Brownridge explains that at 0°C, water only becomes supercooled, 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, such as dust particles, bacteria and dissolved salts, have a characteristic nucleation temperature when ice crystals form around crystallization centers. When there are several elements in water at once, the freezing point is determined by the one that has the most high temperature nucleation.

For the experiment, Brownridge took two water samples of the same temperature and placed them in the freezer. He discovered that one of the specimens always froze before the other, presumably due to a different combination of impurities.

Brownridge says hot water cools faster because there is a greater difference between the temperature of 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.