Why does boiling water freeze faster in cold weather? Why does hot water freeze faster than cold water? Mpemba effect. The question has been around for a long time

Mpemba effect(Mpemba paradox) - a paradox that says that hot water under certain conditions, it freezes faster than cold water, although it must pass the temperature of cold water in the process of freezing. This paradox is an experimental fact that contradicts the usual ideas, according to which, under the same conditions, a hotter body needs more time to cool down to a certain temperature than a cooler body to cool down to the same temperature.

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

As a student of the Magamba high school in Tanzania, Erasto Mpemba did practical work in the culinary arts. He had to make homemade ice cream - boil milk, dissolve sugar in it, cool it to room temperature, and then put it in the refrigerator to freeze. Apparently, Mpemba was not a particularly diligent student and procrastinated on the first part of the assignment. 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 a given technology.

After that, Mpemba experimented not only with milk, but also with ordinary water. In any case, already being a student at Mkwawa High School, he asked Professor Dennis Osborne from the University College in Dar es Salaam (invited by the director of the school to give a lecture on physics to students) about water: "If you take two identical containers with equal volumes of water so that in one of them the water has a temperature of 35 ° C, and in the other - 100 ° C, and put them in the freezer, then in the second the water will freeze faster. Why? Osborne became interested in this issue and soon in 1969, together with Mpemba, they published the results of their experiments in the journal "Physics Education". Since then, the effect they discovered is 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, should be proportional to the temperature difference between this body and the environment. This law was established by Newton and since then has been confirmed many times in practice. In the same effect, water at 100°C cools down to 0°C faster than the same amount of water at 35°C.

However, this does not yet imply a paradox, since the Mpemba effect can also be explained within 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 with 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. First, the mass of water required for cooling is reduced. And secondly, the temperature decreases due to the fact that the heat of evaporation of the transition from the water phase to the vapor phase decreases.

temperature difference

Because the temperature difference between hot water and more cold air - hence the heat exchange in this case is more intense and hot water cools faster.

hypothermia

When water is cooled below 0 C, it does not always freeze. Under certain conditions, it can undergo supercooling while continuing to remain liquid at temperatures below the freezing point. In some cases, water can remain liquid even at -20 C.

The reason for this effect is that in order for the first ice crystals to begin to form, centers of crystal formation are needed. If they are not in liquid water, then supercooling will continue until the temperature drops enough that crystals begin to form spontaneously. When they start to form in the supercooled liquid, they will start to grow faster, forming an ice slush that will freeze to form ice.

Hot water is most susceptible to hypothermia because heating it eliminates 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, which is not supercooled, the following occurs. 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 cold air and will prevent further evaporation. The rate of formation of ice crystals in this case will be less. In the case of hot water undergoing subcooling, the subcooled 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.

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 the loss of heat, while hot water begins to freeze from below.

This effect is explained by an anomaly in the density of water. 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 4°C, it will stay on the surface, forming a thin cold layer. Under these conditions, a thin layer of ice will form on the surface of the water for a short time, but this layer of ice will serve as an insulator protecting the lower layers of water, which will remain at a temperature of 4 C. Therefore, further cooling will be slower.

In the case of hot water, the situation is completely different. The surface layer of water will cool more quickly due to evaporation and a greater temperature difference. Also, cold water layers are denser than hot water layers, so the cold water layer will sink down, lifting the warm water layer to the surface. This circulation of water ensures a rapid drop in temperature.

But why does this process not reach the equilibrium point? To explain the Mpemba effect from this point of view of convection, it would be necessary to assume that the cold and hot layers of water are separated and the convection process itself continues after the average water temperature drops below 4 C.

However, there is no experimental evidence to support this hypothesis that cold and hot layers of water are separated by convection.

gases dissolved in water

Water always contains gases dissolved in it - oxygen and carbon dioxide. These gases have the ability to lower the freezing point of water. When the water is heated, these gases are released from the water because their solubility in water at high temperature is lower. Therefore, when hot water is cooled, there are always fewer dissolved gases in it 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 are no experimental data confirming this fact.

Thermal conductivity

All these (as well as other) conditions have been studied in many experiments, but an unequivocal answer to the question - which of them provide a 100% reproduction of the Mpemba effect - has not been obtained.

So, for example, in 1995, the German physicist David Auerbach studied the influence of supercooling of 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 the supercooled state faster than hot water, thereby compensating for the previous lag.

In addition, Auerbach's results contradicted earlier data that hot water is able to achieve greater supercooling due to fewer crystallization centers. When water is heated, the gases dissolved in it are removed from it, and when it is boiled, some salts dissolved in it precipitate.

So far, only one thing can be asserted - the reproduction of this effect essentially depends on the conditions under which the experiment is carried out. Precisely because it is not always reproduced.

O. V. Mosin

Literarysources:

"Hot water freezes faster than cold water. Why does it do so?", Jearl Walker in The Amateur Scientist, Scientific American, Vol. 237, no. 3, pp. 246-257; September, 1977.

"The Freezing of Hot and Cold Water", G.S. Kell in American Journal of Physics, Vol. 37, no. 5, pp. 564-565; May 1969.

"Supercooling and the Mpemba effect", David Auerbach, in American Journal of Physics, Vol. 63, no. 10, pp. 882-885; Oct, 1995.

"The Mpemba effect: The freezing times of hot and cold water", Charles A. Knight, in American Journal of Physics, Vol. 64, no. 5, p 524; May, 1996.

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. this stage can be skipped, and, accordingly, she 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, in the popular scientific journal published a joint article of these two people 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 begins to freeze from upper layers when hot does it the other way around - it starts to freeze from the bottom up. At the same time, it turns out that cold water, having a cold layer on top with ice already formed in places, thus impairs convection processes 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 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 when the dissolved chemical elements available only in hot water, while cold water does not have them, then there is a possibility for hot water 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 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.

The phenomenon of hot water solidifying at a faster rate than cold water is known in science as the Mpemba effect. Such great minds as Aristotle, Francis Bacon and Rene Descartes pondered over this paradoxical phenomenon, but for millennia no one has yet been able to offer a reasonable explanation for this phenomenon.

Only in 1963, a schoolboy from the Republic of Tanganyika, Erasto Mpemba, noticed this effect on the example of ice cream, but none of the adults gave him an explanation. Nevertheless, physicists and chemists seriously thought about such a simple, but so incomprehensible phenomenon.

Since then, different versions have been expressed, one of which was as follows: part of the hot water simply evaporates at first, and then, when a smaller amount remains, the water solidifies faster. This version, due to its simplicity, became the most popular, but scientists were not completely satisfied.

Now a team of researchers from University of Technology Nanyang in Singapore (Nanyang Technological University), led by chemist Xi Zhang, said they have solved the age-old mystery of why warm water freezes faster than cold water. As Chinese experts found out, the secret lies in the amount of energy stored in hydrogen bonds between water molecules.

As you know, water molecules are made up of one oxygen atom and two hydrogen atoms held together by covalent bonds, which at the particle level looks like an exchange of electrons. Another known fact is that hydrogen atoms are attracted to oxygen atoms from neighboring molecules - this forms hydrogen bonds.

At the same time, water molecules as a whole repel each other. Scientists from Singapore noticed that the warmer the water, the greater the distance between the molecules of the liquid due to the increase in repulsive forces. As a result, hydrogen bonds are stretched, and therefore store more energy. This energy is released when the water cools - the molecules approach each other. And the return of energy, as you know, means cooling.

As the chemists write in their article, which can be found on the arXiv.org preprint site, hydrogen bonds are stretched more strongly in hot water than in cold water. Thus, it turns out that more energy is stored in the hydrogen bonds of hot water, which means that more of it is released when cooled to sub-zero temperatures. For this reason, freezing is faster.

To date, scientists have solved this riddle only theoretically. When they present convincing evidence of their version, then the question of why hot water freezes faster than cold water can be considered closed.

In 1963, a schoolboy from Tanzania named Erasto Mpemba asked his teacher a stupid question - why does warm ice cream freeze faster than cold ice cream in his freezer?

Erasto Mpemba was a student at Magambin High School in Tanzania doing practical cooking work. He had to make homemade ice cream - boil milk, dissolve sugar in it, cool it to room temperature, and then put it in the refrigerator to freeze. Apparently, Mpemba was not a particularly diligent student and procrastinated on the first part of the assignment. Fearing that he would not be in time 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 a given technology.

He turned to the physics teacher for clarification, but he only laughed at the student, saying the following: "This is not world physics, but the physics of Mpemba." After that, Mpemba experimented not only with milk, but also with ordinary water.

In any case, already being a student of the Mkwawa High School, he asked Professor Dennis Osborne from the University College in Dar es Salaam (invited by the director of the school to give a lecture on physics to the students) about water: “If you take two identical containers with equal volumes of water so that in one of them the water has a temperature of 35 ° C, and in the other - 100 ° C, and put them in the freezer, then in the second the water will freeze faster. Why?" Osborn became interested in this issue and soon in 1969, together with Mpemba, they published the results of their experiments in the journal Physics Education. Since then, the effect they discovered is called the Mpemba effect.

Are you curious to know why this happens? Just a few years ago, scientists managed to explain this phenomenon ...

The Mpemba effect (Mpemba Paradox) is a paradox that states that hot water under certain conditions freezes faster than cold water, although it must pass the temperature of cold water in the process of freezing. This paradox is an experimental fact that contradicts the usual ideas, according to which, under the same conditions, a hotter body needs more time to cool down to a certain temperature than a cooler body to cool down to the same temperature.

This phenomenon was noticed at the time by Aristotle, Francis Bacon and Rene Descartes. 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 the ambient temperature must be proportional to the temperature difference between this body and the environment. This law was established by Newton and since then has been confirmed many times in practice. In the same effect, water at 100°C cools down to 0°C faster than the same amount of water at 35°C.

Now a team of researchers from the Nanyang Technological University in Singapore, led by chemist Xi Zhang, says they have solved the age-old mystery of why warm water freezes faster than cold water. As Chinese experts found out, the secret lies in the amount of energy stored in hydrogen bonds between water molecules.

As you know, water molecules consist of one oxygen atom and two hydrogen atoms held together by covalent bonds, which at the particle level looks like an exchange of electrons. Another well-known fact is that hydrogen atoms are attracted to oxygen atoms from neighboring molecules - in this case, hydrogen bonds are formed.

Here are the hypotheses put forward by scientists:

Evaporation

Hot water evaporates faster from the container, thereby reducing its volume, and a smaller volume of water with 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. First, the mass of water required for cooling is reduced. And secondly, due to evaporation, its temperature decreases.

temperature difference

Due to the fact that the temperature difference between hot water and cold air is greater - therefore, heat transfer in this case is more intense and hot water cools faster.

hypothermia
When water is cooled below 0°C, it does not always freeze. Under certain conditions, it can undergo supercooling while continuing to remain liquid at temperatures below the freezing point. In some cases, water can remain liquid even at -20°C. The reason for this effect is that in order for the first ice crystals to begin to form, centers of crystal formation are needed. If they are not in liquid water, then supercooling will continue until the temperature drops enough that crystals begin to form spontaneously. When they start to form in the supercooled liquid, they will start to grow faster, forming an ice slush that will freeze to form ice. Hot water is most susceptible to hypothermia because heating it eliminates 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, what happens is that a thin layer of ice forms on its surface, which acts as an insulator between the water and the cold air, and thus prevents further evaporation. The rate of formation of ice crystals in this case will be less. In the case of hot water undergoing subcooling, the subcooled 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 the loss of heat, while hot water begins to freeze from below. This effect is explained by an anomaly in the density of water. Water has a maximum density at 4°C. If you cool water to 4°C and place it in an environment with a lower temperature, the surface layer of water will freeze faster. Because this water is less dense than water at 4°C, it will stay on the surface, forming a thin cold layer. Under these conditions, a thin layer of ice will form on the surface of the water for a short time, but this layer of ice will serve as an insulator protecting the lower layers of water, which will remain at 4°C. Therefore, the further cooling process will be slower. In the case of hot water, the situation is completely different. The surface layer of water will cool more rapidly due to evaporation and greater temperature differences. Also, cold water layers are denser than hot water layers, so the cold water layer will sink down, lifting the warm water layer to the surface. This circulation of water ensures a rapid drop in temperature. But why does this process not reach the equilibrium point? To explain the Mpemba effect from the point of view of convection, one should assume that the cold and hot layers of water are separated and the convection process itself continues after the average water temperature drops below 4°C. However, there is no experimental evidence to support this hypothesis that cold and hot water layers are separated by convection.

gases dissolved in water

Thermal conductivity

This mechanism can play a significant role when water is placed in a refrigerator freezer in small containers. Under these conditions, it has been observed that the container with hot water melts the ice of the freezer underneath, thereby improving thermal contact with the wall of the freezer and thermal conductivity. As a result, heat is removed from the hot water container faster than from the cold one. In turn, the container with cold water does not melt snow under it. All these (as well as other) conditions have been studied in many experiments, but an unambiguous answer to the question - which of them provide a 100% reproduction of the Mpemba effect - has not been obtained. So, for example, in 1995, the German physicist David Auerbach studied the influence of supercooling of 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 earlier data that hot water is 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. So far, only one thing can be asserted - the reproduction of this effect significantly depends on the conditions under which the experiment is carried out. Precisely because it is not always reproduced.

And here's the most likely reason.

The British Royal Society of Chemistry is offering a £1,000 reward to anyone who can explain with scientific point see 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 he wrote ancient greek in 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 about this inconvenient fact for a long time "forgotten".

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. Employee of the Department of Radiation Safety State University of New York City, James Brownridge 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.