Why does cold water freeze more slowly than hot water? The Mpemba effect or why hot water freezes faster than cold water
Water- a rather simple substance from a chemical point of view, however, it has a number of unusual properties that never cease to amaze scientists. Below are a few facts that few people know about.
1. Which water freezes faster - cold or hot?
Let's take two containers with 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 to the cold temperature, and then turn into ice, while cold water does not need to cool. Why is this happening?
In 1963, a Tanzanian student named Erasto B. Mpemba, while freezing an 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 his 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 It really freezes faster than cold.
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 still do not fully understand the nature of this phenomenon, explaining it either by the difference in supercooling, evaporation, ice formation, convection, or by the effect of liquefied gases on hot and cold water.
2. It can freeze instantly
Everyone knows that water always turns to ice when cooled to 0°C... with some exceptions! Such a case, for example, is supercooling, which is a property of very clean water remain liquid even when cooled to below freezing. This phenomenon is made possible due to the fact that the environment does not contain centers or nuclei of crystallization that could trigger the formation of ice crystals. And so water remains in liquid form even when cooled to below zero degrees Celsius.
Crystallization process can be caused, for example, by gas bubbles, impurities (contaminants), or an uneven surface of the container. Without them, water will remain in a liquid state. When the crystallization process starts, you can watch the super-cooled water instantly turn 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 does water have? If you answered three: solid, liquid, gas, then you were wrong. 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-chilled water will suddenly turn into ice. What will happen 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 “vitreous” water - a solid substance that lacks crystalline structure.
4. Water surprises physicists
At the molecular level, water is even more surprising. In 1995, a neutron scattering experiment conducted by scientists yielded an unexpected result: physicists discovered that neutrons aimed at water molecules “see” 25% fewer hydrogen protons than expected.
It turned out that at a speed of one attosecond (10 -18 seconds) an unusual quantum effect takes place, and the chemical formula of water instead H2O, becomes H1.5O!
5. Water memory
Alternative to official medicine homeopathy states that a dilute solution medicinal product can have a healing effect on the body, even if the dilution factor is so high that there is nothing left in the solution except 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 the substance that was 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 of Queen's University of Belfast, who had criticized the principles of homeopathy, conducted an experiment in 2002 to disprove the concept once and for all. The result was the opposite. After which, scientists stated that they were able to prove the reality of the effect “ water memory" However, experiments carried out 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 did not talk about in this article. For example, the density of water changes depending on temperature (the density of ice is less than the density of water); water has a fairly high surface tension; in the liquid state, water is a complex and dynamically changing network of water clusters, and it is the behavior of the 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", authored by Martin Chaplin, professor at the University of London.
In this article we will look at the question of why hot water freezes faster than cold water.
Heated water freezes much faster than cold water! This amazing property of water, for which scientists still cannot find an exact explanation, has been known since ancient times. For example, even in Aristotle there is a description of winter fishing: fishermen inserted fishing rods into holes in the ice, and so that they would freeze faster, they poured warm water on the ice. This phenomenon was named after Erasto Mpemba in the 60s of the 20th century. Mnemba noticed a strange effect while making ice cream and turned to his physics teacher, Dr. Denis Osborne, for an explanation. Mpemba and Dr. Osborne experimented with water at different temperatures and concluded that almost boiling water begins to freeze much faster than water at room temperature. Other scientists conducted their own experiments and each time obtained similar results.
Explanation of a physical phenomenon
There is no generally accepted explanation for why this happens. Many researchers suggest that the whole point is in the supercooling of the liquid, which occurs when its temperature drops below the freezing point. In other words, if water freezes at a temperature below 0°C, then supercooled water can have a temperature of, for example, -2°C and still remain liquid without turning into ice. When we try to freeze cold water, there is a chance that it will first become supercooled and only harden after some time. Other processes occur in heated water. Its faster transformation into ice is associated with convection.
Convection- this is a physical phenomenon in which the warm lower layers of a liquid rise, and the upper, cooled ones, fall.
Hello dear lovers interesting facts. Today we will talk to you about. But I think that the question posed in the title may seem simply absurd - but should one always completely trust the notorious “ common sense", rather than a strictly defined test experiment. Let's try to figure out why hot water freezes faster than cold water?
Historical reference
That in the issue of freezing cold and hot water, “not everything is pure” was mentioned in the works of Aristotle, then similar notes were made by F. Bacon, R. Descartes and J. Black. IN modern history This effect was given the name “Mpemba’s paradox” - after the Tanganyika schoolboy Erasto Mpemba, who asked the same question to a visiting physics professor.
The boy’s question did not arise out of nowhere, but from purely personal observations of the process of cooling ice cream mixtures in the kitchen. Of course, the classmates who were present there, together with the school teacher, made Mpemba laugh - however, after an experimental test personally by Professor D. Osborne, the desire to make fun of Erasto “evaporated” from them. Moreover, Mpemba, together with a professor, published a detailed description of this effect in Physics Education in 1969 - and since then the above-mentioned name has been fixed in the scientific literature.
What is the essence of the phenomenon?
The setup of the experiment is quite simple: all other things being equal, identical thin-walled vessels are tested, containing strictly equal amounts of water, differing only in temperature. The vessels are loaded into the refrigerator, after which the time until ice forms in each of them is recorded. The paradox is that in a vessel with an initially hotter liquid this happens faster.
How does modern physics explain this?
The paradox does not have a universal explanation, since several parallel processes occur together, the contribution of which may vary depending on the specific initial conditions - but with a uniform result:
- the ability of a liquid to supercool - initially cold water is more prone to supercooling, i.e. remains liquid when its temperature is already below freezing point
- accelerated cooling - steam from hot water is transformed into ice microcrystals, which, when falling back, accelerate the process, working as an additional “external heat exchanger”
- insulation effect - unlike hot water, cold water freezes from above, which leads to a decrease in heat transfer by convection and radiation
There are a number of other explanations (the last time the British Royal Society of Chemistry held a competition for the best hypothesis was recently, in 2012) - but there is still no unambiguous theory for all cases of combinations of input conditions...
Mpemba effect(Mpemba's Paradox) - a paradox that states that hot water under some 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 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.
As a student at Magambi Secondary 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 to room temperature, and then put it 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 ordinary water. In any case, already as a student at Mkwava 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 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, 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 temperature difference between hot water and there is more cold air - 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 cooling process 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 bigger difference temperatures In addition, cold water layers are denser than hot water layers, so the cold water layer will sink down, raising the warm water layer 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 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 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 high temperature below. 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 has been observed that a container of hot water melts the ice in the freezer underneath, 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.
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.
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 of northern latitudes knows. In fact, scientifically, it turns out that in any case, cold water is simply bound to freeze faster.
The physics teacher, who was approached by schoolboy Erasto Mpemba 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 Rene Descartes and even Aristotle also thought about it at one time.
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 from the upper layers, when hot water 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 one 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 when dissolved chemical elements are available only in hot water, and cold water does not have them, then there is a possibility for hot water 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 a solid state is 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 - all this is written in beautiful formulas and seems plausible. But when the experimental data on which water freezes faster - hot or cold - will be put in a practical sense, and their results will be presented, then the question of Mpemba’s paradox can be considered closed.