Hello dear lovers interesting facts... Today we will talk about. But I think that the question in the title may seem simply absurd - but should one always rely completely on the notorious "common sense", and not strictly formulated testing experience. Let's try to figure out why hot water freezes faster than cold water?
History reference
That in the issue of freezing cold and hot water "not everything is clean" was mentioned in the writings of Aristotle, then similar notes were made by F. Bacon, R. Descartes and J. Black. IN recent history this effect was named "Mpemba's paradox" - after the name of a student from Tanganyika, Erasto Mpemba, who asked the same question to a visiting professor of physics.
The boy's question arose not from scratch, but from purely personal observations of the process of cooling ice cream mixes in the kitchen. Of course, the classmates who were present at the same place together with the school teacher laughed at Mpemba - however, after the experimental verification personally by Professor D. Osborn, their desire to make fun of Erasto "evaporated" from them. Moreover, Mpemboy, together with a professor in Physics Education in 1969, published detailed description this effect - and since then the aforementioned name has stuck in the scientific literature.
What is the essence of the phenomenon?
The setting of the experiment is quite simple: all other things being equal, the same thin-walled vessels are tested, in which there are strictly equal amounts of water, differing only in temperature. The vessels are loaded into the refrigerator, after which the time until the formation of ice in each of them is recorded. The paradox is that in a vessel with an initially hotter liquid, this happens faster.
How does modern physics explain this?
The paradox does not have a universal explanation, since several parallel processes proceed together, the contribution of which may differ from specific initial conditions - but with a consistent result:
- the ability of a liquid to hypothermia - initially cold water is more prone to hypothermia, i.e. remains liquid when its temperature is already below freezing point
- accelerated cooling - steam from hot water is transformed into ice microcrystals, which, when falling back, accelerate the process, working as an additional "external heat exchanger"
- insulation effect - unlike hot water, cold water freezes from above, which leads to a decrease in heat transfer by convection and radiation
There are a number of other explanations (the last time a competition for the best hypothesis was held by the British Royal Chemical Society was recently, in 2012) - but there is still no unambiguous theory for all cases of combinations of input conditions ...
One of my favorite subjects at school was chemistry. Once a chemistry teacher gave us a very strange and difficult task. He gave us a list of questions that we had to answer in terms of chemistry. We were given several days for this assignment and allowed to use libraries and other available sources of information. One of these questions concerned the freezing point of water. I don't remember exactly how the question sounded, but it was about the fact that if you take two wooden buckets of the same size, one with hot water, the other cold (with exactly the specified temperature), and place them in an environment with a specific temperature, which one will freeze faster? Of course, the answer immediately suggested itself - a bucket of cold water, but it seemed to us too simple. But this was not enough to give a complete answer, we needed to prove it from a chemical point of view. Despite all my thinking and research, I was unable to draw a logical conclusion. On this day, I even decided to skip this tutorial, so I never found out the solution to this riddle.
Years passed, and I learned a lot of household myths about the boiling point and freezing point of water, and one myth said: "hot water freezes faster." I looked at many websites, but the information was too conflicting. And these were just opinions, unfounded from the point of view of science. And I decided to conduct my own experience. Since I couldn't find wooden buckets, I used a freezer, stovetop, some water, and a digital thermometer. I will talk about the results of my experience a little later. First, I'll share with you some interesting arguments about water:
Hot water freezes faster than cold water. Most experts claim that cold water will freeze faster than hot water. But one funny phenomenon (the so-called Memb effect), for unknown reasons, proves the opposite: Hot water freezes faster than cold water. One of several explanations is the evaporation process: if very hot water is placed in a cold environment, the water will begin to evaporate (the remaining amount of water will freeze faster). And according to the laws of chemistry, this is not a myth at all, and most likely this is what the teacher wanted to hear from us.
Boiled water freezes faster than tap water. Despite the previous explanation, some experts argue that boiled water that has cooled to room temperature should freeze faster because boiling reduces oxygen.
Cold water boils faster than hot water. If hot water freezes faster, then the cold water may boil faster! This is contrary to common sense and scientists argue that this simply cannot be. Hot tap water should actually boil faster than cold water. But by using hot water for boiling, you are not saving energy. You may use less gas or light, but the water heater will use the same amount of energy that is needed to heat cold water. (This is a little different with solar power.) As a result of the heating of the water by the water heater, sediment may appear, so the water will take longer to heat.
If you add salt to the water, it will boil faster. Salt increases the boiling point (and, accordingly, lowers the freezing point - which is why some housewives add a little rock salt to ice cream). But in this case, we are interested in another question: how long will the water boil and whether the boiling point in this case can rise above 100 ° C). Despite what they write in cookbooks, scientists argue that the amount of salt we add to boiling water is not enough to affect the boiling time or temperature.
But here's what I got:
Cold water: I used three 100 ml glass beakers of purified water: one at room temperature (72 ° F / 22 ° C), one with hot water (115 ° F / 46 ° C), and one with boiled water (212 ° F / 100 ° C). I placed all three glasses in the freezer at –18 ° C. And since I knew that water would not immediately turn into ice, I determined the degree of freezing by the "wooden float". When the stick, placed in the center of the glass, no longer touched the base, I assumed that the water had frozen. I checked the glasses every five minutes. And what are my results? The water in the first glass froze after 50 minutes. The hot water froze after 80 minutes. Boiled - after 95 minutes. My findings: Given the conditions in the freezer and the water I used, I was unable to reproduce the Memb effect.
I also tried this experiment with previously boiled water cooled to room temperature. She froze after 60 minutes - it still took longer than freezing cold water.
Boiled water: I took a liter of water at room temperature and put it on fire. It boiled in 6 minutes. Then I cooled it down to room temperature again and added it to the hot one. With the same heat, hot water boiled in 4 hours and 30 minutes. Conclusion: as expected, hot water boils much faster.
Boiled water (with salt): I added 2 large tablespoons of table salt to 1 liter of water. It boiled after 6 minutes 33 seconds, and the thermometer showed it reached a temperature of 102 ° C. Undoubtedly, salt affects the boiling point, but not much. Conclusion: salt in water does not strongly affect the temperature and boiling time. I honestly admit that my kitchen can hardly be called a laboratory, and perhaps my conclusions are contrary to reality. My freezer compartment may freeze food unevenly. My glass glasses could be irregular shape, Etc. But whatever happens in the laboratory, when it comes to freezing or boiling water in the kitchen, the most important thing is common sense.
link with fun facts about vaudews about water
as suggested on the forum.ixbt.com this effect (the effect of freezing hot water faster than cold water) is called the "Aristotle-Mpemba effect"
Those. boiled water (chilled) freezes faster than "raw"
Mpemba effect (Mpemba paradox) - a paradox that says that hot water freezes faster under certain conditions 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 concepts, according to which, under the same conditions, a more heated body to cool to a certain temperature takes longer than a less heated body to cool to the same temperature.
This phenomenon was noticed at the time by Aristotle, Francis Bacon and Rene Descartes, but it was not until 1963 that a Tanzanian schoolboy Erasto Mpemba found that a hot ice cream mixture freezes faster than a cold one.
As a student of Magambinskaya high school in Tanzania, Erasto Mpemba did practical work on 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 he delayed completing the first part of the assignment. Fearing that he would not be in time by the end of the lesson, he put the hot milk in the refrigerator. To his surprise, it froze even 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 of the Mkvavskaya high school, he asked a question to Professor Dennis Osborne from the University College in Dar es Salaam (invited by the headmaster to give the students a lecture on physics) specifically about water: “If we 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 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 a 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 this effect, water with a temperature of 100 ° C cools down to a temperature of 0 ° C faster than the same amount of water with a temperature of 35 ° C.
However, this does not yet suggest a paradox, since the Mpemba effect can be explained within the framework of well-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.
Evaporation effect - double effect. First, the amount of water required for cooling is reduced. And secondly, the temperature decreases due to the fact that the heat of vaporization of the transition from the water phase to the vapor phase decreases.
Temperature difference
Due to the fact that the temperature difference between hot water and cold air is greater - therefore, 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 some conditions, it can undergo hypothermia, continuing to remain liquid at temperatures below the freezing point. 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, centers of crystal formation are needed. If they are not present in liquid water, then hypothermia will continue until the temperature drops so much that crystals begin to form spontaneously. When they begin to form in a supercooled liquid, they will begin to grow faster, forming an ice slush, which, freezing, will 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 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. In this case, the rate of formation of ice crystals will be slower. In the case of hot water subject to supercooling, supercooled water does not have a protective surface layer of ice. Therefore, it loses heat much faster through the open top.
When the hypothermia process ends and the water freezes, much more heat is lost and therefore more ice forms.
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 the water density anomaly. 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 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 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. 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 fail to reach an equilibrium point? To explain the Mpemba effect from this point of view of convection, it should be assumed that 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 data that would 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 reduce the freezing point of water. When the water is heated, these gases are released from the water because their solubility in water at high temperatures is lower. Therefore, when hot water is cooled, there are always less 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
This mechanism can play a significant role when water is placed in a freezer in a refrigerator compartment in small containers. Under these conditions, it was noticed that the container with hot water melts the ice of the freezer under it, thereby improving thermal contact with the freezer wall and thermal conductivity. As a result, heat is removed from the container with hot water faster than from cold water. In turn, a container with cold water does not thaw snow under it.
All these (and other) conditions have been studied in many experiments, but an unambiguous answer to the question - which of them provide one hundred percent reproduction of the Mpemba effect - has not been obtained.
For example, in 1995 the German physicist David Auerbach studied the influence of supercooling of water on this effect. He found that hot water, reaching a supercooled state, freezes at a higher temperature than cold water, which means 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 previously obtained data that hot water can achieve more 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 essentially depends on the conditions in which the experiment is carried out. Precisely because it is not always reproduced.
Have you ever wondered why water heated to 82 degrees C freezes faster than cold? Most likely not, I am even more than sure that the question never crossed your mind - which water freezes faster, hot or cold?
However, this surprising discovery was made by an ordinary African schoolboy Erasto Mpemba back in 1963. It was the usual experience of a curious boy, of course, he could not correctly interpret the meaning of his own, and moreover, scientists from all over the world until 1966 could not give a clear and reasonable the answer to the question - why hot water freezes faster than cold.
Why does hot water freeze at 4 degrees Celsius, and cold water at 0?
There is a lot of dissolved oxygen in cold water, it is he who maintains the freezing point of water at 0 degrees. If oxygen is removed from the water, and this is what happens when the water is heated, air bubbles dissolved in water, as it is fashionable to say now, collapse, the water turns into ice not at zero degrees, as usual, and already at 4 ° C... It is oxygen dissolved in water that breaks the bonds between water molecules preventing water from passing from liquid state into solid, it will simply turn into
Internet marketer, editor of the site "On accessible language"
Date of publication: 21.11.2017
« Which water freezes faster cold or hot?»- try to ask a question to your friends, most likely most of them will answer that cold water freezes faster - and make a mistake.
In fact, if you simultaneously put two vessels of the same shape and volume in the freezer, one of which will have cold water and the other hot, then the hot water will freeze faster.
Such a statement may seem absurd and unreasonable. If you follow the logic, then hot water must first cool down to a cold temperature, and cold at this time should have already turned into ice.
So why does hot water overtake cold water on its way to freezing? Let's try to figure it out.
History of observations and research
People have observed the paradoxical effect since ancient times, but no one attached much importance to it. This is how Arrestotel, as well as René Descartes and Francis Bacon, noted in their notes that there is no coincidence in the freezing rate of cold and hot water. An unusual phenomenon often manifested itself in everyday life.
For a long time, the phenomenon was not studied in any way and did not arouse much interest among scientists.
The study of the unusual effect began in 1963 when an inquisitive student from Tanzania, Erasto Mpemba, noticed that hot 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.
6 years later - in 1969, Mpemba asked this question to professor of physics Dennis Osborne who came to his school. The professor was interested in the observation of the young man, as a result, an experiment was carried out, which confirmed the presence of the effect, but the reasons for this phenomenon were not established.
Since then, the phenomenon has been called mpemba effect.
Throughout the history of scientific observation, many hypotheses have been put forward about the causes of the phenomenon.
So in 2012, the British Royal Chemical Society would have announced a competition for hypotheses explaining the Mpemba effect. Scientists from all over the world took part in the competition, in total 22,000 were registered scientific works... Despite such an impressive number of articles, none of them clarified the Mpemba paradox.
The most common version was that hot water freezes faster, because it simply evaporates faster, its volume becomes smaller, and as the volume decreases, its cooling rate increases. The most common version was eventually disproved as an experiment was carried out in which evaporation was excluded, but the effect was nevertheless confirmed.
Other scientists believed that the cause of the Mpemba effect is the evaporation of gases dissolved in water. In their opinion, gases dissolved in water evaporate during heating, due to which it acquires a higher density than cold water. As you know, an increase in density leads to a change physical properties water (increase in 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 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 it was not possible to scientifically confirm them due to the lack of initial data, contradictions in other experiments, or due to the fact that the revealed factors were simply not comparable with the rate of water cooling. Some scientists in their works questioned the existence of the effect.
In 2013, researchers from Technological University Nanyang in Singapore said they had solved the mystery of the Mpemba effect. According to their research, the reason for the phenomenon lies in the fact that the amount of energy stored in hydrogen bonds between the molecules of cold and hot water is significantly different.
Computer simulation methods have shown the following results: the higher the water temperature, the greater the distance between the molecules due to the fact that the repulsive forces increase. Consequently, the hydrogen bonds of molecules are stretched, storing more energy. When cooled, the molecules begin to approach each other, releasing energy from hydrogen bonds. In this case, the release of energy is accompanied by a decrease in temperature.
In October 2017, Spanish physicists in the course of their next study found that it is the removal of matter from equilibrium (strong heating before strong cooling) plays a large role in the formation of the effect. They identified the conditions under which the likelihood of the effect being greatest. In addition, scientists from Spain have confirmed the existence of the opposite Mpemba effect. They found that when heated, a colder sample can reach a high temperature faster than a warmer one.
Despite exhaustive information and numerous experiments, scientists intend to continue studying the effect.
The Mpemba effect in real life
Have you ever wondered why in winter the ice rink is filled with hot water, and not cold? As you already understood, they do this because an ice rink filled with hot water will freeze faster than if it was filled with cold water. For the same reason, slides in winter ice towns are flooded with hot water.
Thus, knowledge of the existence of the phenomenon allows people to save time when preparing sites for winter sports.
In addition, the Mpemba effect is sometimes used in industry - to reduce the freezing time of products, substances and materials containing water.