Nadezhda Konstantinovna Krupskaya. Nadezhda Krupskaya
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. These could be, for example, very small particles of 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. At 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 cooled 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 else with school days The composition of water is known - 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 that of 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 of covalent bonds occurs in the middle of the molecules 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 quickly. By at least, this is shown by theoretical calculations carried out by Singaporean scientists.
Instantly freezing water - 5 incredible tricks: Video
It seems obvious that cold water freezes faster than hot water, since under equal conditions hot water takes longer to cool and subsequently freeze. However, thousands of years of observations, as well as modern experiments, have shown that the opposite is also true: under certain conditions, hot water freezes faster than cold water. The Sciencium Science Channel explains this phenomenon:
As explained in the video above, the phenomenon of hot water freezing faster than cold water is known as the Mpemba effect, named after Erasto Mpemba, a Tanzanian student who made ice cream as part of a school project in 1963. Students had to bring a mixture of cream and sugar to a boil, let it cool, and then put it in the freezer.
Instead, Erasto put his mixture in immediately, hot, without waiting for it to cool. As a result, after 1.5 hours his mixture was already frozen, but the other students’ mixtures were not. Interested in the phenomenon, Mpemba began studying the issue with physics professor Denis Osborne, and in 1969 they published a paper stating that warm water freezes faster than cold water. This was the first peer-reviewed study of its kind, but the phenomenon itself is mentioned in the papers of Aristotle, dating back to the 4th century BC. e. Francis Bacon and Descartes also noted this phenomenon in their studies.
The video lists several options for explaining what is happening:
- Frost is a dielectric, and therefore frosty cold water stores heat better than a warm glass, which melts ice when it comes into contact with it
- Cold water has more dissolved gases than warm water, and researchers speculate that this may play a role in the rate of cooling, although it is not yet clear how
- Hot water loses more water molecules through evaporation, so there are fewer left to freeze
- Warm water can cool faster due to increased convective currents. These currents occur because the water in the glass cools first at the surface and sides, causing cold water to sink and hot water to rise. In a warm glass, convective currents are more active, which can affect the cooling rate.
However, in 2016, a carefully controlled study was conducted that showed the opposite: hot water froze much more slowly than cold water. At the same time, scientists noticed that changing the location of the thermocouple - a device that determines temperature changes - by just a centimeter leads to the appearance of the Mpemba effect. A study of other similar studies showed that in all cases where this effect was observed, there was a displacement of the thermocouple within a centimeter.
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 task and were 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 with a cold one (with exactly the specified temperature), and place them in an environment with a certain temperature, which one will freeze faster? Of course, the answer immediately suggested itself - a bucket of cold water, but we thought it was 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 could not come to a logical conclusion. I even decided to skip this lesson that day, so I never learned the solution to this riddle.
Years passed, and I learned many everyday 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 a scientific point of view. And I decided to spend own experience. Since I couldn't find wooden buckets, I used the freezer, stove, some water and a digital thermometer. I will tell you about the results of my experience a little later. First, I will share with you some interesting arguments about water:
Hot water freezes faster than cold water. Most experts say that cold water will freeze faster than hot water. But one funny phenomenon (the so-called Memba effect), for unknown reasons, proves the opposite: Hot water freezes faster than cold water. One of several explanations is the process of evaporation: 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, cooled down to room temperature, should freeze faster because boiling reduces the amount of oxygen.
Cold water boils faster than hot water. If hot water freezes faster, then maybe cold water boils faster! This is contrary to common sense and scientists say that this simply cannot be. Hot tap water should actually boil faster than cold water. But using hot water to boil does not save energy. You may use less gas or light, but the water heater will use the same amount of energy needed to heat cold water. (WITH solar energy things are a little different). As a result of heating the water by the water heater, sediment may appear, so the water will take longer to heat up.
If you add salt to water, it will boil faster. Salt increases the boiling point (and accordingly lowers the freezing point - which is why some housewives add a little salt to their ice cream). rock salt). 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 is written in cookbooks, scientists say 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 glasses of purified water: one glass with 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 using a “wooden float”. When the stick placed in the center of the glass no longer touched the base, I considered the water to be frozen. I checked the glasses every five minutes. And what are my results? The water in the first glass froze after 50 minutes. 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 Memba effect.
I also tried this experiment with previously boiled water that had cooled to room temperature. It froze within 60 minutes - still took longer than cold water to freeze.
Boiled water: I took a liter of water at room temperature and put it on the fire. It boiled in 6 minutes. I then cooled it back down to room temperature and added it to it while it was hot. With the same fire, 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 per 1 liter of water. It boiled in 6 minutes 33 seconds, and as 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 greatly affect the temperature and boiling time. I honestly admit that my kitchen can hardly be called a laboratory, and perhaps my conclusions contradict reality. My freezer may not freeze food evenly. My glass glasses could be irregular shape , Etc. But no matter what happens in laboratory conditions , When we're talking about
When it comes to freezing or boiling water in the kitchen, the most important thing is common sense. link with interesting facts
about waterall about water
as suggested on the forum.ixbt.com, this effect (the effect of hot water freezing faster than cold water) is called the “Aristotle-Mpemba effect”
Those. Boiled water (chilled) freezes faster than “raw”
In 1963, a Tanzanian schoolboy named Erasto Mpemba asked his teacher a stupid question - why did the warm ice cream in his freezer freeze faster than the cold one? Being a student of Magambinskaya high school 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.
He turned to the physics teacher for clarification, but he only laughed at the student, saying the following: “This is not universal physics, but Mpemba physics.” 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 the Mpemba effect.
Are you interested in knowing 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 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 in their 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 when different temperatures. The paradox of the Mpemba effect is that the time during which a body cools down to the ambient temperature should be proportional to the difference in temperature 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.
Since then they have spoken out different versions, one of which sounded as follows: part of the hot water first simply evaporates, and then, when less of it remains, the water freezes faster. This version, due to its simplicity, became the most popular, but did not completely satisfy scientists.
Now a team of researchers from University of Technology Nanyang Technological University in Singapore, 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 have 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 known fact lies in the fact that hydrogen atoms are attracted to oxygen atoms from neighboring molecules - and hydrogen bonds are formed.
At the same time, water molecules generally repel each other. Scientists from Singapore noticed: the warmer the water, the greater the distance between the molecules of the liquid due to an 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 move closer to each other. And the release of energy, as is known, means cooling.
Here are the assumptions put forward by scientists:
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, 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, 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: a thin layer of ice forms on its surface, which acts as an insulator between the water and the cold air, and thereby prevents 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 is lost. more heat and therefore is formed 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 heat loss, while hot water begins to freeze from below. This effect is explained by an anomaly in water density. Water has its 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 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 act as an insulator, protecting the lower layers of water, which will remain at a temperature of 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 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, 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 the point of view of convection, it would be necessary to assume that 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 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 an unambiguous answer to the question - which of them ensure 100% 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 can achieve greater supercooling due to smaller quantity 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.
But as they say, the most likely reason.
As the chemists write in their article, which can be found on the preprint website arXiv.org, hydrogen bonds are stronger 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 subzero temperatures. For this reason, hardening occurs faster.
To date, scientists have solved this mystery only theoretically. When they present convincing evidence of their version, the question of why hot water freezes faster than cold water can be considered closed.
Water is one of the most amazing liquids in the world, which has unusual properties. For example, ice is a solid state of liquid, has specific gravity lower than the water itself, which did a lot possible occurrence and the development of life on Earth. In addition, in pseudo-scientific, and scientific world There are discussions about which water freezes faster - hot or cold. Anyone who can prove that hot liquid freezes faster under certain conditions and scientifically substantiates their solution will receive a £1,000 reward from the British Royal Society of Chemists.
Background
The fact that under a number of conditions, hot water freezes faster than cold water was noticed back in the Middle Ages. Francis Bacon and René Descartes spent a lot of effort explaining this phenomenon. However, from the point of view of classical heat engineering, this paradox cannot be explained, and they tried to bashfully hush up about it. The impetus for the continuation of the debate was a somewhat curious story that happened to Tanzanian schoolboy Erasto Mpemba in 1963. One day, during a lesson on making desserts at a chef school, the boy, distracted by other things, did not have time to cool the ice cream mixture in time and put a hot solution of sugar in milk into the freezer. To his surprise, the product cooled somewhat faster than that of his fellow practitioners who observed temperature regime making ice cream.
Trying to understand the essence of the phenomenon, the boy turned to a physics teacher, who, without going into details, ridiculed his culinary experiments. However, Erasto was distinguished by enviable tenacity and continued his experiments not on milk, but on water. He became convinced that in some cases hot water freezes faster than cold water.
Having entered the University of Dar es Salaam, Erasto Mpembe attended a lecture by Professor Dennis G. Osborne. After its completion, the student puzzled the scientist with a problem about the rate of freezing of water depending on its temperature. D.G. Osborne ridiculed the very posing of the question, declaring with aplomb that any poor student knows that cold water will freeze faster. However, the young man’s natural tenacity made itself felt. He made a bet with the professor, proposing to conduct an experimental test right here in the laboratory. Erasto placed two containers of water in the freezer, one at 95°F (35°C) and the other at 212°F (100°C). Imagine the surprise of the professor and the surrounding “fans” when the water in the second container froze faster. Since then, this phenomenon has been called the “Mpemba Paradox”.
However, to date there is no coherent theoretical hypothesis explaining the “Mpemba Paradox”. It's not clear which external factors, chemical composition water, the presence of dissolved gases in it and minerals influence the rate of freezing of liquids at different temperatures. The paradox of the “Mpemba Effect” is that it contradicts one of the laws discovered by I. Newton, which states that the cooling time of water is directly proportional to the temperature difference between the liquid and the environment. And if all other liquids completely obey this law, then water in some cases is an exception.
Why does hot water freeze faster?T
There are several versions of why hot water freezes faster than cold water. The main ones are:
- hot water evaporates faster, while its volume decreases, and a smaller volume of liquid cools faster - when cooling water from + 100°C to 0°C, volumetric losses atmospheric pressure reach 15%;
- intensity of heat exchange between liquid and environment the higher the greater the temperature difference, therefore heat losses boiling water passes faster;
- when hot water cools, a crust of ice forms on its surface, preventing the liquid from completely freezing and evaporating;
- at high water temperatures, convection mixing occurs, reducing the freezing time;
- Gases dissolved in water lower the freezing point, removing energy for crystal formation - there are no dissolved gases in hot water.
All these conditions have been repeatedly tested experimentally. In particular, the German scientist David Auerbach discovered that the crystallization temperature of hot water is slightly higher than that of cold water, which makes it possible for the former to freeze more quickly. However, later his experiments were criticized and many scientists are convinced that the “Mpemba Effect”, which determines which water freezes faster - hot or cold, can only be reproduced under certain conditions, which no one has been searching for and specifying until now.