21.11.2017 11.10.2018 Alexander Firtsev
« Which water freezes faster cold or hot?”- try asking your friends a question, most likely most of them will answer that 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 contain cold water and the other hot, then hot water will freeze faster.
Such a statement may seem absurd and unreasonable. Logically, hot water must first cool down to cold temperature, and cold water should already turn into ice at this time.
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. So inconsistencies in the rate of freezing of cold and hot water were noted in their notes by Arestotel, as well as by Rene Descartes and Francis Bacon. 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.
Six years later, in 1969, Mpemba asked this question to physics professor Dennis Osborne, who came to his school. The professor was interested in the observation of the young man, as a result, an experiment was conducted that confirmed the presence of the effect, but the reasons for this phenomenon were not established.
Since then, the phenomenon has been called Mpemba effect.
Throughout the history of scientific observations, many hypotheses have been put forward about the causes of the phenomenon.
So in 2012, the British Royal Society of Chemistry would announce a competition of hypotheses to explain the Mpemba effect. Scientists from all over the world participated in the competition, in total 22,000 scientific papers were registered. Despite such an impressive number of articles, none of them clarified the Mpemba paradox.
The most common version was according to which, hot water freezes faster, since it simply evaporates faster, its volume becomes smaller, and as the volume decreases, its cooling rate increases. The most common version was eventually refuted, since an experiment was conducted in which evaporation was excluded, but the effect was nevertheless confirmed.
Other scientists believed that the reason for the Mpemba effect is the evaporation of gases dissolved in water. In their opinion, during the heating process, gases dissolved in water evaporate, due to which it acquires a higher density than cold water. As is known, an increase in density leads to a change in the physical properties of water (an increase in thermal conductivity), and hence an increase in the cooling rate.
In addition, a number of hypotheses have been put forward that describe 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 they could not be scientifically confirmed due to a lack of initial data, contradictions in other experiments, or due to the fact that the identified 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 at the Nanyang Technological University in Singapore claimed to have solved the mystery of the Mpemba effect. According to their study, the reason for the phenomenon lies in the fact that the amount of energy stored in hydrogen bonds between cold and hot water molecules differs significantly.
Computer simulation methods have shown the following results: the higher the temperature of the water, 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 another study, found out that it is the removal of matter from equilibrium (strong heating before strong cooling) that plays a large role in the formation of the effect. They determined the conditions under which the likelihood of the effect is maximum. In addition, scientists from Spain have confirmed the existence of the reverse Mpemba effect. They found that when heated, a colder sample can reach a high temperature faster than a warm one.
Despite exhaustive information and numerous experiments, scientists intend to continue studying the effect.
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 a skating rink filled with hot water will freeze faster than if it were filled with cold water. For the same reason, slides in winter ice towns are poured with hot water.
Thus, knowledge about 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.
This story began more than half a century ago, but has not received an outcome to this day. And all because, no matter how hard thousands of inquisitive minds from all over the planet try, they can’t find the only right solution for Mpemba.
In 1963, an inconspicuous African student named Erasto Mpemba (Erasto Mpemba) noticed one oddity: warm ice cream mix hardens faster than chilled.
The observation seemed so improbable that the teacher of physics could only laugh at the discovery of the unlucky experimenter. However, Erasto was sure he was right and was not afraid to become a laughing stock again: a little later he raised a slippery question with Denis Osborne, a professor at the University of Dar es Salaam, Tanzania. The scientist did not rush to conclusions and decided to study the problem. After that, in 1969, the journal Physics Education published a material describing the Mpemba paradox.
In scientific circles, they immediately recalled that something similar had already been said by the greatest minds of the past. For example, he also mentioned the inhabitants of ancient Greek Pontus, who, during winter fishing, heated water and soaked reeds in it so that it would harden faster. Centuries later, Francis Bacon wrote: "Slightly cool water freezes much more easily than completely cold water."
In general, the question is as old as the world, but this only fuels interest in the solution. Over the past few decades, many theories have been put forward to explain the Mpemba effect. The most probable of them were announced in 2013 at a gala event held by the Royal Society of Chemistry of Great Britain. The professional association studied 22,000 (!) Opinions and singled out among them only one belonging to Nikola Bregović.
The Croatian chemist pointed out the importance of the processes of convection and supercooling of a liquid when it freezes.
Here is how these phenomena are described in Wikipedia:
- Cold water begins to freeze from above, thereby slowing down the processes of thermal radiation and convection, and hence the loss of heat, while hot water begins to freeze from below.
- A supercooled liquid is a liquid that has a temperature below the crystallization temperature at a given pressure. A supercooled liquid is obtained from a normal liquid by cooling in the absence of crystallization centers.
The universal and a check for 1,000 pounds were a good reward. By the way, the winner was greeted by Erasto Mpemba and Denis Osborne.
scienceblogs.comWhat should be the temperature of the water before freezing
There is still no clear answer to this question. The Royal Society of Chemistry, although it was determined, did not completely stop the disputes. Until now, new hypotheses are put forward and denials are heard.
Although there is a small clue: the popular science magazine New Scientist did research and came to the conclusion that the best conditions for replicating the Mpemba effect are two containers of water with a temperature of 35 and 5 ° C.
Thus, if there is very little time left before the party, pour it into water, the temperature of which is comparable to room temperature in the hot summer. Well or cool tap water is better not to use.
Hello, dear lovers of interesting facts. Today we will talk about. But I think that the question posed in the title may seem simply absurd - but is it always necessary to completely trust the notorious "common sense", and not strictly set 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 pure” was mentioned in the works of Aristotle, then similar notes were made by F. Bacon, R. Descartes and J. Black. In recent history, the name “Mpemba paradox” has been attached to this effect - after the name of a schoolboy 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 mixtures in the kitchen. Of course, the classmates who were present there, together with the school teacher, laughed at Mpemba - however, after an experimental check personally by Professor D. Osborne, the desire to make fun of Erasto "evaporated" from them. Moreover, Mpemba, together with the professor, published a detailed description of this effect in 1969 in Physics Education - and since then the above name has been fixed in the scientific literature.
What is the essence of the phenomenon?
The setup of the experiment is quite simple: other things being equal, identical 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 is recorded before the formation of ice in each of them. The paradox is that in a vessel with an initially hotter liquid, this happens faster.
How does modern physics explain this?
The paradox has no universal explanation, since several parallel processes proceed together, the contribution of which may differ from specific initial conditions - but with a uniform result:
- the ability of a liquid to supercool - initially cold water is more prone to hypothermia, i.e. remains liquid when its temperature is already below the 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"
- isolation 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 competition for the best hypothesis was held by the British Royal Society of Chemistry recently, in 2012) - but there is still no unambiguous theory for all cases of combinations of input conditions ...
In 1963, a school student in Tanzania named Erasto Mpemba noticed that if you take two containers with the same amount of milk, in one of which the milk is at room temperature, and in the other - hot, then the hot milk will harden in the freezer much faster than cold. Then he did the same experiment with water, and got exactly the same result. The physics teacher, to whom Mpemba turned for clarification, only laughed in response.
But the inquisitive mind did not give Mpemba rest, and he asked this question to Professor Dennis Osborne, invited from the University College in Dar es Salaam to lecture on physics. Osborn found the question interesting, and in 1969 he and his student published an article about the results of their experiments in the journal Physics Education. Since its publication, this effect has been referred to as the Mpemba effect.
In fact, this effect has been known since ancient times: Aristotle, F. Bacon, R. Descartes were interested in it.
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 formulated by I. Newton, and since that time it has been confirmed many times in practice. And in this case, boiling water, heated to 100 degrees. C cools to 0 deg C faster than the same amount of water with a temperature of, for example, 35 deg. C, although during cooling, boiling water must pass the “threshold” of 35 degrees C.
Here are some of the assumptions that scientists put forward:
- Hot water during cooling actively evaporates, while its volume decreases, respectively, a smaller volume of water cools faster. In addition, due to evaporation, the temperature of the water decreases faster.
- The temperature difference between hot water and air is greater, which means that heat exchange occurs with greater intensity, this leads to the fact that hot water freezes faster.
- A layer of ice forms on the surface of cold water, which not only prevents the evaporation of water, but also acts as a kind of “pillow” that protects the bulk of the water from cooling. And hot water does not have ice on the surface, so the evaporation and cooling process takes longer, heat loss occurs faster, and as a result, hot water turns into ice earlier.
- In hot water, hydrogen bonds are stretched more than in cold water. Since more energy is stored in the hydrogen bonds of hot water, it means that more of it is released when it cools to sub-zero temperatures. This explains why hot water freezes faster.
O:H-O hydrogen bonds in an ice crystal
The good old H2O formula seems to contain no secrets. But in fact, water - the source of life and the most famous liquid in the world - is fraught with many mysteries beyond even scientists. Five of the most famous "strangeness" of water - in front of you.
1. Hot water freezes faster than cold water
Let's take two containers with water: pour hot water into one, and cold water into the other. Let's put them in the freezer. A container with hot water will freeze faster than with cold water, although, logically, the container with cold water should have turned into ice first: after all, hot water must first cool down to cold temperature, and then turn into ice, and cold water does not need to cool down. Why is this happening?
In 1963, high school student Erasto B. Mpemba noticed that a hot mixture would set faster in a freezer than a cold one. The physics teacher, with whom the young man shared his discovery, made him laugh. Fortunately, the student was persistent and convinced the teacher to conduct an experiment, which proved him right. Now the phenomenon of hot water freezing faster than cold water is called the Mpemba effect. Scientists do not fully understand the nature of this phenomenon, explaining it by the difference in hypothermia, evaporation, ice formation, etc.
2. “Supercooling” Prevents Ice Formation
Everyone knows that water always turns to ice when it cools down to zero degrees Celsius... except when it doesn't! “Supercooling” is the tendency of water to remain liquid even when cooled below freezing. This phenomenon becomes possible due to the fact that the environment does not contain crystallization centers or nuclei that could provoke the formation of ice crystals. That is why water remains in liquid form, even when cooled to temperatures below zero degrees Celsius. When the crystallization process starts, one can observe how the “super-cooled” water turns into ice in an instant. See for yourself - watch the video on our website.
3. Glassy water
Quickly, without hesitation, tell me, how many different states does water have? Did you say three? Solid, liquid, gaseous? And here it is not. Scientists identify at least 5 states of “liquid” water and 14 states of ice. Remember the talk about super-cooled water? So, no matter what you do, at -38 ° C, the most super-cooled water will suddenly turn into ice. And the WTO will occur with a further decrease in temperature? At -120 °C, ice becomes viscous like molasses, and at -135 °C and below, it turns into “glassy” or “vitreous” water, a solid with no crystals.
4. Quantum number of water
At the molecular level, water has something to surprise scientists with. In 1995, a neutron scattering experiment conducted by scientists gave an unexpected result: it was found that neutrons directed at water molecules "see" 25% less hydrogen protons. It turned out that at the speed of one attosecond (10 to minus 18 seconds) an unusual quantum effect takes place, and the chemical formula of water from the usual H2O turns into H1.5O!
What is one attosecond, you ask? This is the time it takes for light to travel a distance comparable to the size of a water molecule.
5. Does water have a memory?
Homeopathy, an alternative to official medicine, claims that a weak solution of a drug can have a healing effect on the body and retain the properties of the solution of the original concentration, even if the dilution factor is so large that nothing but water molecules remain in the solution. Adherents of homeopathy as a method of treatment explain this paradox with a concept called “memory of water”. In 2002, an international group of scientists led by Professor Madeline Ennis from Queen's University in Belfast, who had previously criticized the principles of homeopathy, announced that she had been able to prove the reality of the "water memory" effect. However, experiments conducted under the supervision of independent experts did not bring results. The discussion about the phenomenon of “memory of water” continues.
