This is true, although it sounds incredible, because in the process of freezing, preheated water must pass the temperature of cold water. Meanwhile, this effect is widely used. For example, ice rinks and slides are filled with hot water instead of cold water in winter. Experts advise motorists to pour cold rather than hot water into the washer reservoir in winter. The paradox is known worldwide as the "Mpemba Effect".
This phenomenon was mentioned at one time by Aristotle, Francis Bacon and Rene Descartes, but only in 1963 did physics professors pay attention to it and try to investigate it. It all started when Tanzanian schoolboy Erasto Mpemba noticed that the sweetened milk he used to make ice cream solidified faster if it was preheated and suggested that hot water freezes faster than cold water. He turned to the physics teacher for clarification, but he only laughed at the student, saying the following: "This is not world physics, but the physics of Mpemba."
Fortunately, Dennis Osborn, a professor of physics from the University of Dar es Salaam, visited the school one day. And Mpemba turned to him with the same question. The professor was less skeptical, said that he could not judge what he had never seen, and upon returning home asked the staff to conduct appropriate experiments. It looks like they confirmed the boy's words. In any case, in 1969, Osborne spoke about working with Mpemba in the magazine "Eng. PhysicsEducation". In the same year, George Kell of the Canadian National Research Council published an article describing the phenomenon in English. AmericanJournalofPhysics».
There are several possible explanations for this paradox:
- Hot water evaporates faster, thereby reducing its volume, and a smaller volume of water with the same temperature freezes faster. In sealed containers cold water should freeze faster.
- The presence of snow lining. The hot water container melts the snow underneath, thereby improving thermal contact with the cooling surface. Cold water does not melt snow under it. With no snow lining, the cold water container should freeze faster.
- 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. With additional mechanical agitation of the water in the containers, the cold water should freeze faster.
- The presence of crystallization centers in the cooled water - substances dissolved in it. With a small number of such centers in cold water, the transformation of water into ice is difficult, and even its supercooling is possible when it remains in a liquid state, having a sub-zero temperature.
Another explanation has recently been published. Dr. Jonathan Katz of the University of Washington investigated this phenomenon and concluded that substances dissolved in water play an important role and precipitate when heated.
Under dissolved substances dr Katz refers to the calcium and magnesium bicarbonates found in hard water. When the water is heated, these substances precipitate, the water becomes "soft". Water that has never been heated contains these impurities and is "hard". As it freezes and ice crystals form, the concentration of impurities in water increases 50 times. This lowers the freezing point of water.
This explanation does not seem convincing to me, because. we must not forget that the effect was found in experiments with ice cream, and not with hard water. Most likely, the causes of the phenomenon are thermophysical, and not chemical.
So far, no unambiguous explanation of the Mpemba paradox has been received. I must say that some scientists do not consider this paradox worthy of attention. However, it is very interesting that a simple schoolboy has achieved recognition of the physical effect and gained popularity because of his curiosity and perseverance.
Added February 2014
The note was written in 2011. Since then, new studies of the Mpemba effect and new attempts to explain it have appeared. So, in 2012, the Royal Society of Chemistry of Great Britain announced an international competition to unravel the scientific mystery “The Mpemba Effect” with a prize fund of 1000 pounds. The deadline was set on July 30, 2012. The winner was Nikola Bregovik from the laboratory of the University of Zagreb. He published his work, in which he analyzed previous attempts to explain this phenomenon and came to the conclusion that they were not convincing. The model he proposed is based on the fundamental properties of water. Those interested can find a job at http://www.rsc.org/mpemba-competition/mpemba-winner.asp
The research didn't end there. In 2013, physicists from Singapore theoretically proved the cause of the Mepemba effect. The work can be found at http://arxiv.org/abs/1310.6514.
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Comments:
Alexey Mishnev. , 06.10.2012 04:14
Why does hot water evaporate faster? Scientists have practically proven that a glass of hot water freezes faster than cold water. Scientists cannot explain this phenomenon for the reason that they do not understand the essence of phenomena: heat and cold! Heat and cold are physical sensations caused by the interaction of particles of Matter, in the form of counter compression of magnetic waves that move from the side of space and from the center of the earth. Therefore, the greater the potential difference of this magnetic voltage, the faster the energy exchange is carried out by the method of counter-penetration of one wave into another. That is, by diffusion! In response to my article, one opponent writes: 1) “..Hot water evaporates FASTER, as a result of which there is less of it, so it freezes faster” Question! What energy makes water evaporate faster? 2) In my article, we are talking about a glass, and not about a wooden trough, which the opponent cites as a counterargument. What is not correct! I answer the question: “FOR WHAT REASON DOES WATER EVAPORATION IN NATURE?” Magnetic waves, which always move from the center of the earth into space, overcoming the counter pressure of magnetic compression waves (which always move from space to the center of the earth), at the same time, spray water particles, since moving into space, they increase in volume. That is, expand! In case of overcoming the magnetic waves of compression, these water vapors are compressed (condensed) and under the influence of these magnetic compression forces, the water returns to the ground in the form of precipitation! Sincerely! Alexey Mishnev. October 6, 2012.
Alexey Mishnev. , 06.10.2012 04:19
What is temperature. Temperature is the degree of electromagnetic stress of magnetic waves with the energy of compression and expansion. In the case of an equilibrium state of these energies, the temperature of the body or substance is in a stable state. If the equilibrium state of these energies is disturbed, towards the energy of expansion, the body or substance increases in the volume of space. In case of exceeding the energy of magnetic waves in the direction of compression, the body or substance decreases in the volume of space. The degree of electromagnetic stress is determined by the degree of expansion or contraction of the reference body. Alexey Mishnev.
Moiseeva Natalia, 23.10.2012 11:36 | VNIIM
Alexey, you are talking about some article that outlines your thoughts on the concept of temperature. But no one read it. Please give me a link. In general, your views on physics are very peculiar. I have never heard of "electromagnetic expansion of the reference body".
Yuri Kuznetsov , 04.12.2012 12:32
A hypothesis is proposed that this is the work of intermolecular resonance and the ponderomotive attraction between molecules generated by it. In cold water, molecules move and vibrate randomly, with different frequencies. When water is heated, with an increase in the oscillation frequency, their range narrows (the frequency difference from liquid hot water to the point of vaporization decreases), the oscillation frequencies of the molecules approach each other, as a result of which a resonance occurs between the molecules. When cooled, this resonance is partially preserved, it does not die out immediately. Try pressing one of the two guitar strings that are in resonance. Now let go - the string will begin to vibrate again, the resonance will restore its vibrations. So in frozen water, the outer cooled molecules try to lose the amplitude and frequency of vibrations, but the “warm” molecules inside the vessel “pull” the vibrations back, act as vibrators, and the outer ones act as resonators. It is between the vibrators and the resonators that the ponderomotive attraction* arises. When the ponderomotive force becomes greater than the force caused by the kinetic energy of the molecules (which not only vibrate, but also move linearly), accelerated crystallization occurs - the "Mpemba Effect". The ponderomotive connection is very unstable, the Mpemba effect strongly depends on all accompanying factors: the volume of water to be frozen, the nature of its heating, freezing conditions, temperature, convection, heat exchange conditions, gas saturation, vibration of the refrigeration unit, ventilation, impurities, evaporation, etc. Perhaps even from lighting... Therefore, the effect has a lot of explanations and is sometimes difficult to reproduce. For the same “resonance” reason, boiled water boils faster than unboiled water - resonance for some time after boiling preserves the intensity of vibrations of water molecules (energy loss during cooling is mainly due to the loss of kinetic energy of the linear motion of molecules). With intense heating, vibrator molecules change roles with resonator molecules in comparison with freezing - the frequency of the vibrators is less than the frequency of the resonators, which means that there is not attraction between the molecules, but repulsion, which accelerates the transition to another state of aggregation (pair).
Vlad, 11.12.2012 03:42
Broke my brain...
Anton , 04.02.2013 02:02
1. Is this ponderomotive attraction really so great that it affects the heat transfer process? 2. Does this mean that when all bodies are heated to a certain temperature, their structural particles enter into resonance? 3. Why does this resonance disappear upon cooling? 4. Is this your guess? If there is a source, please indicate. 5. According to this theory, the shape of the vessel will play an important role, and if it is thin and flat, then the difference in freezing time will not be large, i.e. you can check it.
Gudrat , 11.03.2013 10:12 | METAK
Cold water already has nitrogen atoms and the distances between water molecules are closer than in hot water. That is, the conclusion: Hot water absorbs nitrogen atoms faster and at the same time it quickly freezes than cold water - this is comparable to the hardening of iron, since hot water turns into ice and hot iron hardens upon rapid cooling!
Vladimir , 03/13/2013 06:50
or maybe this: the density of hot water and ice is less than the density of cold water, and therefore water does not need to change its density, losing some time on this and it freezes.
Alexey Mishnev , 03/21/2013 11:50 am
Before talking about resonances, attraction and vibrations of particles, it is necessary to understand and answer the question: What forces make particles vibrate? Since, without kinetic energy, there can be no compression. Without compression, there can be no expansion. Without expansion, there can be no kinetic energy! When you start talking about the resonance of strings, you first made an effort to make one of these strings start to vibrate! When talking about attraction, you must first of all indicate the force that makes these bodies attract! I affirm that all bodies are compressed by the electromagnetic energy of the atmosphere and which compresses all bodies, substances and elementary particles with a force of 1.33 kg. not per cm2, but per elementary particle. Since the pressure of the atmosphere cannot be selective! Do not confuse it with the amount of force!
Dodik , 05/31/2013 02:59
It seems to me that you have forgotten one truth - "Science begins where measurements begin." What is the temperature of the "hot" water? What is the temperature of "cold" water? The article doesn't say a word about it. From this we can conclude - the whole article is bullshit!
Grigory, 06/04/2013 12:17
Dodik, before calling an article nonsense, one must think to learn, at least a little. And not just measure.
Dmitry , 12/24/2013 10:57 AM
Hot water molecules move faster than in cold water, because of this there is a closer contact with the environment, they seem to absorb all the cold, quickly slowing down.
Ivan, 10.01.2014 05:53
It is surprising that such an anonymous article appeared on this site. The article is completely unscientific. Both the author and the commentators vying with each other set off in search of an explanation of the phenomenon, not bothering to find out whether the phenomenon is observed at all and, if so, under what conditions. Moreover, there is not even an agreement on what we actually observe! So the author insists on the need to explain the effect of rapid freezing of hot ice cream, although from the entire text (and the words "the effect was discovered in experiments with ice cream") it follows that he himself did not set up such experiments. From the variants of "explanation" of the phenomenon listed in the article, it can be seen that completely different experiments are described, set up under different conditions with different aqueous solutions. Both the essence of the explanations and the subjunctive mood in them suggest that even an elementary verification of the ideas expressed was not carried out. Someone accidentally heard a curious story and casually expressed his speculative conclusion. I'm sorry but it's not physical Scientific research, and conversation in a smoking-room.
Ivan , 01/10/2014 06:10
Regarding the comments in the article about filling the rollers with hot water and cold washer reservoirs. Everything is simple from the point of view of elementary physics. The skating rink is filled with hot water just because it freezes more slowly. The rink must be level and smooth. Try to fill it with cold water - you will get bumps and "influxes", because. water will _quickly_ freeze without having time to spread in a uniform layer. And the hot one will have time to spread in an even layer, and it will melt the existing ice and snow bumps. With a washer, it’s also not difficult: there is no point in pouring clean water in frost - it freezes on glass (even hot); and hot non-freezing liquid can lead to cracking of cold glass, plus it will have an increased freezing point on the glass due to the accelerated evaporation of alcohols on the way to the glass (does everyone know the principle of the moonshine still? - alcohol evaporates, water remains).
Ivan , 01/10/2014 06:34
But in fact the phenomenon, it is silly to ask why two different experiments in different conditions proceed differently. If the experiment is set up cleanly, then you need to take hot and cold water of the same chemical composition - we take pre-chilled boiling water from the same kettle. Pour into identical vessels (for example, thin-walled glasses). We put not on the snow, but on the same even, dry base, for example, a wooden table. And not in a microfreezer, but in a sufficiently voluminous thermostat - I conducted an experiment a couple of years ago in the country, when there was stable frosty weather outside, about -25C. Water crystallizes at a certain temperature after the release of the heat of crystallization. The hypothesis boils down to the statement that hot water cools faster (this is true, in accordance with classical physics, the heat transfer rate is proportional to the temperature difference), but maintains an increased cooling rate even when its temperature is equal to the temperature of cold water. The question is, how does water that has cooled to a temperature of +20C outside differ from exactly the same water that has cooled to a temperature of +20C an hour before, but in a room? Classical physics (by the way, based not on chatter in a smoking room, but on hundreds of thousands and millions of experiments) says: yes, nothing, further cooling dynamics will be the same (only boiling water will reach the +20 point later). And the experiment shows the same thing: when there is already a solid crust of ice in a glass of initially cold water, hot water did not even think of freezing. P.S. To the comments of Yuri Kuznetsov. The presence of a certain effect can be considered established when the conditions for its occurrence are described and it is stably reproduced. And when we have incomprehensible experiments with unknown conditions, it is premature to build theories of their explanation and this does not give anything from a scientific point of view. P.P.S. Well, it’s impossible to read Alexei Mishnev’s comments without tears of emotion - a person lives in some kind of fictional world that has nothing to do with physics and real experiments.
Grigory, 01/13/2014 10:58 AM
Ivan, I understand that you refute the Mpemba effect? It does not exist, as your experiments show? Why is it so famous in physics, and why do many try to explain it?
Ivan , 02/14/2014 01:51
Good afternoon, Gregory! The effect of an impurely staged experiment exists. But, as you understand, this is not a reason to look for new patterns in physics, but a reason to improve the skill of the experimenter. As I already noted in the comments, in all the mentioned attempts to explain the “Mpemba effect”, researchers cannot even clearly articulate what exactly and under what conditions they are measuring. And you want to say that these are experimental physicists? Do not make me laugh. The effect is known not in physics, but in pseudo-scientific discussions on various forums and blogs, of which the sea is now. As a real physical effect (in the sense as a consequence of some new physical laws, and not as a consequence of an incorrect interpretation or just a myth), people who are far from physics perceive it. So there is no reason to speak as a single physical effect about the results of different experiments set up under completely different conditions.
Pavel, 02/18/2014 09:59
hmm, guys... article for "Speed Info"... No offense... ;) Ivan is right about everything...
Gregory, 02/19/2014 12:50 pm
Ivan, I agree that there are a lot of pseudo-scientific sites publishing unverified sensational material now.? After all, the effect of Mpemba is still being studied. Moreover, scientists from universities are researching. For example, in 2013 this effect was studied by a group of University of Technology in Singapore. Look at the link http://arxiv.org/abs/1310.6514. They believe they have found an explanation for this effect. I will not write in detail about the essence of the discovery, but in their opinion, the effect is associated with the difference in energies stored in hydrogen bonds.
Moiseeva N.P. , 02/19/2014 03:04
For everyone interested in research on the Mpemba effect, I have slightly supplemented the material of the article and provided links where you can get acquainted with the latest results (see text). Thanks for the comments.
Ildar , 02/24/2014 04:12 | it makes no sense to list everything
If this Mpemba effect really takes place, then the explanation must be sought, I think, in the molecular structure of water. Water (as I learned from the popular science literature) exists not as individual H2O molecules, but as clusters of several molecules (even dozens). With an increase in water temperature, the speed of movement of molecules increases, the clusters break up against each other and the valence bonds of the molecules do not have time to assemble large clusters. It takes a little more time to form clusters than to slow down the speed of molecules. And since the clusters are smaller, the formation of the crystal lattice is faster. In cold water, apparently, large, fairly stable clusters prevent the formation of a lattice; it takes some time for their destruction. I myself saw on TV a curious effect, when cold water standing quietly in a jar remained liquid for several hours in the cold. But as soon as the jar was picked up, that is, slightly moved from its place, the water in the jar immediately crystallized, became opaque, and the jar burst. Well, the priest who showed this effect explained it by the fact that the water was consecrated. By the way, it turns out that water greatly changes its viscosity depending on temperature. We, as large creatures, do not notice this, but at the level of small (mm and less) crustaceans, and even more so bacteria, the viscosity of water is a very significant factor. This viscosity, I think, is also given by the size of the water clusters.
GREY , 03/15/2014 05:30
everything around that we see is superficial characteristics (properties), so we take for energy only what we can measure or prove existence in any way, otherwise it's a dead end. This phenomenon, the Mpemba effect, can only be explained by a simple volumetric theory that will unite all physical models into a single structure of interaction. actually it's simple
Nikita, 06/06/2014 04:27 | car
but how to make the water stay cold and not be warm when you go in the car!
alexey, 03.10.2014 01:09
And here is another "discovery", on the go. Water in a plastic bottle freezes much faster with an open stopper. For the sake of fun, I experimented many times in severe frost. The effect is obvious. Hello theorists!
Eugene , 12/27/2014 08:40
The principle of an evaporative cooler. We take two hermetically sealed bottles with cold and hot water. We put it in the cold. Cold water freezes faster. Now we take the same bottles with cold and hot water, open it and put it in the cold. Hot water will freeze faster than cold water. If we take two basins with cold and hot water, then hot water will freeze much faster. This is due to the fact that we increase contact with the atmosphere. The more intense the evaporation, the faster the temperature drop. Here it is necessary to mention the factor of humidity. The lower the humidity, the stronger the evaporation and the stronger the cooling.
gray TOMSK, 03/01/2015 10:55
GREY, 15.03.2014 05:30 - continued What you know about temperature is not everything. There is something else. If you correctly compose a physical model of temperature, then it will become the key to describing energy processes from diffusion, melting and crystallization to such scales as an increase in temperature with an increase in pressure, an increase in pressure with an increase in temperature. Even the physical model of the Sun's energy will become clear from the above. I am in winter. . in the early spring of 20013, after looking at the temperature models, I compiled a general temperature model. After a couple of months, I remembered the temperature paradox, and then I realized ... that my temperature model also describes the Mpemba paradox. This was in May - June 2013. A year late, but that's for the best. My physical model is a freeze frame and it can be scrolled both forward and backward and it has the motor skills of activity, the very activity in which everything moves. I have 8 classes of school and 2 years of college with a repetition of the topic. 20 years have passed. So I can’t ascribe any kind of physical models of famous scientists, as well as formulas. So sorry.
Andrey , 08.11.2015 08:52
In general, I have an idea about why hot water freezes faster than cold water. And in my explanations everything is very simple if you are interested then write me an email: [email protected]
Andrey , 08.11.2015 08:58
I'm sorry, I gave the wrong mailbox here is the correct email: [email protected]
Victor , 12/23/2015 10:37 AM
It seems to me that everything is simpler, snow falls with us, it is evaporated gas, cooled, and maybe in frost it cools faster hot because it evaporates and immediately crystallizes far from rising, and water in a gaseous state cools faster than in liquid)
Bekzhan , 01/28/2016 09:18
Even if someone revealed these laws of the world that are associated with this effect, he would not write here. From my point of view, it would not be logical to reveal his secrets to Internet users when he can publish it in famous scientific journals and prove it himself in front of the people. So, what will be written about this effect here, all this majority is not logical.)))
Alex , 02/22/2016 12:48 PM
Hello Experimenters You are right in saying that Science begins where... not Measurements, but Calculations. "Experiment" - an eternal and indispensable argument for those deprived of Imagination and Linear thinking Offended everyone, now in the case of E \u003d mc2 - does everyone remember? The speed of molecules flying out of cold water into the atmosphere determines the amount of energy they carry away from water (cooling - loss of energy) The speed of molecules from hot water is much higher and the energy carried away is squared (the rate of cooling of the remaining mass of water) That's all, if you leave from " experimentation" and remember the Basics of Science
Vladimir , 04/25/2016 10:53 AM | Meteo
In those days when antifreeze was a rarity, the water from the cooling system of cars in an unheated garage of a car fleet was drained after a working day so as not to defrost the cylinder block or radiator - sometimes both together. Hot water was poured in the morning. In severe frost, the engines started without problems. Somehow, due to the lack of hot water, water was poured from the tap. The water immediately froze. The experiment was expensive - exactly as much as it costs to buy and replace the cylinder block and radiator of a ZIL-131 car. Who does not believe, let him check. and Mpemba experimented with ice cream. In ice cream, crystallization proceeds differently than in water. Try biting off a piece of ice cream and a piece of ice with your teeth. Most likely it did not freeze, but thickened as a result of cooling. And fresh water, whether it is hot or cold, freezes at 0*C. Cold water is fast, but hot water needs time to cool.
Wanderer , 06.05.2016 12:54 | to Alex
"c" - speed of light in vacuum E=mc^2 - formula expressing the equivalence of mass and energy
Albert , 07/27/2016 08:22
First, an analogy with solids (there is no evaporation process). Recently soldered copper water pipes. The process occurs by heating the gas burner to the melting temperature of the solder. The heating time of one joint with the coupling is approximately one minute. I soldered one joint with the coupling and after a couple of minutes I realized that I soldered it wrong. It took a little to scroll the pipe in the coupling. I began to heat the joint again with a burner and, surprisingly, it took 3-4 minutes to heat the joint to the melting point. How so!? After all, the pipe is still hot and it would seem that much less energy is needed to heat it to the melting point, but everything turned out to be the opposite. It's all about the thermal conductivity, which is much higher for an already heated pipe and the boundary between the heated and cold pipes managed to move far from the junction in two minutes. Now about water. We will operate with the concepts of hot and semi-heated vessel. In a hot vessel, a narrow temperature boundary is formed between hot, highly mobile particles and slow-moving, cold ones, which moves relatively quickly from the periphery to the center, because at this boundary, fast particles quickly give up their energy (cool) by particles on the other side of the boundary. Since the volume of the outer cold particles is larger, the fast particles, giving up their thermal energy, cannot significantly heat the outer cold particles. Therefore, the process of cooling hot water occurs relatively quickly. Semi-heated water, on the other hand, has a much lower thermal conductivity, and the width of the boundary between semi-heated and cold particles is much wider. The displacement to the center of such a wide boundary occurs much more slowly than in the case of a hot vessel. As a result, a hot vessel cools faster than a warm one. I think it is necessary to trace the process of cooling water of different temperatures in dynamics by placing several temperature sensors from the middle to the edge of the vessel.
Max , 11/19/2016 05:07
It has been verified: in Yamal, in frost, a pipe with hot water freezes and it has to be warmed up, but not cold!
Artem, 09.12.2016 01:25
It is difficult, but I think that cold water is denser than hot water, even better than boiled water, and then there is an acceleration in cooling, i.e. hot water reaches the cold temperature and overtakes it, and given that hot water freezes from the bottom and not from the top as written above, this speeds up the process a lot!
Alexander Sergeev, 21.08.2017 10:52
There is no such effect. Alas. In 2016, a detailed article on the topic was published in Nature: https://en.wikipedia.org/wiki/Mpemba_effect From it it is clear that if the experiments are carried out carefully (if the samples of warm and cold water are the same in everything except temperature), the effect is not observed .
Headlab, 08/22/2017 05:31
Victor , 10/27/2017 03:52 AM
"It really is." - if the school did not understand what heat capacity and the law of conservation of energy are. It's easy to check - for this you need: a desire, a head, hands, water, a refrigerator and an alarm clock. And the skating rinks, as experts write, are frozen (filled) with cold water, and with warm water they level the cut ice. And in the winter you need to pour anti-freeze fluid into the washer reservoir, not water. Water will freeze anyway, and cold water will freeze faster.
Irina , 01/23/2018 10:58
Scientists all over the world have been struggling with this paradox since the time of Aristotle, and Viktor, Zavlab and Sergeev turned out to be the smartest.
Denis , 02/01/2018 08:51
Everything is right in the article. But the reason is somewhat different. In the process of boiling, the air dissolved in it is evaporated from the water, therefore, as the boiling water cools, as a result, its density will be less than that of raw water of the same temperature. There are no other reasons for different thermal conductivity except for different density.
Headlab, 03/01/2018 08:58 | head lab
Irina :), "scientists of the whole world" do not fight this "paradox", for real scientists this "paradox" simply does not exist - this is easily verified in well-reproducible conditions. The "paradox" appeared due to the irreproducible experiments of the African boy Mpemba and was inflated by similar "scientists" :)
miroland, 03/23/2019 07:20 AM
a Tanzanian boy living in the heart of Africa, who, very likely, has never seen snow in his eyes ... ;-D I'm not confusing anything ???)))
Sergey , 04/14/2019 02:02
We take two elastic bands, stretch both, and one is larger than the other (an analogy with internal energy cold and warm water) at the same time release one end of the rubber bands. Which rubber will shrink faster?
Artanis , 05/08/2019 03:34
Just had this experience myself. I put two identical cups of hot and cold water in the freezer. The cold one froze much faster. The hot one was still a little warm. What is wrong in my experience?
Headlab, 05/09/2019 06:21 |
Artanis, With your experience, "everything is right" :) - The "Mpemba effect" does not exist with a correctly performed experiment, which ensures the identity of the cooling conditions for identical volumes of water only with different initial temperatures. Congratulations - you have switched to the side of light, reason and the triumph of basic physical laws and began to move away from the "Mpemba sect", and fans of YouTube videos in the style of "what they lied to us about in physics lessons" ... :)
Moiseeva N.P. , 05/16/2019 04:30 | Ch. editor
You are right, a lot depends on the conditions of the experiment. But if the effect were not observed at all, then there would be no research and publications in serious journals. Did you read the note to the end? There is no mention of YouTube videos here.
Headlab, 08/06/2019 05:26 | SlavOilGas-SouthNorthWestVostok-SintezWhatever
Natalya Petrovna, we are living in an era of a “reproducibility crisis” in science, when, in order to increase the citation index under the slogan “publish or perish”, “unfortunate scientists” prefer to compete in inventing crazy theories to substantiate obviously dubious experimental data instead of spending a little time and resources to verify these data before sitting down to a purely theoretical article. An example of such "unfortunate scientists" is just the "physicists from Singapore" that you mentioned in the article - their publication does not contain their own experimental data, but only bare theoretical arguments about the possible influence of the abstruse phenomenon "O:H-O Bond Anomalous Relaxation" on the process of anomalous freezing of water, which was observed by both Francis Bacon and Rene Descartes and even Aristotle as early as 350 years BC. ... And personally, I am very glad that Nikola Bregovic from the University of Zagreb received his prize 1000 pounds from the Royal Society of Chemistry of Great Britain after he measured quite physically explicable results without any anomalies on good equipment in reproducible conditions and questioned them as clumsy measurements boy Mpemba and his adepts and the adequacy of those who tried to bring these clumsy experiments "theoretical base".
In this article, we will look at why hot water freezes faster than cold water.
Heated water freezes much faster than cold water! This amazing property of water, the exact explanation for which scientists still cannot find, has been known since ancient times. For example, even in Aristotle there is a description of winter fishing: the fishermen inserted fishing rods into holes in the ice, and in order for them to freeze faster, they poured warm water on the ice. The name of this phenomenon was named after Erasto Mpemba in the 60s of the XX century. Mnemba noticed the strange effect while making ice cream and turned to his physics teacher, Dr. Denis Osborn, 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 have carried out their own experiments and each time they have obtained similar results.
Explanation of a physical phenomenon
There is no generally accepted explanation as to why this is happening. Many researchers suggest that it's all about the supercooling of a liquid, which occurs when its temperature drops below freezing. 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 become supercooled at first, and will only harden after some time. In heated water, other processes take place. Its faster transformation into ice is associated with convection.
Convection- This is a physical phenomenon in which the warm lower layers of the liquid rise, and the upper, cooled ones, fall.
Mpemba effect(Mpemba's paradox) is a paradox that states that hot water, under certain conditions, freezes faster than cold water, although it must pass the temperature of cold water in the process of freezing. This paradox is an experimental fact that contradicts the usual ideas, according to which, under the same conditions, a hotter body needs more time to cool down to a certain temperature than a cooler body to cool down to the same temperature.
This phenomenon was noticed at the time by Aristotle, Francis Bacon and Rene Descartes, but only in 1963, the Tanzanian schoolboy Erasto Mpemba found that a hot ice cream mixture freezes faster than a cold one.
As a student of the Magamba high school in Tanzania, Erasto Mpemba did practical cooking work. He had to make homemade ice cream - boil milk, dissolve sugar in it, cool it to room temperature, and then put it in the refrigerator to freeze. Apparently, Mpemba was not a particularly diligent student and procrastinated on the first part of the assignment. Fearing that he would not be in time by the end of the lesson, he put the still hot milk in the refrigerator. To his surprise, it froze even earlier than the milk of his comrades, prepared according to a given technology.
After that, Mpemba experimented not only with milk, but also with ordinary water. In any case, already being a student at Mkwawa High School, he asked Professor Dennis Osborne from the University College in Dar es Salaam (invited by the director of the school to give a lecture on physics to students) about water: "If you take two identical containers with equal volumes of water so that in one of them the water has a temperature of 35 ° C, and in the other - 100 ° C, and put them in the freezer, then in the second the water will freeze faster. Why? Osborne became interested in this issue and soon in 1969, together with Mpemba, they published the results of their experiments in the journal "Physics Education". Since then, the effect they discovered is called Mpemba effect.
Until now, no one knows exactly how to explain this strange effect. Scientists do not have a single version, although there are many. It's all about the difference in the properties of hot and cold water, but it is not yet clear which properties play a role in this case: the difference in supercooling, evaporation, ice formation, convection, or the effect of liquefied gases on water at different temperatures.
The paradox of the Mpemba effect is that the time during which the body cools down to the ambient temperature must be proportional to the temperature difference between this body and the environment. This law was established by Newton and since then has been confirmed many times in practice. In the same effect, water at 100°C cools down to 0°C faster than the same amount of water at 35°C.
However, this does not yet imply a paradox, since the Mpemba effect can also be explained within known physics. Here are some explanations for the Mpemba effect:
Evaporation
Hot water evaporates faster from the container, thereby reducing its volume, and a smaller volume of water with the same temperature freezes faster. Water heated to 100 C loses 16% of its mass when cooled to 0 C.
The evaporation effect is a double effect. First, the mass of water required for cooling is reduced. And secondly, the temperature decreases due to the fact that the heat of evaporation of the transition from the water phase to the vapor phase decreases.
temperature difference
Due to the fact that the temperature difference between hot water and cold air is greater - hence the heat exchange in this case is more intense and hot water cools faster.
hypothermia
When water is cooled below 0 C, it does not always freeze. Under certain conditions, it can undergo supercooling while continuing to remain liquid at temperatures below the freezing point. In some cases, water can remain liquid even at -20 C.
The reason for this effect is that in order for the first ice crystals to begin to form, centers of crystal formation are needed. If they are not in liquid water, then supercooling will continue until the temperature drops enough that crystals begin to form spontaneously. When they start to form in the supercooled liquid, they will start to grow faster, forming an ice slush that will freeze to form ice.
Hot water is most susceptible to hypothermia because heating it eliminates dissolved gases and bubbles, which in turn can serve as centers for the formation of ice crystals.
Why does hypothermia cause hot water to freeze faster? In the case of cold water, which is not supercooled, the following occurs. In this case, a thin layer of ice will form on the surface of the vessel. This layer of ice will act as an insulator between the water and cold air and will prevent further evaporation. The rate of formation of ice crystals in this case will be less. In the case of hot water undergoing subcooling, the subcooled water does not have a protective surface layer of ice. Therefore, it loses heat much faster through the open top.
When the supercooling process ends and the water freezes, much more heat is lost and therefore more ice is formed.
Many researchers of this effect consider hypothermia to be the main factor in the case of the Mpemba effect.
Convection
Cold water begins to freeze from above, thereby worsening the processes of heat radiation and convection, and hence the loss of heat, while hot water begins to freeze from below.
This effect is explained by an anomaly in the density of water. Water has a maximum density at 4 C. If you cool water to 4 C and put it at a lower temperature, the surface layer of water will freeze faster. Because this water is less dense than water at 4°C, it will stay on the surface, forming a thin cold layer. Under these conditions, a thin layer of ice will form on the surface of the water for a short time, but this layer of ice will serve as an insulator protecting the lower layers of water, which will remain at a temperature of 4 C. Therefore, further cooling will be slower.
In the case of hot water, the situation is completely different. The surface layer of water will cool more quickly due to evaporation and a greater temperature difference. Also, cold water layers are denser than hot water layers, so the cold water layer will sink down, lifting the warm water layer to the surface. This circulation of water ensures a rapid drop in temperature.
But why does this process not reach the equilibrium point? To explain the Mpemba effect from this point of view of convection, it would be necessary to assume that the cold and hot layers of water are separated and the convection process itself continues after the average water temperature drops below 4 C.
However, there is no experimental evidence to support this hypothesis that cold and hot layers of water are separated by convection.
gases dissolved in water
Water always contains gases dissolved in it - oxygen and carbon dioxide. These gases have the ability to lower the freezing point of water. When the water is heated, these gases are released from the water because their solubility in water at high temperature is lower. Therefore, when hot water is cooled, there are always fewer dissolved gases in it than in unheated cold water. Therefore, the freezing point of heated water is higher and it freezes faster. This factor is sometimes considered as the main one in explaining the Mpemba effect, although there are no experimental data confirming this fact.
Thermal conductivity
This mechanism can play a significant role when water is placed in a refrigerator freezer in small containers. Under these conditions, it has been observed that the container with hot water melts the ice of the freezer under itself, thereby improving thermal contact with the wall of the freezer and thermal conductivity. As a result, heat is removed from the hot water container faster than from the cold one. In turn, the container with cold water does not melt snow under it.
All these (as well as other) conditions have been studied in many experiments, but an unequivocal answer to the question - which of them provide a 100% reproduction of the Mpemba effect - has not been obtained.
So, for example, in 1995, the German physicist David Auerbach studied the influence of supercooling of water on this effect. He discovered that hot water, reaching a supercooled state, freezes at a higher temperature than cold water, and therefore faster than the latter. But cold water reaches the supercooled state faster than hot water, thereby compensating for the previous lag.
In addition, Auerbach's results contradicted earlier data that hot water is able to achieve more supercooling due to fewer crystallization centers. When water is heated, the gases dissolved in it are removed from it, and when it is boiled, some salts dissolved in it precipitate.
So far, only one thing can be asserted - the reproduction of this effect essentially depends on the conditions under which the experiment is carried out. Precisely because it is not always reproduced.
Which water freezes faster, hot or cold, is influenced by many factors, but the question itself seems a little strange. It is implied, and it is known from physics, that hot water still needs time to cool down to the temperature of comparable cold water in order to turn into ice. this stage can be skipped, and, accordingly, she wins in time.
But the answer to the question of which water freezes faster - cold or hot - on the street in frost, any inhabitant of the northern latitudes knows. In fact, scientifically, it turns out that in any case, cold water simply has to freeze faster.
So did the teacher of physics, who was approached by the schoolboy Erasto Mpemba in 1963 with a request to explain why the cold mixture of future ice cream freezes longer than a similar, but hot one.
"This is not world physics, but some kind of Mpemba physics"
At that time, the teacher only laughed at this, but Deniss Osborne, a professor of physics, who at one time went to the same school where Erasto studied, experimentally confirmed the existence of such an effect, although there was no explanation for this then. In 1969 a popular scientific journal published a joint article by the two men who described this peculiar effect.
Since then, by the way, the question of which water freezes faster - hot or cold, has its own name - the effect, or paradox, Mpemba.
The question has been around for a long time
Naturally, such a phenomenon has taken place before, and it was mentioned in the works of other scientists. Not only the schoolboy was interested in this question, but Rene Descartes and even Aristotle thought about it at one time.
Here are just approaches to solving this paradox began to look only at the end of the twentieth century.
Conditions for a paradox to occur
As with ice cream, it's not just ordinary water that freezes during the experiment. Certain conditions must be present in order to start arguing which water freezes faster - cold or hot. What influences this process?
Now, in the 21st century, several options have been put forward that can explain this paradox. Which water freezes faster, hot or cold, may depend on the fact that it has a higher evaporation rate than cold water. Thus, its volume decreases, and with a decrease in volume, the freezing time becomes shorter than if we take a similar initial volume of cold water.
Freezer has long been defrosted
Which water freezes faster, and why it does so, can be affected by the snow lining that may be present in the freezer of the refrigerator used for the experiment. If you take two containers that are identical in volume, but one of them will have hot water and the other cold water, the container with hot water will melt the snow under it, thereby improving the contact of the thermal level with the refrigerator wall. A cold water container can't do that. If there is no such lining with snow in the refrigerator, cold water should freeze faster.
Top - bottom
Also, the phenomenon of which water freezes faster - hot or cold, is explained as follows. Following certain laws, cold water starts to freeze from the upper layers, when hot water does it the other way around - it starts to freeze from the bottom up. It turns out that cold water, having a cold layer on top with ice already formed in some places, thus worsens the processes of convection and thermal radiation, thereby explaining which water freezes faster - cold or hot. A photo from amateur experiments is attached, and here it is clearly visible.
The heat goes out, tending upwards, and there it meets a very cool layer. There is no free path for heat radiation, so the cooling process becomes difficult. Hot water has absolutely no such barriers in its path. Which freezes faster - cold or hot, on which the probable outcome depends, you can expand the answer by saying that any water has certain substances dissolved in it.
Impurities in the composition of water as a factor influencing the outcome
If you do not cheat and use water with the same composition, where the concentrations of certain substances are identical, then cold water should freeze faster. But if a situation occurs when the dissolved chemical elements available only in hot water, while cold water does not have them, then there is a possibility for hot water to freeze earlier. This is explained by the fact that 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. Even supercooling of water is possible, in the sense that at sub-zero temperatures it will be in a liquid state.
But all these versions, apparently, did not suit the scientists to the end, and they continued to work on this issue. In 2013, a team of researchers in Singapore said they had solved the age-old mystery.
A group of Chinese scientists claim that the secret this effect consists in the amount of energy that is stored between water molecules in its bonds, called hydrogen bonds.
The answer from Chinese scientists
Further information will follow, for the understanding of which it is necessary to have some knowledge in chemistry in order to figure out which water freezes faster - hot or cold. As you know, it consists of two H (hydrogen) atoms and one O (oxygen) atom held together by covalent bonds.
But hydrogen atoms of one molecule are also attracted to neighboring molecules, to their oxygen component. These bonds are called hydrogen bonds.
At the same time, it is worth remembering that at the same time, water molecules act repulsively on each other. Scientists noted that when water is heated, the distance between its molecules increases, and this is facilitated by repulsive forces. It turns out that occupying one distance between molecules in a cold state, one can say that they stretch, and they have a greater supply of energy. It is this energy reserve that is released when water molecules begin to approach each other, that is, cooling occurs. It turns out that a larger supply of energy in hot water, and its greater release when cooled to sub-zero temperatures, occurs faster than in cold water, which has a smaller supply of such energy. So which water freezes faster - cold or hot? On the street and in the laboratory, the Mpemba paradox should occur, and hot water should turn into ice faster.
But the question is still open
There is only theoretical confirmation of this clue - all this is written in beautiful formulas and seems plausible. But when the experimental data, which water freezes faster - hot or cold, will be put in a practical sense, and their results will be presented, then it will be possible to consider the question of the Mpemba paradox closed.
The properties of water never cease to amaze scientists. Water is a fairly simple substance from a chemical point of view, but at the same time it has a number of unusual properties that never cease to amaze scientists. Below are some facts that few people know about.
1. Which water freezes faster - cold or hot?
Take two containers of 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 down to cold temperature, and then turn into ice, while cold water does not need to cool down. Why is this happening?
In 1963, a Tanzanian student named Erasto B. Mpemba, while freezing a prepared 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 a 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 really freezes faster than cold water.
Now this phenomenon of hot water freezing faster than cold water is called the Mpemba effect. True, long before him, this unique property of water was noted by Aristotle, Francis Bacon and Rene Descartes.
Scientists do not fully understand the nature of this phenomenon, explaining it either by the difference in hypothermia, evaporation, ice formation, convection, or the effect of liquefied gases on hot and cold water.
2. She is able to freeze instantly
Everyone knows that water always turns to ice when it cools down to 0°C...except in some cases! Such a case is, for example, supercooling, which is the property of very pure water to remain liquid even when cooled to a temperature below freezing. This phenomenon is made possible by the fact that environment does not contain centers or nuclei of crystallization, which could provoke the formation of ice crystals. And so water remains in liquid form, even when cooled to temperatures below zero degrees Celsius.
The crystallization process can be triggered, for example, by gas bubbles, impurities (pollution), uneven surface of the container. Without them, water will remain in a liquid state. When the crystallization process starts, you can watch how the super-cooled water instantly turns 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 water has? If you answered three: solid, liquid, gaseous, then you are mistaken. Scientists distinguish at least 5 different states of water in liquid form and 14 states in frozen form.
Remember the conversation about super-cooled water? So, no matter what you do, at -38 ° C, even the purest super-cooled water will suddenly turn into ice. What happens 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 "glassy" water - a solid substance that lacks crystalline structure.
4. Water surprises physicists
On the molecular level water surprises even more. In 1995, scientists conducted an experiment on neutron scattering gave an unexpected result: physicists found that neutrons aimed at water molecules “see” 25% less hydrogen protons than expected.
It turned out that at the speed of one attosecond (10 -18 seconds) an unusual quantum effect takes place, and chemical formula water instead of H2O becomes H1.5O!
5. Water memory
Homeopathy, an alternative to official medicine, claims that a dilute solution of a drug can have a healing effect on the body, even if the dilution factor is so large that nothing is left in the solution but water molecules. Proponents of homeopathy explain this paradox by a concept called "memory of water", according to which water at the molecular level has a "memory" of the substance 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 criticized the principles of homeopathy, conducted an experiment in 2002 to disprove the concept once and for all. The result was the opposite. After that, the scientists said that they were able to prove the reality of the "memory of water" effect. However, experiments conducted under the supervision of independent experts did not bring results. Disputes about the existence of the phenomenon of "memory of water" continue.
Water has many other unusual properties that we have not covered in this article. For example, the density of water changes with temperature (the density of ice is less than the density of water)
water has a fairly high surface tension
in a liquid state, water is a complex and dynamically changing network of water clusters, and it is the behavior of clusters that affects the structure of water, etc.
You can read about these and many other unexpected features of water in the article "The Anomalous Properties of Water" by Martin Chaplin, a professor at the University of London.