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 with water: pour hot water into one and cold water into the other, and place them in the freezer. Hot water will freeze faster than cold water, although logically, cold water should have been the first to turn into ice: after all, hot water must first cool down to a 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 concoction, noticed that the hot concoction would freeze faster in the freezer than the cold concoction. When the young man shared his discovery with the physics teacher, he only laughed at him. Fortunately, the student was persistent and persuaded the teacher to conduct an experiment, which confirmed his discovery: under certain conditions, hot water does freeze faster than cold water.
Now this phenomenon of hot water freezing faster than cold water is called the "Mpemba effect." True, long before him it unique property water was noted by Aristotle, Francis Bacon and Rene Descartes.
Scientists still do not fully understand the nature of this phenomenon, explaining it either by the difference in hypothermia, evaporation, ice formation, convection, or by 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 cooled to 0 ° C ... except in some cases! Such a case, for example, is supercooling, which is a property of very pure water remain liquid even when cooled to below freezing point. 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. And therefore, 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 (impurities), or an uneven container surface. Without them, water will remain in liquid state... When the crystallization process starts, you can observe how supercooled 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 supercooled water? So, no matter what you do, at a temperature of -38 ° C, even the purest supercooled water will suddenly turn into ice. What happens when the temperature drops further? At -120 ° C, something strange starts to happen with water: it becomes super-viscous or viscous, like molasses, and at temperatures below -135 ° C, it turns into "glass" or "glassy" water - a solid that lacks crystalline structure.
4. Water surprises physicists
On molecular level water surprises even more. In 1995, a neutron scattering experiment conducted by scientists gave an unexpected result: physicists discovered that neutrons aimed at water molecules "see" 25% fewer hydrogen protons than expected.
It turned out that at a speed of one attosecond (10 -18 seconds) an unusual quantum effect takes place, and chemical formula water instead of H2O becomes H1.5O!
5. Memory of water
Homeopathy, alternative to official medicine, claims that a diluted solution of a drug can have a healing effect on the body, even if the dilution factor is so great that nothing but water molecules is left in the solution. 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 a substance that was once dissolved in it and retains the properties of a solution of its original concentration after not a single molecule of an ingredient remains in it.
An international group 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 refute this 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 carried out under the supervision of independent experts did not bring results. The debate about the existence of the phenomenon of "memory of water" continues.
Water has many other unusual properties that we haven't covered in this article. For example, the density of water changes with temperature (ice density is less than water density)
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 the clusters that affects the structure of water, etc.
You can read about these and many other unexpected features of water in the article "Abnormal Properties of Water" by Martin Chaplin, professor at the University of London.
Many factors influence which water freezes faster, hot or cold, 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 to turn into ice. this stage can be skipped, and, accordingly, it wins in time.
But the answer to the question of which water freezes faster - cold or hot - outside in frost, knows any inhabitant of northern latitudes. In fact, in a scientific way, it turns out that in any case, cold water simply must freeze faster.
The physics teacher, who was approached by schoolboy Erasto Mpemba in 1963 with a request to explain why the cold mixture of the future ice cream freezes longer than the same, but hot, thought the same way.
"This is not world physics, but some kind of Mpemba physics"
At that time, the teacher only laughed at this, but Deniss Osborne, a physics professor who at one time drove into the same school where Erasto studied, experimentally confirmed the existence of such an effect, although there was no explanation for this then. In 1969 in the popular scientific journal a joint article was published by these two people 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, of Mpemba.
The question arose for a long time
Naturally, such a phenomenon took place before, and it was mentioned in the works of other scientists. Not only the schoolboy was interested in this issue, but Rene Descartes and even Aristotle thought about it in their 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 the course of this process?
Now, in the 21st century, several options have been put forward that can explain this paradox. Which water freezes faster, hot or cold, may depend on the fact that it has a faster 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.
Defrost the freezer for a long time
Which water freezes faster, and why it happens, can be influenced by the snow lining that can be found in the freezer of the refrigerator used for the experiment. If you take two containers that are identical in volume, but one of them contains hot water, and the other contains cold water, a container with hot water melts the snow underneath, thereby improving the contact of the thermal level with the wall of the refrigerator. A cold water container cannot do that. If there is no such lining with snow in the refrigerator compartment, cold water should freeze faster.
Top - Bottom
Also, the phenomenon of which water freezes faster - hot or cold, is explained as follows. Following certain laws, cold water begins to freeze from the upper layers, when hot water does it the other way around - it begins to freeze from the bottom up. It turns out that cold water, having a cold layer on top with ice already formed in places, thus worsens the processes of convection and thermal radiation, thereby explaining which water freezes faster - cold or hot. Photo from amateur experiments is attached, and this is clearly visible here.
The heat goes out, tending upward, and there it meets a very cooled layer. There is no free path for heat radiation, so the cooling process becomes difficult. Hot water has no such obstacles on its way. Which one freezes faster - cold or hot, on which the probable outcome depends, you can expand the answer by the fact that any water has certain substances dissolved in it.
Impurities in water as a factor affecting 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 dissolved chemical elements available only in hot water, and cold water does not have them, then there is an opportunity for hot water to freeze earlier. This is explained by the fact that solutes in water create crystallization centers, and with a small number of these centers, the transformation of water into a solid state is difficult. It is even possible overcooling of water, in the sense that at sub-zero temperatures it will be in a liquid state.
But all these versions, apparently, did not completely suit the scientists and they continued to work on this issue. In 2013, a team of researchers in Singapore said they had solved an age-old mystery.
A group of Chinese scientists argues that the secret of this effect lies in the amount of energy that is stored between water molecules in its bonds, called hydrogen bonds.
The answer from Chinese scientists
This is followed by information, for understanding 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 atoms (hydrogen) and one O (oxygen) atom, held together by covalent bonds.
But also hydrogen atoms of one molecule are attracted to neighboring molecules, to their oxygen component. It is these bonds that are called hydrogen bonds.
It is worth remembering that at the same time, water molecules are repulsive to each other. Scientists noted that when water is heated, the distance between its molecules increases, and this is facilitated by just repulsive forces. It turns out that occupying one distance between the molecules in the cold state, one might say, they stretch, and they have a greater supply of energy. It is this store of energy that is released when the water molecules begin to approach each other, that is, cooling occurs. It turns out that a greater supply of energy in hot water, and its greater release when cooled to subzero temperatures, occurs faster than in cold water, which has less 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 a 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 into practical sense, and their results will be presented, then it will be possible to consider the question of the Mpemba paradox closed.
Mpemba effect (The Mpemba paradox) is a paradox that states that hot water freezes faster under certain conditions than cold water, although it must pass the temperature of cold water during the freezing process. This paradox is an experimental fact that contradicts the usual concepts, according to which, under the same conditions, a more heated body to cool to a certain temperature takes longer than a less heated body to cool to the same temperature.
This phenomenon was noticed at the time by Aristotle, Francis Bacon and Rene Descartes, but it was not until 1963 that the Tanzanian schoolboy Erasto Mpemba found that a hot ice cream mixture freezes faster than a cold one.
As a student of Magambinskaya high school in Tanzania, Erasto Mpemba did practical work on cooking. He needed to make homemade ice cream - boil milk, dissolve sugar in it, cool it to room temperature, and then put it in the refrigerator to freeze. Apparently, Mpemba was not a particularly diligent student and he delayed completing the first part of the assignment. Fearing that he would not be in time by the end of the lesson, he put the hot milk in the refrigerator. To his surprise, it froze even earlier than the milk of his comrades, prepared according to a given technology.
After that, Mpemba experimented not only with milk, but also with ordinary water. In any case, already being a student of the Mkvavskaya high school, he asked a question to Professor Dennis Osborne from the University College in Dar es Salaam (invited by the headmaster to give the students a lecture on physics) specifically about water: “If we take two identical containers with equal volumes of water so that in one of them the water has a temperature of 35 ° C, and in the other - 100 ° C, and put them in the freezer, then in the second the water will freeze faster. Why? " Osborne became interested in this issue and soon in 1969 he and Mpemba published the results of their experiments in the journal "Physics Education". Since then, the effect they discovered is called mpemba effect.
Until now, no one knows exactly how to explain this strange effect. Scientists do not have a single version, although there are many. It's all about the difference in the properties of hot and cold water, but it is not yet clear which properties play a role in this case: the difference in supercooling, evaporation, ice formation, convection, or the effect of liquefied gases on water at different temperatures.
The paradox of the Mpemba effect is that the time during which the body cools down to a temperature environment, should be proportional to the temperature difference between this body and the environment. This law was established by Newton and has since been confirmed many times in practice. In this effect, water with a temperature of 100 ° C cools down to a temperature of 0 ° C faster than the same amount of water with a temperature of 35 ° C.
However, this does not yet suggest a paradox, since the Mpemba effect can be explained within the framework of well-known physics. Here are some explanations for the Mpemba effect:
Evaporation
Hot water evaporates faster from the container, thereby reducing its volume, and a smaller volume of water with the same temperature freezes faster. Water heated to 100 C loses 16% of its mass when cooled to 0 C.
Evaporation effect - double effect. First, the amount of water needed for cooling is reduced. And secondly, the temperature decreases due to the fact that the heat of vaporization of the transition from the water phase to the vapor phase decreases.
Temperature difference
Due to the fact that the temperature difference between hot water and cold air is greater - therefore, heat exchange in this case is more intense and hot water cools faster.
Hypothermia
When water is cooled below 0 C, it does not always freeze. Under some conditions, it can undergo hypothermia, continuing to remain liquid at temperatures below the freezing point. In some cases, water can remain liquid even at a temperature of -20 C.
The reason for this effect is that in order for the first ice crystals to begin to form, centers of crystal formation are needed. If they are not present in liquid water, then hypothermia will continue until the temperature drops so much that crystals begin to form spontaneously. When they begin to form in a supercooled liquid, they will begin to grow faster, forming an ice slush, which, freezing, will form ice.
Hot water is most susceptible to hypothermia because heating it removes dissolved gases and bubbles, which in turn can serve as centers of ice crystal formation.
Why does hypothermia cause hot water to freeze faster? In the case of cold water that is not supercooled, the following occurs. In this case, a thin layer of ice will form on the surface of the vessel. This layer of ice will act as an insulator between water and cold air and will prevent further evaporation. The rate of formation of ice crystals in this case will be lower. In the case of hot water subject to supercooling, supercooled water does not have a protective surface layer of ice. Therefore, it loses heat much faster through the open top.
When the hypothermia process ends and the water freezes, much more heat is lost and therefore more ice forms.
Many researchers of this effect consider hypothermia to be the main factor in the case of the Mpemba effect.
Convection
Cold water begins to freeze from above, thereby worsening the processes of heat radiation and convection, and hence the loss of heat, while hot water begins to freeze from below.
This effect is explained by the water density anomaly. Water has a maximum density at 4 ° C. If you cool water to 4 ° C and put it at a lower temperature, the surface layer of water will freeze faster. Because this water is less dense than water at 4 ° C, it will remain on the surface, forming a thin, cold layer. Under these conditions, a thin layer of ice will form on the surface of the water for a short time, but this layer of ice will serve as an insulator protecting the lower layers of water, which will remain at a temperature of 4 C. Therefore, 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. In addition, cold water layers are denser than hot water layers, so the cold water layer will sink down, raising the warm water layer to the surface. This circulation of water ensures a rapid drop in temperature.
But why does this process fail to reach an equilibrium point? To explain the Mpemba effect from this point of view of convection, one should assume that cold and hot layers of water are separated and the convection process itself continues after the average water temperature drops below 4 C.
However, there is no experimental data that would support this hypothesis that cold and hot layers of water are separated by convection.
Gases dissolved in water
Water always contains gases dissolved in it - oxygen and carbon dioxide. These gases have the ability to reduce the freezing point of water. When the water is heated, these gases are released from the water, since their solubility in water at high temperatures is lower. Therefore, when hot water is cooled, there is always less dissolved gases in it than in unheated cold water. Therefore, the freezing point of heated water is higher and it freezes faster. This factor is sometimes considered as the main one in explaining the Mpemba effect, although there is no experimental data confirming this fact.
Thermal conductivity
This mechanism can play a significant role when water is placed in a freezer in a refrigerator compartment in small containers. Under these conditions, it has been observed that the container with hot water melts the ice of the freezer under it, thereby improving thermal contact with the freezer wall and thermal conductivity. As a result, heat is removed from the container with hot water faster than from cold water. In turn, a container with cold water does not thaw snow under it.
All these (as well as others) conditions were studied in many experiments, but an unambiguous answer to the question - which of them provide one hundred percent reproduction of the Mpemba effect - has not been obtained.
For example, in 1995 the German physicist David Auerbach studied the effect of supercooling of water on this effect. He found that hot water, reaching a supercooled state, freezes at a higher temperature than cold water, which means faster than the latter. But cold water reaches a supercooled state faster than hot water, thereby compensating for the previous lag.
In addition, Auerbach's results contradicted previously obtained data that hot water can achieve more supercooling due to fewer crystallization centers. When water is heated, gases dissolved in it are removed from it, and when it is boiled, some of the salts dissolved in it precipitate.
So far, only one thing can be asserted - the reproduction of this effect essentially depends on the conditions in which the experiment is carried out. Precisely because it is not always reproduced.
Hello dear lovers interesting facts... Today we will talk about. But I think that the question in the title may seem simply absurd - but should you always rely completely on the notorious "common sense", and not strictly posed testing experience. Let's try to figure out why hot water freezes faster than cold water?
Historical reference
That in the issue of freezing cold and hot water "not everything is clean" was mentioned in the writings of Aristotle, then F. Bacon, R. Descartes and J. Black made similar notes. IN recent history this effect was named "Mpemba's paradox" - after the name of a student from Tanganyika, Erasto Mpemba, who asked the same question to a visiting professor of physics.
The boy's question arose not from scratch, but from purely personal observations of the process of cooling ice cream mixes in the kitchen. Of course, the classmates who were present at the same place together with the school teacher laughed at Mpemba - however, after the experimental verification personally by Professor D. Osborn, their desire to make fun of Erasto "evaporated". Moreover, Mpemboy, together with a professor in Physics Education in 1969, published detailed description this effect - and since then the aforementioned name stuck in the scientific literature.
What is the essence of the phenomenon?
The setting of the experiment is quite simple: all other things being equal, the same thin-walled vessels are tested, in which there are strictly equal amounts of water, differing only in temperature. The vessels are loaded into the refrigerator, after which the time until ice forms in each of them is timed. 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 consistent result:
- the ability of a liquid to hypothermia - initially cold water is more prone to hypothermia, i.e. remains liquid when its temperature is already below freezing point
- accelerated cooling - steam from hot water is transformed into ice microcrystals, which, when falling back, accelerate the process, working as an additional "external heat exchanger"
- insulation effect - unlike hot water, cold water freezes from above, which leads to a decrease in heat transfer by convection and radiation
There are a number of other explanations (the last time a competition for the best hypothesis was held by the British Royal Chemical Society was recently, in 2012) - but an unambiguous theory for all cases of combinations of input conditions still does not exist ...
One of my favorite subjects in school was chemistry. Once a chemistry teacher gave us a very strange and difficult task. He gave us a list of questions that we had to answer in terms of chemistry. We were given several days for this assignment and allowed to use libraries and other available sources of information. One of these questions related to 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 cold (with exactly the specified temperature), and place them in an environment with a certain temperature, which of will they freeze faster? Of course, the answer immediately suggested itself - a bucket of cold water, but it seemed to us too simple. But this was not enough to give a complete answer, we needed to prove it from a chemical point of view. Despite all my thoughts and research, I was unable to draw a logical conclusion. On this day, I even decided to skip this tutorial, so I never found out the solution to this riddle.
Years passed, and I learned a lot of 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 the point of view of science. And I decided to conduct my own experience. Since I couldn't find any wooden buckets, I used a freezer, stovetop, some water, and a digital thermometer. I will talk about the results of my experience a little later. First, I will share with you some interesting arguments about water:
Hot water freezes faster than cold water. Most experts claim that cold water will freeze faster than hot water. But one funny phenomenon (the so-called Membe effect), for unknown reasons, proves the opposite: Hot water freezes faster than cold water. One of several explanations is the evaporation process: if very hot water is placed in a cold environment, the water will begin to evaporate (the remaining amount of water will freeze faster). And according to the laws of chemistry, this is not a myth at all, and most likely this is what the teacher wanted to hear from us.
Boiled water freezes faster than tap water. Despite the previous explanation, some experts argue that boiled water that has cooled down to room temperature should freeze faster because boiling reduces oxygen.
Cold water boils faster than hot water. If hot water freezes faster, then cold water may boil faster! This is contrary to common sense and scientists argue that this simply cannot be. Hot tap water should actually boil faster than cold water. But by using hot water for boiling, you are not saving energy. You may use less gas or light, but the water heater will use the same amount of energy that is needed to heat cold water. (This is a little different with solar power.) As a result of the heating of the water by the water heater, sediment may appear, so the water will take longer to heat up.
If you add salt to the water, it will boil faster. Salt increases the boiling point (and, accordingly, lowers the freezing point - which is why some housewives add a little rock salt to ice cream). But in this case, we are interested in another question: how long will the water boil and whether the boiling point in this case can rise above 100 ° C). Despite what they write in cookbooks, scientists argue that the amount of salt we add to boiling water is not enough to affect the boiling time or temperature.
But here's what I got:
Cold water: I used three 100 ml glass beakers of purified water: one at room temperature (72 ° F / 22 ° C), one with hot water (115 ° F / 46 ° C), and one with boiled water (212 ° F / 100 ° C). I placed all three glasses in the freezer at –18 ° C. And since I knew that water would not immediately turn into ice, I determined the degree of freezing by the "wooden float". When the stick, placed in the center of the glass, no longer touched the base, I assumed that the water had frozen. I checked the glasses every five minutes. And what are my results? The water in the first glass froze after 50 minutes. The hot water froze after 80 minutes. Boiled - after 95 minutes. My findings: Given the conditions in the freezer and the water I used, I was unable to reproduce the Memb effect.
I also tried this experiment with previously boiled water cooled to room temperature. She froze in 60 minutes - it still took longer than freezing cold water.
Boiled water: I took a liter of water at room temperature and put it on fire. It boiled in 6 minutes. Then I cooled it down to room temperature again and added it to the hot one. At the same heat, hot water boiled in 4 hours and 30 minutes. Conclusion: as expected, hot water boils much faster.
Boiled water (with salt): I added 2 large tablespoons of table salt to 1 liter of water. It boiled after 6 minutes 33 seconds, and as shown by the thermometer reached a temperature of 102 ° C. Undoubtedly, salt affects the boiling point, but not much. Conclusion: salt in water does not strongly affect the temperature and boiling time. I honestly admit that my kitchen can hardly be called a laboratory, and perhaps my conclusions are contrary to reality. My freezer compartment may freeze food unevenly. My glass glasses could be irregular shape, Etc. But whatever happens in the laboratory, when it comes to freezing or boiling water in the kitchen, the most important thing is common sense.
link with fun facts about vaudews about water
as suggested on the forum.ixbt.com this effect (the effect of freezing hot water faster than cold water) is called the "Aristotle-Mpemba effect"
Those. boiled water (chilled) freezes faster than "raw"