Mpemba effect(Mpemba's Paradox) - a paradox that states that hot water under some 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 more heated body takes more time to cool to a certain temperature than a less heated body to cool to the same temperature.
This phenomenon was noticed at one time by Aristotle, Francis Bacon and Rene Descartes, but it was only in 1963 that Tanzanian schoolboy Erasto Mpemba discovered that a hot ice cream mixture freezes faster than a cold one.
As a student at Magambi High School in Tanzania, Erasto Mpemba did practical work as a cook. He needed to make homemade ice cream - boil milk, dissolve sugar in it, cool it to room temperature, and then put it in the refrigerator to freeze. Apparently, Mpemba was not a particularly diligent student and delayed completing the first part of the task. Fearing that he would not make it by the end of the lesson, he put still hot milk in the refrigerator. To his surprise, it froze even earlier than the milk of his comrades, prepared according to the given technology.
After this, Mpemba experimented not only with milk, but also with ordinary water. In any case, already as a student at Mkwava Secondary School, he asked Professor Dennis Osborne from the University College in Dar Es Salaam (invited by the school director to give a lecture on physics to the students) specifically about water: “If you take two identical containers with equal volumes of water so that in one of them the water has a temperature of 35°C, and in the other - 100°C, and put them in the freezer, then in the second the water will freeze faster. Why?" Osborne became interested in this issue and soon, in 1969, he and Mpemba published the results of their experiments in the journal Physics Education. Since then, the effect they discovered has been called 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 a body cools down to the ambient temperature 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 to a temperature of 0°C faster than the same amount of water with a temperature of 35°C.
However, this does not yet imply a paradox, since the Mpemba effect can be explained within the framework of 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 at the same temperature freezes faster. Water heated to 100 C loses 16% of its mass when cooled to 0 C.
The evaporation effect is a double effect. Firstly, the mass of water required for cooling decreases. And secondly, the temperature decreases due to the fact that the heat of evaporation of the transition from the water phase to the steam phase decreases.
Temperature difference
Due to the fact that the temperature difference between hot water and cold air is greater, therefore the heat exchange in this case is more intense and the hot water cools faster.
Hypothermia
When water cools below 0 C, it does not always freeze. Under some conditions, it can undergo supercooling, continuing to remain liquid at temperatures below freezing. In some cases, water can remain liquid even at a temperature of –20 C.
The reason for this effect is that in order for the first ice crystals to begin to form, crystal formation centers are needed. If they are not present in liquid water, then supercooling will continue until the temperature drops enough for crystals to form spontaneously. When they begin to form in the supercooled liquid, they will begin to grow faster, forming slush ice, which will freeze to form ice.
Hot water is most susceptible to hypothermia because heating it removes dissolved gases and bubbles, which in turn can serve as centers for the formation of ice crystals.
Why does hypothermia cause hot water to freeze faster? In the case of cold water that is not supercooled, the following happens. 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 the 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 subjected to supercooling, the supercooled water does not have a protective surface layer of ice. Therefore, it loses heat much faster through the open top.
When the supercooling process ends and the water freezes, much 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 heat loss, while hot water begins to freeze from below.
This effect is explained by an anomaly in water density. 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 a temperature of 4 C, it will remain on the surface, forming a thin cold layer. Under these conditions, a thin layer of ice will form on the surface of the water within a short time, but this layer of ice will serve as an insulator, protecting the lower layers of water, which will remain at a temperature of 4 C. Therefore, 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 not reach an 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 the process of convection.
Gases dissolved in water
Water always contains gases dissolved in it - oxygen and carbon dioxide. These gases have the ability to reduce the freezing point of water. When water is heated, these gases are released from the water because their solubility in water is lower at high temperatures. Therefore, when hot water cools, it always contains less dissolved gases than in unheated cold water. Therefore, the freezing point of heated water is higher and it freezes faster. This factor is sometimes considered as the main one in explaining the Mpemba effect, although there is no experimental data confirming this fact.
Thermal conductivity
This mechanism can play a significant role when water is placed in the refrigerator compartment freezer in small containers. Under these conditions, it has been observed that a container of hot water melts the ice in the freezer underneath, thereby improving thermal contact with the freezer wall and thermal conductivity. As a result, heat is removed from a hot water container faster than from a cold one. In turn, a container with cold water does not melt the snow underneath.
All these (as well as other) conditions were studied in many experiments, but a clear answer to the question - which of them provide one hundred percent reproduction of the Mpemba effect - was never obtained.
For example, in 1995, German physicist David Auerbach studied the effect of supercooling water on this effect. He discovered that hot water, reaching a supercooled state, freezes at a higher temperature than cold water, and therefore faster than the latter. But cold water reaches a supercooled state faster than hot water, thereby compensating for the previous lag.
In addition, Auerbach's results contradicted previous data that hot water was able to achieve greater supercooling due to fewer crystallization centers. When water is heated, gases dissolved in it are removed from it, and when it is boiled, some salts dissolved in it precipitate.
For now, only one thing can be stated - the reproduction of this effect significantly depends on the conditions under which the experiment is carried out. Precisely because it is not always reproduced.
O. V. Mosin
Literarysources:
"Hot water freezes faster than cold water. Why does it do so?", Jearl Walker in The Amateur Scientist, Scientific American, Vol. 237, No. 3, pp 246-257; September, 1977.
"The Freezing of Hot and Cold Water", G.S. Kell in American Journal of Physics, Vol. 37, No. 5, pp 564-565; May, 1969.
"Supercooling and the Mpemba effect", David Auerbach, in American Journal of Physics, Vol. 63, No. 10, pp 882-885; Oct 1995.
"The Mpemba effect: The freezing times of hot and cold water", Charles A. Knight, in American Journal of Physics, Vol. 64, No. 5, p 524; May, 1996.
Weak pressure from a faucet can upset even the most level-headed homeowner. After all, the duration of filling a kettle or coffee maker and the performance of a washing machine or dishwasher depend on the pressure.
In addition, if the pressure is poor, it is almost impossible to use either the toilet, shower or bath. In a word, if there is no pressure in the tap, then there will be no comfortable living in the house.
We understand the reasons for low water pressure in the tap
What weakens the water pressure in the tap?
We have already discussed why weak water pressure in the tap can ruin even the happiest life even in the most perfect house or apartment. However, moaning will not help the grief. Moreover, this problem is not as terrible as it seems. You just have to understand what weakened the pressure, and you will get an almost ready-made recipe for eliminating this trouble.
In this case, the list of TOP 3 reasons for a drop in hot or cold water pressure is as follows:
- Faucet clogged . In this case, the intensity of the water jet is weakened by a plug of rust and scale that has clogged the aerator, filter insert (mesh) or axle box. Moreover, only one tap in the house suffers from this problem. That is, if your tap water flows poorly, for example, in the kitchen, but there are no problems in the bathroom, then you will have to disassemble and clean the problem point of consumption.
- . In this case, the same particles of silt, rust or scale are to blame. Only now they are not blocking the faucet aerator or faucet mesh, but the filter built into the water supply. In the worst case, such deposits can block the flow diameter of the connecting fitting or the pipeline fitting itself.
- . In this case, the cause of the weakening may be either a failure at the level of the pumping station or depressurization of the pipeline. A failure at a station can only be corrected by utility service repair teams. An indicator of this breakdown is the lack of water in the entire neighborhood. Loss of tightness is diagnosed visually - by a stream of water gushing from the body of the water supply fittings. Any mechanic from a service company can fix this breakdown.
- In addition, speaking about the reasons for the weakening of pressure, it is necessary to mention possible miscalculations when installing a specific water supply line . Incorrect diameter (larger than the previous branch), excessive length (inappropriate for the characteristics of the pressure equipment) - these are the most important reasons for the drop in pressure in the new water supply network.
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If you don’t want to deal with them, order a water supply project from professionals.
Well, now that you already know the reasons for the drop in pressure in the tap, it’s time to figure out ways to eliminate this water supply defect.
What to do if cold and hot water from the tap does not flow well?
It all depends on the reason for the drop in pressure.
For example, if your faucet is clogged, you will have to do the following:
Removing the faucet aerator for cleaning
- Take an adjustable wrench and unscrew it from the spout of the faucet. – foaming water jet nozzle. This part has very small nozzles. Therefore, aerators are clogged every six months. And if we are talking about a faucet mixer with hot/cold water, then the frequency of cleaning the nozzles is reduced to 2-3 months. The dismantled aerator is washed under running water.
- If the aerator is clean and the water flows weakly, you will have to dive even deeper into the faucet design . Indeed, in this case you need to get close to the locking unit - the axle box. To do this, you need to dismantle the valve (faucet handle) and unscrew the lock washer holding the locking element in the body seat. Next, you remove the locking assembly from the body and clean off any deposits of silt or scale from its surface. In the final, you will have to assemble the crane using the reverse procedure.
Before dismantling the faucet shut-off unit, be sure to shut off the water supply by closing the water valve closest to the point of consumption. Otherwise you will flood the entire apartment.
- If the source of the problem is not the faucet, but the “spray” in the shower stall or bathroom, you'll have to do things a little differently. First, turn off the supply to the sprayer. Then remove it from the stand or metal hose using an adjustable wrench. Immerse the removed part of the sprayer in a saucepan with vinegar. Warm up this medium on the hotplate. Rinse off the scale with water. Return the nozzle to its place.
If the smell of vinegar irritates you, try a 10% citric acid solution. To prepare it, it is enough to dissolve 100 grams of dry acid powder - it is sold in any confectionery department - in a liter of water.
If you don’t want to tinker with the crane, call a mechanic from the management company. He will solve this problem right before your eyes.
We hope you already understand what to do if there is poor water pressure in the tap.
Now let's move on to the pipes:
- First, turn off the water by turning the central valve near the meter.
- Next, remove the coarse filter plug. Remove the wire cassette and wash it in a container. Then return the filter element to its place, renew the seal and screw in the plug.
- After checking the coarse filter, proceed to checking the fine cleaning system. First, disconnect it from the water supply and check the pressure in the free pipe by slightly opening the central valve. If everything is in order, change the liner, simultaneously rinsing the filter glass from particles of accumulated dirt. In the final, everything, of course, is mounted in its original place.
- If the filters are cleaned, but water still does not come out of the tap with the required force, then the reason for the drop in pressure is a blockage in the pipes themselves. Locating this problem and eliminating it is an extremely time-consuming task. Therefore, after cleaning the filters without results, you will have to call the management company and report a problem with the passage of pipes in the water supply.
If you have not changed the wiring of the water supply system in the apartment, the management company will pay for cleaning the pipes. After all, it is she who must monitor the performance of the “native” engineering communications.
There are many factors that influence which water freezes faster, hot or cold, but the question itself seems a little strange. The implication, and this is known from physics, is that hot water still needs time to cool to the temperature of the cold water being compared 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 - outside in the cold, any resident of northern latitudes knows. In fact, scientifically, it turns out that in any case, cold water is simply bound to freeze faster.
The physics teacher, who was approached by schoolboy Erasto Mpemba in 1963, thought the same thing with a request to explain why the cold mixture of future ice cream takes longer to freeze than a similar, but hot one.
“This is not universal 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 visited the same school where Erasto studied, experimentally confirmed the presence of such an effect, although there was no explanation for it then. In 1969, a joint article by these two people was published in a popular scientific journal, 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 Mpemba effect, or paradox.
The question has been around for a long time
Naturally, such a phenomenon took place before, and it was mentioned in the works of other scientists. Not only the schoolchild was interested in this issue, but Rene Descartes and even Aristotle also thought about it at one time.
But they began to look for approaches to solving this paradox only at the end of the twentieth century.
Conditions for a paradox to occur
As with ice cream, it's not just plain water that freezes during the experiment. Certain conditions must be present in order to start arguing about 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 higher evaporation rate than cold water. Thus, its volume decreases, and as the volume decreases, the freezing time becomes shorter than if we take the same initial volume of cold water.
It's been a while since you defrosted the freezer.
Which water freezes faster and why this happens can be influenced 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 contains hot water and the other cold, the container with hot water will melt the snow underneath, thereby improving the contact of the thermal level with the wall of the refrigerator. A container of cold water cannot do this. 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 the opposite - 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. Photos from amateur experiments are attached, and this is clearly visible here.
The heat goes out, rushing upward, 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 obstacles in its path. Which one freezes faster - cold or hot, what determines the likely outcome? You can expand the answer by saying that any water has certain substances dissolved in it.
Impurities in water as a factor influencing the outcome
If you don't 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 where dissolved chemical elements are present only in hot water, and cold water does not have them, then the hot water has the opportunity 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. It is even possible that the water will be supercooled, 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 claim 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
What follows is information, to understand which you need to have some knowledge of chemistry in order to understand which water freezes faster - hot or cold. As is known, it consists of two H (hydrogen) atoms and one O (oxygen) atom, held together by covalent bonds.
But also the hydrogen atoms of one molecule are attracted to neighboring molecules, to their oxygen component. These bonds are called hydrogen bonds.
It is worth remembering that at the same time, water molecules have a repulsive effect 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 by occupying the same distance between the molecules in a cold state, they can be said to stretch, and they have a greater supply of energy. It is this energy reserve that is released when water molecules begin to move closer to each other, that is, cooling occurs. It turns out that a greater reserve of energy in hot water, and its greater release when cooling to sub-zero temperatures, occurs faster than in cold water, which has a smaller reserve of such energy. So which water freezes faster - cold or hot? On the street and in the laboratory, Mpemba's paradox should occur, and hot water should turn into ice faster.
But the question is still open
There is only theoretical confirmation of this solution - all this is written in beautiful formulas and seems plausible. But when the experimental data on which water freezes faster - hot or cold - are put into practical use, and their results are presented, then the question of Mpemba’s paradox can be considered closed.
Why does water freeze? Water is an amazing miracle of nature. It is necessary for all life on earth. It was in water, according to scientists, that life originated. It is surprising that water can exist in three states: liquid, solid and gaseous. At the same time, it can move from one state to another. The vast majority of water on the planet is liquid. The solid state of water is ice.
Why does water freeze in the cold?
The ability of water to transform into different states is affected by its composition. Water molecules are weakly bonded to each other; they always move and group, but at the same time they cannot form a certain structure. Water takes the shape of the vessel in which it is placed, but on its own it cannot hold any particular model. For example, we pour water into a pan, and the liquid will take its shape, but will not be able to hold it outside the container.
When heated, water molecules begin to move relative to each other even faster and more chaotically, losing connection with each other to a greater extent. In this case, the water becomes steam.
When low temperatures influence water, the movement of molecules is inhibited, the connection between them is strengthened, and then they can build a structure - hexagonal crystals. The state of transformation of moisture into ice is called crystallization, solidification.
In such a strong state it can retain the various forms it acquires for a long time. Water begins to freeze at a temperature of 0 degrees Celsius. Thus, the transition of water from a liquid state to a solid state, to ice, is determined by the physical properties of water, its composition.
Why does hot water freeze faster than cold water?
Speaking about the “transformation” of water into ice, curious phenomena are observed. A hot one freezes faster than a cold one, no matter how unlikely this may seem. This fact has been known for a long time, but for a long time it was not possible to reveal the secret of the mysterious properties of water. Only in the twentieth century did scientists around the world try to explain the reason why hot water freezes faster than cold water.
In 1963, a boy named Mpemba from Tanzania noticed while making ice cream that the delicious delicacy hardened faster if it was made from warm rather than cold milk. They started making fun of him when he shared his observations with his teacher and friends. Only one person, Professor Dennis Osborne, whom Mpemba met as an adult, paid attention to this fact.
Many hypotheses have been put forward about hot water freezing faster than cold water, but they all remained assumptions. The “strange” behavior of water is called the “Mpemba Effect”. Research is still being carried out. Scientists from many countries are trying to prove the “Mpemba Effect”, but so far to no avail.
Many researchers consider this fact not worthy of attention, since ice cream has different properties compared to hard water. Physicists from Singapore in 2013 theoretically proved the mystery of the Mpemba effect, but confirmation of laboratory studies of the incomprehensible phenomenon still does not exist.
Water freezes from above, not from below
Almost everyone knows that on reservoirs at low temperatures, a thin ice crust first forms, which becomes thicker and stronger as frost intensifies. And if it were not for this amazing property of water, it is unlikely that anyone would be able to skate, since the ice would simply sink to the bottom of the reservoir.
Water, like most similar substances, contracts and decreases in volume when cooled, but to a temperature not lower than 3 degrees Celsius. At lower temperatures, water, on the contrary, expands and its density increases. Ice is lighter than water, and this keeps it on top.
Why doesn't distilled water freeze?
Distilled water is called pure; it is “freed” from all impurities and oxygen. Impurities are the fragments to which water molecules attach. When transitioning from a liquid state to ice, the impurities present in the water are compressed. Distilled water, due to the absence of other substances, expands, and the distance between the molecules increases.
The resulting ice will float on the surface because it is lighter than water. Still, distilled water can freeze, but its freezing point is much lower than ordinary water. At the same time, it was noticed that if you hit, for example, a bottle of distilled water or shake it, the water will immediately begin to freeze. This is explained by the adhesion of molecules upon impact.
Freezing point of mineral water
Mineral water is saturated with salts and chemicals that are beneficial to humans. The freezing point of mineral water is lower than that of ordinary water. Striking or shaking a container of water will speed up the freezing process in the same way as with distilled water. Water molecules will adhere to each other and structure into crystals, accordingly, the water will freeze.
Does salt water freeze?
There are people who believe that it does not freeze. This statement is not entirely true. Salt water also tends to freeze, but its freezing point is significantly below zero. The explanation for this lies in the molecular composition of water.
Salt, or rather its small crystals, does not allow water molecules to connect. The freezing of salt water depends on the concentration of salt it contains. The more salt in the water, the lower the freezing point. Why are Antarctic ice and icebergs reserves of fresh water? According to scientists, these are fragments of the continent that broke away millions of years ago. Their education was not facilitated by the place where they are located.
Sea water also freezes at very low temperatures. Ice crystals formed on the surface of the water push out salt crystals, so the deeper the brine becomes richer. If you take ice from the water surface of the sea and melt it, the melted water will be almost fresh.
Does Epiphany water freeze?
Epiphany water is called “holy”. There is an opinion that on Epiphany night and for the next three days, the water in all reservoirs becomes “holy”, possessing magical healing properties. It can indeed be stored for a long time without changing its taste, but it freezes. Anyone can verify this. Place in the cold 2 bottles filled with plain water collected on Epiphany night. The water will freeze equally in both bottles.
Does water in a well freeze?
People prefer to drink water from a well, considering it more beneficial and suitable for the body. Does well water freeze in winter? The answer to this question is obvious. If the well is deep enough, the water level does not rise above the freezing point of the ground, which means that the water in the well will not freeze. If the well is shallow, then the top layer of water may be covered with an ice crust or a significant layer of ice.
Water is an amazing substance that can change from one state to another due to its chemical composition. The freezing point of water varies. Water is probably the only exceptional substance that can expand at low temperatures.
frozen water
Everyone knows about the importance and benefits of water for life. It turns out that water thawed after freezing has healing properties on the human body. It changes its structure after freezing and thawing processes. Many people attribute the longevity of the mountaineers to their consumption of melt water from springs flowing in the mountains.