MPEMBA effect, also known as Mpemba effect, refers to the phenomenon that the liquid with slightly higher temperature freezes before the liquid with slightly lower temperature in the same volume, mass and cooling environment.

Aristotle, Bacon and Descartes all described this phenomenon in different ways, but they failed to attract widespread attention. 1963 Baba, a junior middle school student in Makamba, Tanzania, often makes ice cream with his classmates. In the process of cooking, they always boil raw milk, add sugar, cool it, pour it into ice cubes, and then put it in the refrigerator for freezing. One day, when Mpamba was making ice cream, there was not much room in the freezer. In order to seize the remaining space in the refrigerator, Mpan couldn't wait to boil the milk in a hurry, put sugar in it, and couldn't wait to cool it, so he poured the boiled milk into the ice and sent it to the refrigerator. An hour and a half later, Mpamba found a phenomenon that puzzled him very much: the hot milk he put in had turned into ice, while the cold milk put in by other students was still a very thick liquid. It stands to reason that the lower the water temperature, the faster it freezes, and milk contains a lot of water. It should be that cold milk freezes faster than hot milk, but how can the fact be reversed? Mpamba brought this question from junior high school to senior high school. He consulted several physics teachers successively, but he didn't get the answer. A teacher thought the question he asked was almost absurd, so he said sarcastically, what you said is called the Mpumba effect! However, the persistent Mpamba doesn't think his question is ridiculous. He seized the opportunity of Dr. Osborne, head of the Physics Department of Dar es Salaam University, to visit their school and put forward his own questions. The doctor didn't laugh at his question. After returning to the laboratory, the doctor did the physics experiments of hot and cold milk and hot and cold water according to Mpamba's statement. As a result, he observed the strange phenomenon of subverting common sense described by Mpamba. So he invited Mpamba to study this phenomenon with him. 1969, he and Dr. Denis G. Osborne)*** wrote a paper on this phenomenon, so the phenomenon was named after it.

Can the "mpemba effect" really subvert our previous common sense about water freezing? For more than forty years, many papers and experiments have tried to confirm the principle behind this phenomenon, but due to the lack of scientific experimental data and quantitative analysis, there is no conclusion so far.

[Edit this paragraph] An unexplained phenomenon

The doctor who first confirmed the existence of the "Mbamba effect" found that there was no temperature difference between the upper surface and the bottom of the hot water when it was put into the refrigerator for cooling, but once it was cooled sharply, the temperature difference immediately appeared, in which the temperature difference between high and low in hot water with an initial temperature of 70℃ was close to 14℃, while that in hot water with an initial temperature of 47℃ was only 65,438. This shows that in the cooling process before freezing, the temperature difference of hot liquid is greater than that of relatively cold liquid for a period of time. But why should water with large temperature difference freeze first? There is only one reasonable explanation, that is, the higher the temperature of the upper surface of the water body, the more heat the upper surface emits, so the faster the temperature drops, the faster the ice freezes. This is the secret that hot milk freezes before cold milk.

But later, the experimental results of other researchers were quite different from the above. Some researchers have repeatedly done similar experiments with pure water, and the results have never found the "Mpumba effect". Some researchers who are interested in this have confirmed through experiments that the "Mpumba effect" will only occur when there is obvious temperature difference in the refrigerator, or the sugar content of milk is different, or the sugar is not dissolved, or the liquid for making ice cream contains more non-liquid components such as starch. In other words, the "Mpumba effect" is an individual phenomenon, and the physical phenomena it contains cannot deny our common sense.

[Edit this paragraph] Something hard is at work.

Recently, Jonathan of the University of Washington, USA? Through the in-depth study of mpemba effect, Katz caught the hidden ghost. He confirmed that this phenomenon is not only real, but also the ghost that caused it. However, ghosts are just some ordinary "hard things" hidden in the water.

In the process of deciphering mpemba effect, Katz fixed his eyes on the water. We know that when water is heated, some soluble hard substances hidden in water, such as calcium carbonate and carbonic acid mirror, will be discharged to form sediments. The scale attached to the inner wall of the kettle, which is common in our daily life, is the evidence that they are driven out. After reaching the boiling point, water will be softened by removing most hard objects. Katz found that the freezing point of unheated hard water is lower than that of heated soft water because there are hard objects in it, which slows down the freezing speed of hard water. This principle is just like sprinkling salt on the road surface after snow will prevent freezing. The mixing of salt will lower the freezing point of snow, so the process of snow freezing will be lengthened.

However, this discovery alone cannot directly solve the mpemba effect, because MP emba students cook raw milk first in the process of making ice cream. Then why does Mpamba's hot milk freeze first? Katz found that the reason was hard objects in the water: in order to eat delicious ice cream, they all added sugar to the milk, which actually hardened the milk liquid. However, the hardness of hot milk is actually lower than that of cold milk, which leads to the difference of freezing point between them. The freezing point of cold milk with higher hardness is relatively low. In this way, hot milk with a slightly higher freezing point naturally freezes before cold milk with a slightly lower freezing point.

Of course, there is another reason to slow down the freezing speed of low-temperature water, because experiments have proved that the speed of water heat dissipation depends on the temperature difference, that is to say, in the same low-temperature environment, water with relatively high temperature dissipates heat faster than water with relatively low temperature. So is milk.

So why doesn't mpemba effect appear every time in many experiments? Katz believes that the reason is that the experimenter used soft water at first. Using the same soft water for hot and cold experiments, because the freezing point of water is the same, the cooling speed has little effect on the freezing speed, so the mpemba effect is not so obvious.

Some scientists have pointed out that Katz's discovery may not be the final answer to the mpemba effect, but compared with the existing answers, this answer is the most convincing.

The misunderstanding of "hard water theory";

First, the hardness of water that can meet the requirements of human life in nature cannot be very high, otherwise it will endanger health. Therefore, even if the hard water used by human beings is boiled, the freezing point temperature will not increase significantly. Under the condition of general refrigerator refrigeration, it is difficult for hot water to freeze first. Otherwise, hot water often freezes before cold water, and physics teachers can't say that mpemba effect is deceptive. If the "hard water theory" is established, the premise is that all researchers who have completed the boiling water freezing experiment first choose water with extremely high hardness, which is harmful to human beings, which obviously does not conform to common sense.

Secondly, in theory, there are many situations in nature that can make the freezing point rise after water boils. For example, when water or milk is contaminated by microorganisms, the freezing point temperature will drop, but it will also rise after boiling, and so on.

Thirdly, according to the basic physical characteristics of water, the experiment of mpemba effect can also be completed with soft water, and there are many examples of using soft water to complete this experiment in reality.

[Edit this paragraph] Get rid of the shackles of common sense

Now it seems that mpemba effect, as a special case of icing, has not subverted our previous common sense, but after all, it has posed a fierce challenge to our common sense and enriched our understanding of water. If we are bound by common sense and regard this strange phenomenon as absurd, then we will not make new discoveries about the freezing characteristics of water under special conditions. On the contrary, if we respect common sense and are good at getting rid of it, we will make new discoveries.

Taking water as an example, American researchers found that water molecules can be used to make water films, which can play a waterproof role like wax. They put a water film on the surface of uranium and found that the newly spilled water was quickly driven away by the water film like raindrops on a waxing car.

Also, as common sense, everyone knows that the freezing point of water is 0℃. But a South Korean research team found that water can also condense into ice at 20℃. These researchers made this unexpected discovery when they used scanning tunneling microscope to observe how electrons passed through the water film and reached the electrode under the water film. During the observation, they learned from the abnormal data displayed by the testing instrument that the charged metal tip of the scanning tunneling microscope was blocked when it vibrated up and down in the water film. The reason for this is that the water molecules under the falling metal tip instantly solidify, which forms an obstacle to the tip. Later, after repeated experiments, it was confirmed that the closer the charged metal tip of the scanning tunneling microscope is to the electrode below the water film, the stronger the electric field formed between them. When the distance is about 2 water molecules, water is transformed into solid form under the action of strong electric field.

If researchers stick to the common sense that only cooling can turn water into solid, it will be difficult for them to make this important discovery.

In addition, in the past, we thought that the images of water molecules were tetrahedrons holding hands like pyramids, but scientists' recent research on water molecules showed that their images were not single tetrahedrons, but varied. It is also found that water can be frozen into 13 typical crystal.

Just ordinary water has so many strange characteristics, and there must be countless strange phenomena in nature that challenge our common sense.

Reasons for the Mpumba effect:

According to the physics theory of middle school, not only the temperature of hot water and cold water is changing under the condition of the same mass and the same external environment temperature, but also their respective densities, volumes, masses and air pressures are changing in the sealed state, which makes the cooling speed of water with high initial temperature always faster than that of water with low initial temperature. As long as the external environment temperature continues to drop, the water with high initial temperature will eventually have a lower temperature. (Note: Under normal pressure, the initial temperature of both is not lower than 4 degrees Celsius; It is not valid when the initial temperature of both is not higher than 4 degrees Celsius; When one of them is not higher than 4 degrees Celsius and the other is not lower than 4 degrees Celsius, it is necessary to discuss their initial temperature, density, volume, mass and air pressure in the sealed state. ) discuss the Mpamba problem. Water with an initial temperature of 35 degrees Celsius and water with an initial temperature of 100 degrees Celsius are higher than 4 degrees Celsius, so mpemba effect will occur.

1. The temperature of the refrigerator is uneven. If Mpamba happens to put its ice box near the cooling pipe, or even contact with the cooling pipe, hot milk may freeze before cold milk;

2. If Mpamba doesn't like sweet food, he puts less sugar in the ice cream, or because he doesn't have time to stir in a hurry, the sugar particles sink to the bottom of the box to form a solid, and the experiment proves that it can freeze first;

3.Mpamba's homemade ice cream not only adds sugar to the milk, but also adds starch. If you put less sugar and less milk, it will freeze first;

4. Put it near the heat pipe of the refrigerator.

[Edit this paragraph] Other notes:

At present, this phenomenon has been proved to be the coincidence of the above four factors by three female students in China, Ming Middle School. In general, cold water still freezes first. The three students spent most of their winter vacation in the laboratory with Mr. Huang Zengxin. More than 100 experiments eventually brought tens of thousands of valuable data. Before the start of school, the experimental stage ended, and the research group ushered in a more boring data analysis stage. Although there are advanced automation instruments to help, it is quite troublesome to sort out, analyze and summarize thousands of data. For the time being, no matter how long it took the research group to draw the schematic diagram of 1 1, we only need to extract the "data record analysis" part of the paper, and its complexity can be seen: the comparison between cold and hot pure milk; Comparison of hot and cold sugar milk; Comparison between cold milk and hot milk without sugar and starch; Comparison of cold milk and hot milk containing sugar and starch: comparison of cold and hot pure water; Comparison of hot and cold sugar water; Comparison of cold brine and hot brine; Cold purified water compared with pure milk; Comparison between sugar cold and hot starch and sugar-free cold and hot starch ... After strict analysis, the conclusion is naturally drawn: under the same quality and the same external temperature, there will be no mpemba effect, and it is impossible for hot liquid to freeze first. Recently, Xiangming Middle School will invite relevant experts to evaluate and identify this experimental subject.

From June last year 165438+ 10, under the guidance of Huang Zengxin, a famous science and technology teacher in Xiangming Middle School, three female middle school students in Shanghai-Yu Shunxi, Ye Shasha and Shanghai Middle School Director-began to study the mpemba effect. In the last four months, it was concluded that the hot liquid could not freeze before the cold liquid under the conditions of homogeneous equivalence and external temperature, and three possibilities for misunderstanding were put forward. They believe that the mpemba effect can only occur when there is a temperature difference in the refrigerator, the sugar content in milk is different, or the sugar is not dissolved and contains more non-liquid components such as starch. (CCTV V2 broadcast Mpamba at 20: 30 on July 6, 2005)

Proof of Mpumba Effect

Mpemba effect, known as the world physics problem, has been denied by physics teachers for more than 40 years, and even described as a lie by the media. However, according to the theory of middle school physics, it can be found that Mpamba problem is only a comprehensive problem of middle school students' knowledge, and every middle school student can master the method of proof.

It is proved that if hot water can freeze before cold water, the necessary conditions are either that the freezing point of hot water is higher than that of cold water or that the cooling speed of hot water is faster than that of cold water. Because the freezing point of pure hot water is the same as that of cold water under normal pressure, it must be proved that the cooling speed of hot water is faster than that of cold water to prove the mpemba effect.

According to the basic theory of physics, the evaporation intensity of hot water is greater than that of cold water, and the density is less than that of cold water. If you take two identical unsealed containers, put the same amount of water, one is hot water and the other is cold water, and put them at the same external ambient temperature. In the process of cooling, hot water loses more water due to evaporation than cold water, so the final mass of water with high initial temperature must be less than that with low initial temperature, and the cooling speed of hot water must always be faster than that of cold water.

If you take two identical sealed containers, put the same amount of water, one is hot water and the other is cold water, and put them at the same external ambient temperature. In the process of cooling, the air pressure in the container formed by the increase of the density and the decrease of the volume of hot water is inevitably lower than that of cold water. The boiling point temperature of hot water is lower than that of cold water, and the convection intensity is higher than that of cold water. Hot water always loses more heat per unit time than cold water, so the cooling speed of hot water is always faster than cold water. At the same time, according to the three-phase diagram theory of water, when the air pressure of water decreases, the freezing point temperature increases. The freezing point of boiling water with high initial temperature is higher than that of cold water because it is subjected to lower air pressure than cold water with low initial temperature.

Because the cooling speed of hot water is always faster than that of cold water under the same mass and the same external environment temperature, when the external environment temperature is continuously cooled, the temperature of hot water will be lower than that of cold water; When the external environment temperature is in a cooling state within a specific time or temperature range, the temperature of hot water will be equal to or higher than that of cold water. Therefore, it is a common phenomenon that the temperature of hot water will be lower than that of cold water under the condition of homogeneous quantity and external environment temperature, and it is a special phenomenon that cold water freezes before hot water under the condition of specific external environment temperature. If we choose pure water with the same quality and quantity, one is cold water at 4℃ and the other is boiling water at 100℃, and do experiments under the same external environment temperature conditions that make the cooling speed of both very slow, then no one can make the cold water at 4℃ freeze before the hot water at 100℃. It can be proved by experiments that mpemba effect conforms to the basic theory of physics, and people deny mpemba effect mainly because of their own shortcomings in observing objective things or freezing experiments. Take the experiments of three senior high school students in Shanghai as an example: under the guidance of Mr. Huang Zengxin, they only did the experiments under the condition of rapid freezing, but did not observe the freezing results under the condition of slow cooling, so their experiments cannot be a reason to deny the mpemba effect.

Mpemba effect, as an objective fact, has been doubted and disputed by the world physics community for decades, and there have been negative voices in China in recent years. In fact, it is very simple to complete this proof: put the same amount of boiling water with an initial temperature of 100 degrees Celsius and cold water with an initial temperature of 35 degrees Celsius into the freezer at the same time. If the temperature condition of the freezing room causes the rapid cooling of water, we often see that the water with low initial temperature freezes first, but this is only a one-sided phenomenon. As long as the power supply of the refrigerator is cut off, the temperature in the freezer will rise. When the frozen boiled water and cold water are completely dissolved, the freezing experiment is carried out again, and the result can only be that the original boiled water is frozen first. If this experimental process is repeated, all the results will be the same. So in the state of rapid cooling, cold water can appear, and it only freezes once.

If the temperature conditions in the freezer of the refrigerator form a slow cooling state for water, what we see is that the boiling water with high initial temperature freezes first. If the temperature in the freezer is raised at this time, the boiling water and cold water are completely dissolved and then cooled and frozen again, then the original boiling water will be frozen first regardless of the temperature conditions in the freezer. Repeated operations like this can only make the original boiling water freeze first. Therefore, it is impossible for cold water to freeze first in a slow cooling state. Mpimba effect makes us know more about the characteristics of water, and the experience of Mpimba effect shows the importance of scientific and serious attitude in understanding and mastering nature.

Attachment: first complete the operation points of the frozen boiling water experiment.

For more than forty years, few people in the world have completed the experiment of boiling water freezing first because the experimental method is unscientific. Theoretically speaking, water at 36℃ can freeze before water at 35℃ under the condition of homogeneous quantity and the same external environment temperature. However, if the temperature of the two is close, the evaporation intensity and density are not much different, so it is difficult to show the advantages of hot water in cooling rate by experimental methods. According to the basic theory of physics, 100℃ is the boiling point of water at normal pressure, and the density of water is the highest at 4℃. In order to complete the experiment of hot water freezing first more clearly, we can give the known conditions according to Mpamba problem, choose hot water and cold water as close as possible to 100℃, and reasonable the initial temperature of the external environment experiment to slow down the decline of the external environment temperature. The unsealed container has a large evaporation area, and the sealed container has high sealing performance. Special attention should be paid to the extraction method of boiling water: once boiling water leaves the heating source, the temperature and evaporation intensity drop rapidly, so it is necessary to extract a certain amount of boiling water quickly and accurately, and then extract cold water according to the requirements of uniformity and quantity.

Operation method of completing boiling water icing experiment with unsealed container first: (for reference)

1, control the initial temperature of the experiment in the freezer at 4 degrees Celsius, take two identical plates, put the same amount of water, one is cold water at 4 degrees Celsius and the other is hot water near 100 degrees Celsius, and put them in the freezer at the same time. Control the temperature drop rate of the freezing room to make it drop by 65438 0 degrees Celsius every hour (or drop by 65438 0 degrees Celsius every two hours), and record the final quality of hot water and cold water after freezing.

2. In winter, the experiment is completed by natural cooling. When the outdoor temperature is not lower than 4 degrees Celsius at noon and the lowest temperature at night is -2~3 degrees Celsius, you can choose to take two identical plates at noon and put in the same amount of water, one is hot water close to 100 degrees Celsius and the other is cold water with the same outdoor temperature. Put them in the same outdoor location at the same time, and record the time when hot water and cold water are completely frozen and their final quality.

3. According to the experimental method of three senior high school students in Shanghai, the final quality of hot water and cold water after freezing was recorded. According to the fact that the final mass of hot water is less than that of cold water, it is proved that hot water can freeze before cold water because its cooling speed is always faster than cold water.

Please refer toNo. 1 and No.2 experimental methods for unsealed containers when doing experiments with sealed containers.

In addition, some people think that the description of this phenomenon in Aristotle's original text is like this: "Water that has been heated before will help it freeze faster", and most people probably misunderstand the original intention of this sentence, that is, "comparison between overheated water and unheated water at the same temperature" rather than "comparison between hot water and cold water". Therefore, according to the second understanding, that is, as discussed above, the mpemba effect does not hold; Under the first understanding, the mpemba effect is possible.