It is generally believed that when a cup of cold water and a cup of hot water are put into the refrigerator at the same time, the cold water will freeze quickly. That was not the case. 1963 One day, in a middle school in Tanzania, tropical Africa, a group of students wanted to make some frozen food to cool down. A student named Erasto Mpeba added sugar to hot milk and prepared to put it in the refrigerator to make ice cream. He thought that if the hot milk was cooled and put in the refrigerator, other students would fill the refrigerator, so he put the hot milk in the refrigerator. Not long after, he opened the refrigerator and looked. Surprisingly, his cup of ice cream had become delicious, while the ice cream made by other students with cold water had not frozen. His discovery did not attract the attention of teachers and classmates. On the contrary, it was a joke to them. Mba told Dr. Osborne, a professor of physics at Dar es Salaam University, about this special phenomenon. Osborne was a little surprised after listening to Mba's account, but he believed that what Mba said must be true. Osborne, who respects science, conducted another experiment, and the result was completely consistent with Mbeba's account. This certainly proves that hot water freezes faster than cold water at low temperature. Since then, many scientific magazines in the world have introduced this natural phenomenon and named it "MpembaEffect".
Second, the history of Mba effect.
It has been known for centuries that hot water freezes faster than cold water. The earliest data to mention and record this phenomenon can be traced back to Aristotle in 300 BC. He wrote:
"The previously heated water helps it freeze faster. So when people want to cool hot water, they will put it in the sun first ... "
But before the 20th century, this phenomenon was only regarded as folklore. It was not until 1969 that Mpemba was put forward again in the scientific community. Since then, many experiments have confirmed the existence of Mpemba effect, but there is no unique explanation.
About 146 1 physicist GiovanniMarliani said in a debate about how to cool objects that he had proved that hot water freezes faster than cold water. He said that he put four ounces of boiled water and four ounces of unheated water in two small containers respectively, and put them outside the house in a cold winter, and found that the boiled water froze first. But he could not explain this phenomenon.
By the beginning of the seventeenth century, this phenomenon seemed to be common sense. Bacon wrote in 1620, "When water is slightly heated, it freezes more easily than cold water." Soon after, Descartes said, "Experience shows that water left on a fire for a period of time freezes faster than other water."
Until 1969, 500 years after Marliani's experiment, a middle school student named Mpemba in Tanzania discovered this phenomenon again and published it in New Scientist magazine. This story tells scientists and teachers not to ignore the observations of non-scientists and not to judge prematurely.
1963, Mpemba is making ice cream at school. He mixed boiled milk with sugar. He should have waited for the milk to get cold before putting it in the refrigerator. But because there was not enough space in the refrigerator, he put the milk directly in before it cooled down. To his surprise, he found his hot milk frozen into ice earlier than his classmates. He asked the physics teacher why, and the teacher said that it must be mixed up with other students' ice cream, because his observation was impossible.
Mpemba believed his teacher at that time. But later that year, he met a friend of his who made and sold ice cream in Tanga town. He told Mbamba that when he made ice cream, he would put those hot liquids in the refrigerator to make them freeze faster. Mpumba found that other ice cream sellers in Tanga town had the same practical experience.
Later, Mpemba learned Newton's law of cooling, which describes how hot objects get cold (under some simplified assumptions). Mpumba asked his teacher why hot milk freezes before cold milk. The teacher also replied that Mpemba must be confused. When Mpemba continued to argue, the teacher said, "I can only say that this is your Mpemba physics, not ordinary physics." Since then, teachers and other students have criticized his mistakes with "that's Mpemba's math" or "that's Mpemba's physics". But later, when Mpumba tried to do experiments with hot water and cold water in the school biology laboratory, he found that the hot water froze first again.
Earlier, Dr. Osborne, a professor of physics, visited Mpumba Middle School. Mpemba asked him this question. Dr Osborne said he couldn't think of any explanation, but he would try to do the experiment later. When he returned to the laboratory, he invited a young technician to test the experiment of Mpemba. The technician later reported that hot water was the first to freeze, adding, "But we will continue to repeat this experiment until we get the correct result." However, the experimental report gives the same result. In 1969, Mpemba and Osborne reported their results.
In the same year, one of the most common coincidences in science, Dr. Kyle independently wrote an article about hot water freezing before cold water. Kyle shows that if it is assumed that water is initially cooled by evaporation and kept at a uniform temperature, hot water will lose enough mass and freeze first. Kyle therefore shows that this phenomenon is true (it was a rumor in Canadian cities at that time). ) and can be explained by evaporation. However, he didn't know Osborne's experiment. Osborne measured the lost mass and found that evaporation was not enough to explain this phenomenon. In later experiments, sealed containers were used to eliminate the influence of evaporation, and it was still found that hot water froze first.
Third, various explanations of Mba effect.
What is the Mpemba effect? There are two cups with the same shape and the same volume of water. The only difference is the temperature of water. Now cool two glasses of water in the same environment. Under some conditions, water with higher initial temperature will freeze first, but this is not always the case. For example, hot water at 99.9℃ and cold water at 0.0 1℃ will freeze first. Mpemba effect can not be seen under any initial temperature, container shape and cooling conditions.
Most people will think it seems impossible, others will try to prove it is impossible. This proof is usually like this: it takes 10 minutes for water at 30℃ to cool into ice, it takes some time for water at 70℃ to cool to 30℃, and it takes 10 minutes for water to cool into ice again. Because what cold water must do, hot water must also do, so hot water freezes slowly. Is this proof wrong?
The mistake of this proof is that it implicitly assumes that the freezing of water is only affected by the average temperature. But in fact, besides the average temperature, other factors are also very important. A cup of water with a uniform initial temperature of 70℃ changes when it is cooled to an average temperature of 30℃, which is different from a cup of water with a uniform initial temperature of 30℃. The former has less mass, less dissolved gas and less convection, which leads to uneven temperature distribution. These factors will change the environment inside the refrigerator and around the container. These four factors will be considered separately below.
1. Evaporation-In the process of cooling hot water to the initial temperature of cold water, hot water will lose some moisture due to evaporation. The smaller mass makes it easier for water to cool and freeze. In this way, hot water may freeze earlier than cold water, but it freezes less. If it is assumed that water only dissipates heat through evaporation, theoretical calculation can show that evaporation can explain Mpemba effect. This explanation is credible and intuitive, and evaporation is indeed a very important factor. However, this is not the only mechanism. Evaporation cannot explain the experiment done in a closed container. In a closed container, no water vapor can leave. Many scientists claim that evaporation alone is not enough to explain their experiments.
2. Dissolved gas-hot water can retain less dissolved gas than cold water, and with boiling, a lot of gas will escape from the water surface. Dissolved gas will change the properties of water. Or make it easier to form convection (and thus easier to cool), or reduce the heat required for freezing water per unit mass, or change the boiling point. Some experiments support this explanation, but there is no theoretical calculation.
3. Convection-due to cooling, water will form convection and the temperature distribution will be uneven. With the increase of temperature, the density of water will decrease, so the surface of water is hotter than the bottom of water-called "hot top". If the water mainly dissipates heat through the surface, the heat dissipation speed of the "hot top" water will be faster than that of the water with uniform temperature. When hot water is cooled to the initial temperature of cold water, there will be a hot top, so its cooling speed will be faster than that of water with the same average temperature but uniform temperature. Although the hot top and related convection can be seen in the experiment, it is still unknown whether convection can explain Mpemba effect.
Things around-the final difference between two glasses of water has nothing to do with yourself, but with the surrounding environment. Water with high initial temperature may change its surrounding environment in a complicated way, thus affecting the cooling process. For example, if this glass of water is placed on a layer of frost, the thermal conductivity of the frost will be poor. Hot water may melt this layer of frost, thus creating a better cooling system for yourself. Obviously, this explanation is not general enough, and many experiments will not put the container on the frost layer.
Finally, supercooling may be important in this effect. Supercooling is the phenomenon that water freezes below 0℃. An experiment found that the supercooling degree of hot water is less than that of cold water. This means that hot water will freeze first because it freezes at a higher temperature. But this can't fully explain the Mpemba effect, because we still need to explain why the less hot water, the colder it is.
In many cases, hot water freezes before cold water, but this phenomenon can not be observed in all experiments. Moreover, although there are many explanations, there is still no perfect explanation. Therefore, Mpeba effect is still a mystery.