HOT WATER OR COLD WATER ?
When the same volume of hot water or cold water is placed in the refrigerator, which one is more likely to freeze?You may think that it must be cold water, because in order to freeze, hot water must first turn into cold waterBut in fact, hot water will freeze first. This counter-intuitive situation is not difficult to see in life. For example, you must have seen the following picture of water being poured into ice.
To be able to pour water into ice first of all the weather must be cold enough and the second point is very important the thing that is poured out must be hot water. If you pour a ladle of cold water on you, you may drown yourself or pour water on someone else head.
A very important reason behind this is that the volatilization of hot water will form a large amount of water vapor and small droplets, which have a large specific surface area in contact with cold air and can quickly freeze into ice cubes. There is also another influencing factor that you may not expect, that is Hot water is inherently more likely to freeze than cold water.
This phenomenon was discovered more than two thousand years ago.
The first person to record it was Aristotle. He found: “Boiled water freezes more easily.” And he recorded: “In a place called Pontus on the southern coast of the Black Sea, when people go ice fishing in winter, they pour warm water on the reeds. Let the reeds freeze quickly.”
Early Insights into Freezing Water
Francis Bacon also recorded: “Water that is somewhat warm freezes more easily than cold water.” Descartes also mentioned a similar phenomenon in his “Discourse on Method”. Descartes also gave an explanation; some of the small particles that prevent water from freezing evaporate when heated, making it easier to freeze.
But at that time, people did not study this phenomenon in depth. On the one hand, it was because people did not know enough about the microscopic world at that time. You must know that there was no such concept of water molecules at that time. On the other hand, maybe everyone doesn’t think this is a great phenomenon, and it can only be studied as a hobby.
The Mpemba Effect
Finally, in 1963, a Tanzanian middle school student noticed this phenomenon again of hot water and cold water, continued to study it, and published a paper.
The student who discovered this phenomenon is called Mpemba.
At that time, he and his classmates were making milk ice cream together. The teacher told them that the milk must be cooled before putting it in the refrigerator, otherwise the refrigerator may be damaged. You must know that in Tanzania at that time, refrigerators were not as common as they are now, and they were not cheap. Teachers naturally regarded refrigerators as “high-end equipment.”
However, there are too many people making ice cream in Mpemba’s class, and the space in the refrigerator is limited.
Seeing that the space in the refrigerator was occupied little by little, Mpemba took the risk and put the hot milk directly into the refrigerator. After an hour and a half, he opened the refrigerator and saw that not only was the refrigerator not broken, but his milk had even frozen into ice. But the other cold milk that he put in at the same time did not freeze. This phenomenon was very counterintuitive.
Unraveling the Mystery of Hot Water Freezing Faster
Mpemba ran to ask the physics teacher. Without thinking about it, the physics teacher told him: “This is impossible, you must have made a mistake.” After hearing what the teacher said, Mpemba stopped asking further, thinking that he might have made a mistake.
This matter has already passed. But soon, Mpemba heard a similar phenomenon again while chatting with friends.
A friend told him that his brother was an “ice master” at an ice cream shop and could make ice cream faster than anyone else. His secret is to put hot milk in the refrigerator. Later, while chatting with another friend selling ice cream, Mpemba heard him mention this phenomenon again.
At this time, Mpemba thought, maybe the junior high school physics teacher made a mistake?
So, after entering high school, Mpemba asked this question to his high school physics teacher. The answer is still the same: “You are wrong, this is impossible, this phenomenon is not the physics of our universe, but the physics of the Mpemba universe.”
The joke got bigger and bigger. When people got a math problem wrong and wanted to find an excuse, they would say it was “Mpemba’s math”. Mpemba was angry now. If you don’t believe it, I’ll show it to you myself.
He went to the person in charge of the school kitchen and borrowed the use of high-end experimental equipment (refrigerator) for a week, and also roped in a few friends to do experiments together. The experimental results indeed confirmed what Mpemba said.
The incident got bigger and bigger, and finally the principal found out. The principal was very responsible. He specifically consulted Dennis Osborne, a Ph.D. in physics. Of course Osborne didn’t believe it after hearing this, but he was also very responsible. He did an experiment himself and measured the freezing time of water at different water temperatures. The result was that it was really like this (see the picture below). This was an interesting phenomenon, so in 1969, Osborne and Mpemba published a paper describing this phenomenon.
This phenomenon later became known as the Mpemba phenomenon.
Conclusion of Hot Water Or Cold Water
In fact, until today, scientists have not reached a unified conclusion, but there are many guesses. Some people think that this is related to the fact that there are more dissolved gases in cold water than in hot water; Some people believe that the hydrogen bond energy of heated water becomes higher, which is beneficial to the process of forming ice; Some scientists also believe that during freezing, the convection of hot water is more intense than that of cold water, so the overall temperature drops faster.
It may take a long time to find the real answer about hot water or cold water. But the Mpemba phenomenon shows that even today, there are still many magical and unsolved scientific phenomena around us. The road of scientific exploration is far from over. If you also discover any counter-intuitive phenomena, pay more attention, maybe you will be the next Mpemba.