In modern society, ice cubes can be said to be the summer cooling artifact, drink cola to put a handful of ice cubes, eat ice cream to add smoothies. Ice cubes are so popular because of the development of modern science and technology.
In ancient China, although there is no modern high-tech, the ancients can still make ice cubes in the summer, and this method of making ice has confused many modern physicists. So how did the ancients in our country make ice cubes in the hot summer?
Ancient production of "summer ice"
As early as the Spring and Autumn Period and the Warring States Period, the ancients specially made ice cellars, which could store the ice mined in winter until the summer. What's even more amazing is that they also have a way to make their own ice in the summer.
In the book "The Completion of Huainan" co-authored by Liu An, the king of Huainan, and his disciples, this method of "making ice in summer" is recorded: "Take boiling soup and put it in an urn, secretly use a new silk (jiān), and Shen Zhong will become ice in three days." ”
The meaning of this passage is to put the boiling water that has just been boiled into a large bottle. Then seal the bottle with a clean and fine silk fabric. Finally, the bottle of water is hung with a rope, slowly sinks into the deep well, and after three days, it is taken out to make ice cubes.
In the summer, ice cubes are obtained by placing bottles of boiling water in deep wells. This makes one wonder why it is necessary to make ice from boiling water instead of cold water at room temperature. Supposedly, the temperature of cold water is lower, isn't it easier to freeze?
Physicists in ancient China were very concerned about these suspenseful problems, which have been recorded in later dynasties and dynasties, but the anomaly of "hot water" freezing and cold water not freezing is still not well explained.
Can modern science explain "summer ice"?
In this regard, modern physicists have also shown great interest and began to try to use modern physics knowledge for experimental verification. At that time, the famous physicist Mr. Hong Zhenhuan believed that this was the formation of "summer ice" and air pressure had a lot to do with it.
In his article, it is mentioned: "Boiling water is placed in the bottle, the bottle mouth is sealed, sinks into the well water, the temperature inside the bottle drops sharply, the liquid level pressure is greatly reduced, and the freezing point of the water in the bottle is increased accordingly..."
His theoretical basis is that the lower the air pressure, the higher the temperature required for water to freeze, and it is not necessary to reduce the temperature to 0 ° C to freeze, and the temperature in the deep well can basically be maintained below 10 ° C regardless of winter and summer, and some can even reach 3-4 ° C, which is likely to make the hot water freeze.
However, there is a very large limitation, that is, under normal conditions, the influence of the freezing point on air pressure is very small. It is estimated that for every point of air pressure reduction, the freezing point of the water will only increase by 0.008 °C, and even if the air pressure in the bottle is reduced from normal to vacuum, it will only increase the freezing point by less than 1 °C at most.
The study of Li Zhichao, a physicist at the University of Science and Technology of China, believes that "summer ice is caused by the expansion and cooling of gas adiabatics, that is, the Joule-Thomson effect is at work." ”
Combined with the process of "summer ice", it is a bottle filled with boiling water, and a large amount of water vapor will have a pressure mutation when passing through the small bottle mouth, which will cause a temperature change. This is the principle of air conditioning and liquefied gas and other devices, and can also be used to explain why when bicycle tires deflate, the valves will become cold or even condense ice crystals.
Joule-Thomson effect: schematic of a permeable porous plug in the middle of a gas stream
In a small-mouthed bottle, boil the water until the water vapor in it drains most of the air out of the bottle.
When the water in the bottle is about to boil and evaporate, immediately use a fine silk fabric to immerse the water to block the mouth, and immediately sink into the deep well, the temperature in the bottle drops sharply, which will lead to water vapor condensation, coupled with the sudden drop in air pressure inside the bottle, it can be said that it is similar to the vacuum state, and then continue to put the bottle temperature to the outside world, so that a certain amount of ice can be basically created.
Based on this theory, he instructed master's student Zhao Hongjun to do a simulated "ice-making" experiment: take a 1-liter glass ball bottle, fill it with a small amount of water and a thermometer and seal it, and use it to replace the ancient ice-making urn bottle. Then boil the water in the bottle, quickly put it into a bucket filled with cool water, and cool the boiling water.
Could it be that the water temperature in the bucket is not as low as the temperature of the well water? Or is it not enough time to place? After all, the Huainan Wanbi Book says that it will be stored in a deep well for three days. So Li Zhichao left the bottle for another few hours, and the water temperature was reduced to 7 ° C. Then remove the seal of the bottle and let the air flow in slowly. Eventually, the overall water temperature stabilized at about 6 °C, but it still did not freeze.
Li Zhichao later proposed a more detailed explanation:
"A bottle (urn) with a small mouth and a large belly, which does not hold much water, boils for a period of time, the water vapor will fill the whole bottle, most of the original air is squeezed out, burned to the point where there is little water left, and immediately sealed with a fine fabric soaked in water and sinks into a deep well. The bottle is cold, the water vapor condenses, and the air pressure in the bottle drops quickly. ”
"When the bottle temperature is close to equilibrium with the well water, take it out immediately, remove the seal and start breathing. This is the use of decompressive expansion of the heat absorption effect, so that the temperature inside the bottle drops. If the experiment is perfectly controlled, you can get a few traces of ice ballast. ”
Is the lost "summer ice" process a "chicken rib"?
From the research of Hong Zhenhuan and Li Zhichao, two modern scientists, it can be seen that "summer ice" is feasible, and this process is likely to have actually existed.
However, it is not difficult to find that even if modern technology is used to restore the ancient process of "summer ice making", these experimental steps are very complicated and cumbersome, as long as there is a little condition that cannot be met, it is impossible to make ice cubes. And even if ice is really made, the yield is very small, only a little ice ballast.
If you want to make a large number of ice cubes in the hot summer, you must strictly select the best ice-making deep wells, which consumes a lot of manpower, material resources and time. I am afraid that in ancient times, only the relatives of the emperor and the state, rich and powerful people, could use this method to make ice to cool off. Instead of making your own ice so much, it is better to build a few more ice cellars and store more ice cubes in the winter.
Of course, whether it is an ice cellar or an ice-making, it is not something that ordinary people can expect. Until the end of the Tang Dynasty, people inadvertently discovered the "saltpeter", which absorbs heat after dissolving in water, and can quickly cool and freeze water at room temperature. The discovery of this convenient and fast ice-making method really allows ordinary people to touch a piece of ice in the hot summer. From this point of view, according to the "Summer Ice" process described in the "Huainan Completion Technique", compared with the ice-making effect of saltpeter, it is still relatively "chicken ribs".
However, there is also a possibility that there is a large error in the description of this process in the book. The ancient "summer ice" technology may have been very mature, but it was gradually lost because it was monopolized by the royal family and artists.