Is it true that hot water freezes faster? Why does hot water freeze faster than cold water? Mpemba effect
Have you ever wondered why water heated to 82 degrees C freezes faster than cold water? Most likely not, I’m even more than sure that the question has never occurred to you: which water freezes faster, hot or cold?
However, this amazing discovery was made by an ordinary African schoolboy, Erasto Mpemba, back in 1963. This was the usual experience of a curious boy, of course he could not correctly interpret the meaning of his and, moreover, scientists from all over the world until 1966 could not give a clear and substantiated answer to the question - why hot water freezes faster than cold.
Why does hot water freeze at 4 degrees Celsius, and cold water at 0?
Cold water has a lot of dissolved oxygen, it is he who maintains the freezing temperature of water at 0 degrees. If oxygen is removed from the water, and this is what happens when water is heated, the air bubbles dissolved in the water, as it is fashionable to say now, collapse, the water turns into ice not at zero degrees as usual, and already at 4 °C. It is oxygen dissolved in water that breaks the bonds between water molecules, preventing water from going from a liquid to a solid state, and will simply turn into
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Publication date: November 21, 2017
« Which water freezes faster, cold or hot?“- try asking your friends a question, most likely most of them will answer that cold water freezes faster - and they will make a mistake.
In fact, if you simultaneously place two vessels of the same shape and volume in the freezer, one of which contains cold water and the other hot, then it is the hot water that will freeze faster.
Such a statement may seem absurd and unreasonable. If you follow the logic, then hot water must first cool down to the temperature of cold water, and cold water should already turn into ice at this time.
So why does hot water beat cold water on its way to freezing? Let's try to figure it out.
History of observations and research
People have been observing this paradoxical effect since ancient times, but no one attached much importance to it. Thus, Arestotle, as well as Rene Descartes and Francis Bacon, noted in their notes the inconsistencies in the rate of freezing of cold and hot water. An unusual phenomenon often appeared in everyday life.
For a long time, the phenomenon was not studied in any way and did not arouse much interest among scientists.
The study of this unusual effect began in 1963, when an inquisitive schoolboy from Tanzania, Erasto Mpemba, noticed that hot milk for ice cream froze faster than cold milk. Hoping to get an explanation for the reasons for the unusual effect, the young man asked his physics teacher at school. However, the teacher only laughed at him.
Later, Mpemba repeated the experiment, but in his experiment he no longer used milk, but water, and the paradoxical effect was repeated again.
6 years later, in 1969, Mpemba asked this question to physics professor Dennis Osborn, who came to his school. The professor was interested in the young man’s observation, and as a result, an experiment was conducted that confirmed the presence of the effect, but the reasons for this phenomenon were not established.
Since then the phenomenon has been called Mpemba effect.
Throughout the history of scientific observations, many hypotheses have been put forward about the causes of the phenomenon.
So in 2012, the British Royal Society of Chemistry would announce a competition of hypotheses explaining the Mpemba effect. Scientists from all over the world participated in the competition; a total of 22,000 scientific papers were registered. Despite such an impressive number of articles, none of them brought clarity to the Mpemba paradox.
The most common version was according to which hot water freezes faster, since it simply evaporates faster, its volume becomes smaller, and as the volume decreases, its cooling rate increases. The most common version was eventually refuted because an experiment was conducted in which evaporation was excluded, but the effect was nevertheless confirmed.
Other scientists believed that the cause of the Mpemba effect was the evaporation of gases dissolved in water. In their opinion, during the heating process, gases dissolved in water evaporate, due to which it acquires a higher density than cold water. As is known, an increase in density leads to a change in the physical properties of water (an increase in thermal conductivity), and therefore an increase in the cooling rate.
In addition, a number of hypotheses have been put forward describing the rate of water circulation depending on temperature. Many studies have attempted to establish the relationship between the material of the containers in which the liquid was located. Many theories seemed very plausible, but they could not be scientifically confirmed due to a lack of initial data, contradictions in other experiments, or because the identified factors were simply not comparable with the rate of cooling of water. Some scientists in their works questioned the existence of the effect.
In 2013, researchers at Nanyang Technological University in Singapore claimed to have solved the mystery of the Mpemba effect. According to their research, the reason for the phenomenon lies in the fact that the amount of energy stored in hydrogen bonds between cold and hot water molecules is significantly different.
Computer modeling methods showed the following results: the higher the water temperature, the greater the distance between the molecules due to the fact that the repulsive forces increase. Consequently, the hydrogen bonds of molecules stretch, storing more energy. When cooled, the molecules begin to move closer to each other, releasing energy from hydrogen bonds. In this case, the release of energy is accompanied by a decrease in temperature.
In October 2017, Spanish physicists, in the course of another study, found that a major role in the formation of the effect is played by the removal of a substance from equilibrium (strong heating before strong cooling). They determined the conditions under which the likelihood of the effect occurring is maximum. In addition, scientists from Spain confirmed the existence of the reverse Mpemba effect. They found that when heated, a colder sample can reach a high temperature faster than a warmer one.
Despite comprehensive information and numerous experiments, scientists intend to continue studying the effect.
Mpemba effect in real life
Have you ever wondered why in winter the skating rink is filled with hot water and not cold? As you already understand, they do this because a skating rink filled with hot water will freeze faster than if it was filled with cold water. For the same reason, hot water is poured into the slides in winter ice towns.
Thus, knowledge of the existence of the phenomenon allows people to save time when preparing sites for winter sports.
In addition, the Mpemba effect is sometimes used in industry to reduce the freezing time of products, substances and materials containing water.
The properties of water never cease to amaze scientists. Water is a fairly simple substance from a chemical point of view, but it has a number of unusual properties that never cease to amaze scientists. Below are a few facts that few people know about.
1. Which water freezes faster - cold or hot?
Let's take two containers with water: pour hot water into one, and cold water into the other, and place them in the freezer. Hot water will freeze faster than cold water, although logically, cold water should have turned into ice first: after all, hot water must first cool to the cold temperature, and then turn into ice, while cold water does not need to cool. Why is this happening?
In 1963, a Tanzanian student named Erasto B. Mpemba, while freezing an ice cream mixture, noticed that the hot mixture solidified faster in the freezer than the cold one. When the young man shared his discovery with his physics teacher, he only laughed at him. Fortunately, the student was persistent and convinced the teacher to conduct an experiment, which confirmed his discovery: under certain conditions, hot water actually freezes faster than cold water.
Now this phenomenon of hot water freezing faster than cold water is called the “Mpemba effect.” True, long before him this unique property of water was noted by Aristotle, Francis Bacon and Rene Descartes.
Scientists still do not fully understand the nature of this phenomenon, explaining it either by the difference in supercooling, evaporation, ice formation, convection, or by the effect of liquefied gases on hot and cold water.
2. It can freeze instantly
Everyone knows that water always turns into ice when cooled to 0°C... except in some cases! An example of such a case is supercooling, which is the property of very pure water to remain liquid even when cooled to below freezing. This phenomenon is made possible due to the fact that the environment does not contain centers or nuclei of crystallization that could trigger the formation of ice crystals. And so water remains in liquid form even when cooled to below zero degrees Celsius.
The crystallization process can be triggered, for example, by gas bubbles, impurities (contaminants), or an uneven surface of the container. Without them, water will remain in a liquid state. When the crystallization process starts, you can watch the super-cooled water instantly turn into ice.
Note that “superheated” water also remains liquid even when heated above its boiling point.
3. 19 states of water
Without hesitation, name how many different states does water have? If you answered three: solid, liquid, gas, then you were wrong. Scientists distinguish at least 5 different states of water in liquid form and 14 states in frozen form.
Remember the conversation about super-chilled water? So, no matter what you do, at -38 °C even the purest super-chilled water will suddenly turn into ice. What will happen as the temperature drops further? At -120 °C something strange begins to happen to water: it becomes super viscous or viscous, like molasses, and at temperatures below -135 °C it turns into “glassy” or “vitreous” water - a solid substance that lacks crystalline structure.
4. Water surprises physicists
At the molecular level, water is even more surprising. In 1995, a neutron scattering experiment conducted by scientists yielded an unexpected result: physicists discovered that neutrons aimed at water molecules “see” 25% fewer hydrogen protons than expected.
It turned out that at a speed of one attosecond (10 -18 seconds) an unusual quantum effect takes place, and the chemical formula of water instead of H2O becomes H1.5O!
5. Water memory
An alternative to conventional medicine, homeopathy states that a diluted solution of a drug can have a healing effect on the body, even if the dilution factor is so high that there is nothing left in the solution except water molecules. Proponents of homeopathy explain this paradox with a concept called “water memory,” according to which water at the molecular level has a “memory” of the substance once dissolved in it and retains the properties of the solution of the original concentration after not a single molecule of the ingredient remains in it.
An international team of scientists led by Professor Madeleine Ennis of Queen's University of Belfast, who had criticized the principles of homeopathy, conducted an experiment in 2002 to disprove the concept once and for all. The result was the opposite. After which, scientists stated that they were able to prove the reality of the “water memory” effect. However, experiments carried out under the supervision of independent experts did not bring results. The debate about the existence of the “water memory” phenomenon continues.
Water has many other unusual properties that we did not talk about in this article. For example, the density of water changes depending on temperature (the density of ice is less than the density of water)
water has a fairly high surface tension
in the liquid state, water is a complex and dynamically changing network of water clusters, and it is the behavior of the clusters that affects the structure of water, etc.
You can read about these and many other unexpected features of water in the article “Anomalous Properties of Water,” authored by Martin Chaplin, professor at the University of London.
Hello, dear lovers of interesting facts. Today we will talk to you about. But I think that the question posed in the title may seem simply absurd - but should one always undividedly trust the notorious “common sense” and not a strictly established test experiment. Let's try to figure out why hot water freezes faster than cold water?
Historical reference
That in the issue of freezing cold and hot water, “not everything is pure” was mentioned in the works of Aristotle, then similar notes were made by F. Bacon, R. Descartes and J. Black. In recent history, this effect has been given the name “Mpemba’s Paradox” - named after a schoolboy from Tanganyika, Erasto Mpemba, who asked the same question to a visiting physics professor.
The boy’s question did not arise out of nowhere, but from purely personal observations of the process of cooling ice cream mixtures in the kitchen. Of course, the classmates who were present there, together with the school teacher, made Mpemba laugh - however, after an experimental test personally by Professor D. Osborne, the desire to make fun of Erasto “evaporated” from them. Moreover, Mpemba, together with a professor, published a detailed description of this effect in Physics Education in 1969 - and since then the above-mentioned name has been fixed in the scientific literature.
What is the essence of the phenomenon?
The setup of the experiment is quite simple: all other things being equal, identical thin-walled vessels are tested, containing strictly equal amounts of water, differing only in temperature. The vessels are loaded into the refrigerator, after which the time until ice forms in each of them is recorded. The paradox is that in a vessel with an initially hotter liquid this happens faster.
How does modern physics explain this?
The paradox does not have a universal explanation, since several parallel processes occur together, the contribution of which may vary depending on the specific initial conditions - but with a uniform result:
- the ability of a liquid to supercool - initially cold water is more prone to supercooling, i.e. remains liquid when its temperature is already below freezing point
- accelerated cooling - steam from hot water is transformed into ice microcrystals, which, when falling back, accelerate the process, working as an additional “external heat exchanger”
- insulation effect - unlike hot water, cold water freezes from above, which leads to a decrease in heat transfer by convection and radiation
There are a number of other explanations (the last time the British Royal Society of Chemistry held a competition for the best hypothesis was recently, in 2012) - but there is still no unambiguous theory for all cases of combinations of input conditions...
Water is one of the most amazing liquids in the world, which has unusual properties. For example, ice, a solid state of liquid, has a specific gravity lower than water itself, which made the emergence and development of life on Earth largely possible. In addition, in the pseudo-scientific and scientific world there are discussions about which water freezes faster - hot or cold. Anyone who can prove that hot liquid freezes faster under certain conditions and scientifically substantiates their solution will receive a £1,000 reward from the British Royal Society of Chemists.
Background
The fact that under a number of conditions, hot water freezes faster than cold water was noticed back in the Middle Ages. Francis Bacon and René Descartes spent a lot of effort explaining this phenomenon. However, from the point of view of classical heat engineering, this paradox cannot be explained, and they tried to bashfully hush up about it. The impetus for the continuation of the debate was a somewhat curious story that happened to Tanzanian schoolboy Erasto Mpemba in 1963. One day, during a lesson on making desserts at a chef school, the boy, distracted by other things, did not have time to cool the ice cream mixture in time and put a hot solution of sugar in milk into the freezer. To his surprise, the product cooled somewhat faster than that of his fellow students who observed the temperature regime for preparing ice cream.
Trying to understand the essence of the phenomenon, the boy turned to a physics teacher, who, without going into details, ridiculed his culinary experiments. However, Erasto was distinguished by enviable tenacity and continued his experiments not on milk, but on water. He became convinced that in some cases hot water freezes faster than cold water.
Having entered the University of Dar es Salaam, Erasto Mpembe attended a lecture by Professor Dennis G. Osborne. After its completion, the student puzzled the scientist with a problem about the rate of freezing of water depending on its temperature. D.G. Osborne ridiculed the very posing of the question, declaring with aplomb that any poor student knows that cold water will freeze faster. However, the young man’s natural tenacity made itself felt. He made a bet with the professor, proposing to conduct an experimental test right here in the laboratory. Erasto placed two containers of water in the freezer, one at 95°F (35°C) and the other at 212°F (100°C). Imagine the surprise of the professor and the surrounding “fans” when the water in the second container froze faster. Since then, this phenomenon has been called the “Mpemba Paradox”.
However, to date there is no coherent theoretical hypothesis explaining the “Mpemba Paradox”. It is not clear what external factors, the chemical composition of water, the presence of dissolved gases and minerals in it, influence the rate of freezing of liquids at different temperatures. The paradox of the “Mpemba Effect” is that it contradicts one of the laws discovered by I. Newton, which states that the cooling time of water is directly proportional to the temperature difference between the liquid and the environment. And if all other liquids completely obey this law, then water in some cases is an exception.
Why does hot water freeze faster?T
There are several versions of why hot water freezes faster than cold water. The main ones are:
- hot water evaporates faster, while its volume decreases, and a smaller volume of liquid cools faster - when cooling water from + 100°C to 0°C, volumetric losses at atmospheric pressure reach 15%;
- the greater the temperature difference, the greater the temperature difference, the higher the intensity of heat exchange between the liquid and the environment, so the heat loss of boiling water occurs faster;
- when hot water cools, a crust of ice forms on its surface, preventing the liquid from completely freezing and evaporating;
- at high water temperatures, convection mixing occurs, reducing the freezing time;
- Gases dissolved in water lower the freezing point, removing energy for crystal formation - there are no dissolved gases in hot water.
All these conditions have been repeatedly tested experimentally. In particular, the German scientist David Auerbach discovered that the crystallization temperature of hot water is slightly higher than that of cold water, which makes it possible for the former to freeze more quickly. However, later his experiments were criticized and many scientists are convinced that the “Mpemba Effect”, which determines which water freezes faster - hot or cold, can only be reproduced under certain conditions, which no one has been searching for and specifying until now.