Warm water cools faster than cold water. Why does hot water freeze faster than cold water?

In this article we will look at the question of why hot water freezes faster than cold water.

Heated water freezes much faster than cold water! This amazing property of water, for which scientists still cannot find an exact explanation, has been known since ancient times. For example, even in Aristotle there is a description of winter fishing: fishermen inserted fishing rods into holes in the ice, and so that they would freeze faster, they poured warm water on the ice. This phenomenon was named after Erasto Mpemba in the 60s of the 20th century. Mnemba noticed a strange effect while making ice cream and turned to his physics teacher, Dr. Denis Osborne, for an explanation. Mpemba and Dr. Osborne experimented with water at different temperatures and concluded that almost boiling water begins to freeze much faster than water at room temperature. Other scientists conducted their own experiments and each time obtained similar results.

Explanation of a physical phenomenon

There is no generally accepted explanation for why this happens. Many researchers suggest that the whole point is in the supercooling of the liquid, which occurs when its temperature drops below the freezing point. In other words, if water freezes at a temperature below 0°C, then supercooled water can have a temperature of, for example, -2°C and still remain liquid without turning into ice. When we try to freeze cold water, there is a chance that it will first become supercooled and only harden after some time. Other processes occur in heated water. Its faster transformation into ice is associated with convection.

Convection- this is a physical phenomenon in which the warm lower layers of a liquid rise, and the upper, cooled ones, fall.

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.

The British Royal Society of Chemistry is offering a £1,000 reward to anyone who can scientifically explain why hot water freezes faster than cold water in some cases.

“Modern science still cannot answer this seemingly simple question. Ice cream makers and bartenders use this effect in their daily work, but no one really knows why it works. This problem has been known for millennia, with philosophers such as Aristotle and Descartes thinking about it,” said Professor David Phillips, president of the British Royal Society of Chemistry, as quoted in a Society press release.

How a cook from Africa defeated a British physics professor

This is not an April Fool's joke, but a harsh physical reality. Modern science, which easily operates with galaxies and black holes, and builds giant accelerators to search for quarks and bosons, cannot explain how elementary water “works.” The school textbook clearly states that it takes more time to cool a hotter body than to cool a cold body. But for water, this law is not always observed. Aristotle drew attention to this paradox in the 4th century BC. e. Here is what the ancient Greek wrote in his book Meteorologica I: “The fact that water is preheated causes it to freeze. Therefore, many people, when they want to cool hot water faster, first put it in the sun...” In the Middle Ages, Francis Bacon and Rene Descartes tried to explain this phenomenon. Alas, neither the great philosophers nor the numerous scientists who developed classical thermophysics succeeded in this, and therefore such an inconvenient fact was “forgotten” for a long time.

And only in 1968 they “remembered” thanks to the schoolboy Erasto Mpembe from Tanzania, far from any science. While studying at culinary arts school in 1963, 13-year-old Mpembe was given the task of making ice cream. According to the technology, it was necessary to boil milk, dissolve sugar in it, cool it to room temperature, and then put it in the refrigerator to freeze. Apparently, Mpemba was not a diligent student and hesitated. Fearing that he would not make it by the end of the lesson, he put still hot milk in the refrigerator. To his surprise, it froze even earlier than the milk of his comrades, prepared according to all the rules.

When Mpemba shared his discovery with his physics teacher, he laughed at him in front of the whole class. Mpemba remembered the insult. Five years later, already a student at the university in Dar es Salaam, he attended a lecture by the famous physicist Denis G. Osborne. After the lecture, he asked the scientist a question: “If you take two identical containers with equal amounts of water, one at 35 °C (95 °F) and the other at 100 °C (212 °F), and place them in the freezer, then Water in a hot container will freeze faster. Why?" You can imagine the reaction of a British professor to a question from a young man from Godforsaken Tanzania. He made fun of the student. However, Mpemba was ready for such an answer and challenged the scientist to a bet. Their dispute ended with an experimental test that confirmed Mpemba was right and Osborne defeated. Thus, the apprentice cook wrote his name in the history of science, and from now on this phenomenon is called the “Mpemba effect.” It is impossible to discard it, to declare it as “non-existent”. The phenomenon exists, and, as the poet wrote, “it doesn’t hurt.”

Are dust particles and solutes to blame?

Over the years, many have tried to unravel the mystery of freezing water. A whole bunch of explanations for this phenomenon have been proposed: evaporation, convection, the influence of dissolved substances - but none of these factors can be considered definitive. A number of scientists have devoted their entire lives to the Mpemba effect. James Brownridge, a member of the Department of Radiation Safety at the State University of New York, has been studying the paradox in his spare time for a decade. After conducting hundreds of experiments, the scientist claims to have evidence of the “guilt” of hypothermia. Brownridge explains that at 0°C, water only becomes supercooled, and begins to freeze when the temperature drops below. The freezing point is regulated by impurities in the water - they change the rate of formation of ice crystals. Impurities, such as dust particles, bacteria and dissolved salts, have a characteristic nucleation temperature when ice crystals form around crystallization centers. When several elements are present in water at once, the freezing point is determined by the one that has the highest nucleation temperature.

For the experiment, Brownridge took two water samples of the same temperature and placed them in the freezer. He discovered that one of the specimens always froze before the other, presumably due to a different combination of impurities.

Brownridge says hot water cools faster because there is a greater difference between the temperature of the water and the freezer - this helps it reach its freezing point before cold water reaches its natural freezing point, which is at least 5°C lower.

However, Brownridge's reasoning raises many questions. Therefore, those who can explain the Mpemba effect in their own way have a chance to compete for a thousand pounds sterling from the British Royal Society of Chemistry.

21.11.2017 11.10.2018 Alexander Firtsev

« 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.

It seems obvious that cold water freezes faster than hot water, since under equal conditions hot water takes longer to cool and subsequently freeze. However, thousands of years of observations, as well as modern experiments, have shown that the opposite is also true: under certain conditions, hot water freezes faster than cold water. The Sciencium Science Channel explains this phenomenon:

As explained in the video above, the phenomenon of hot water freezing faster than cold water is known as the Mpemba effect, named after Erasto Mpemba, a Tanzanian student who made ice cream as part of a school project in 1963. Students had to bring a mixture of cream and sugar to a boil, let it cool, and then put it in the freezer.

Instead, Erasto put his mixture in immediately, hot, without waiting for it to cool. As a result, after 1.5 hours his mixture was already frozen, but the other students’ mixtures were not. Interested in the phenomenon, Mpemba began studying the issue with physics professor Denis Osborne, and in 1969 they published a paper stating that warm water freezes faster than cold water. This was the first peer-reviewed study of its kind, but the phenomenon itself is mentioned in the papers of Aristotle, dating back to the 4th century BC. e. Francis Bacon and Descartes also noted this phenomenon in their studies.

The video lists several options for explaining what is happening:

1. Frost is a dielectric, and therefore frosty cold water stores heat better than a warm glass, which melts ice when it comes into contact with it
2. Cold water has more dissolved gases than warm water, and researchers speculate that this may play a role in the rate of cooling, although it is not yet clear how
3. Hot water loses more water molecules through evaporation, so there are fewer left to freeze
4. Warm water can cool faster due to increased convective currents. These currents occur because the water in the glass cools first at the surface and sides, causing cold water to sink and hot water to rise. In a warm glass, convective currents are more active, which can affect the cooling rate.

However, in 2016, a carefully controlled study was conducted that showed the opposite: hot water froze much more slowly than cold water. At the same time, scientists noticed that changing the location of the thermocouple - a device that determines temperature changes - by just a centimeter leads to the appearance of the Mpemba effect. A study of other similar studies showed that in all cases where this effect was observed, there was a displacement of the thermocouple within a centimeter.