Which water freezes faster - hot or cold. The Mpemba effect or why hot water freezes faster than cold water

Purified by evaporation, cooling and condensation, the liquid has special physical properties. It is recommended to use it in the heating system, since there are no salts, as well as oxygen. This has a positive effect on the duration of the operation of the equipment.

But many are interested in the question, does distilled water freeze at temperatures below 0˚ C?

It is easy to conduct an experiment at home, and get an answer to this question. We will see that at 0˚ C it will remain liquid. Even if we lower the temperature, its physical state will not change.

So at what temperature does water freeze?

An interesting property of distilled water is observed at a negative temperature. If you lower a piece of ice, snow, air or dust into it, crystals will instantly appear throughout the volume.

This is due to the fact that tap water has many centers of crystallization: salts, air inside, the surface of the container, and so on. Purified liquids do not have such centers. Due to this, it can significantly supercool.

The laws of physics state that the more a liquid is purified from impurities, the lower the threshold for the transition to a solid state.

Distilled water freezes at -10˚C and below. This explains its advantage over other coolants during the heating period. Due to this property, when heating a room, it can compete with antifreeze.

At the same time, there are a number of additional advantages over other coolants:

ecological cleanliness;
safety for human life and health;
careful attitude to pipes;
ease of use;
availability.

Now you know that distilled water freezes at temperatures below 10 degrees, so you can be calm about your heating system.

We hope that the article was useful to you. We will be grateful if you share it on social networks.

Have a nice day!

Read also:

Heat carrier for the heating system - what is used today?
Water heating in a private house - the technology for its implementation
Water pump for heating: principle of operation and installation

Hello, dear lovers of interesting facts. Today we will talk about. But I think that the question posed in the title may seem simply absurd - but is it always necessary to completely trust the notorious "common sense", and not strictly set testing experience. 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Historical reference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, the name “Mpemba paradox” has been attached to this effect - after the name of a schoolboy from Tanganyika, Erasto Mpemba, who asked the same question to a visiting professor of physics.

The boy's question arose not from scratch, 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, laughed at Mpemba - however, after an experimental check personally by Professor D. Osborne, the desire to make fun of Erasto "evaporated" from them. Moreover, Mpemba, together with the professor, published a detailed description of this effect in 1969 in Physics Education - and since then the above name has been fixed in the scientific literature.

What is the essence of the phenomenon?

The setup of the experiment is quite simple: other things being equal, identical thin-walled vessels are tested, in which there are strictly equal amounts of water, differing only in temperature. The vessels are loaded into the refrigerator, after which the time is recorded before the formation of ice in each of them. The paradox is that in a vessel with an initially hotter liquid, this happens faster.

How does modern physics explain this?

The paradox has no universal explanation, since several parallel processes proceed together, the contribution of which may differ from specific initial conditions - but with a uniform result:

the ability of a liquid to supercool - initially cold water is more prone to hypothermia, i.e. remains liquid when its temperature is already below the 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"
isolation 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 competition for the best hypothesis was held by the British Royal Society of Chemistry recently, in 2012) - but there is still no unambiguous theory for all cases of combinations of input conditions ...

Which water freezes faster, hot or cold, is influenced by many factors, but the question itself seems a little strange. It is understood, and it is known from physics, that hot water still needs time to cool down to the temperature of comparable cold water in order to turn into ice. this stage can be skipped, and, accordingly, she wins in time.

But the answer to the question of which water freezes faster - cold or hot - on the street in frost, any inhabitant of the northern latitudes knows. In fact, scientifically, it turns out that in any case, cold water simply has to freeze faster.

So did the teacher of physics, who was approached by the schoolboy Erasto Mpemba in 1963 with a request to explain why the cold mixture of future ice cream freezes longer than a similar, but hot one.

"This is not world physics, but some kind of Mpemba physics"

At that time, the teacher only laughed at this, but Deniss Osborne, a professor of physics, who at one time went to the same school where Erasto studied, experimentally confirmed the existence of such an effect, although there was no explanation for this then. In 1969 a popular scientific journal published a joint article by the two men who described this peculiar effect.

Since then, by the way, the question of which water freezes faster - hot or cold, has its own name - the effect, or paradox, Mpemba.

The question has been around for a long time

Naturally, such a phenomenon has taken place before, and it was mentioned in the works of other scientists. Not only the schoolboy was interested in this question, but Rene Descartes and even Aristotle thought about it at one time.

Here are just approaches to solving this paradox began to look only at the end of the twentieth century.

Conditions for a paradox to occur

As with ice cream, it's not just ordinary water that freezes during the experiment. Certain conditions must be present in order to start arguing which water freezes faster - cold or hot. What influences this process?

Now, in the 21st century, several options have been put forward that can explain this paradox. Which water freezes faster, hot or cold, may depend on the fact that it has a higher evaporation rate than cold water. Thus, its volume decreases, and with a decrease in volume, the freezing time becomes shorter than if we take a similar initial volume of cold water.

Freezer has long been defrosted

Which water freezes faster, and why it does so, can be affected by the snow lining that may be present in the freezer of the refrigerator used for the experiment. If you take two containers that are identical in volume, but one of them will have hot water and the other cold water, the container with hot water will melt the snow underneath, thereby improving the contact of the thermal level with the refrigerator wall. A cold water container can't do that. If there is no such lining with snow in the refrigerator, cold water should freeze faster.

Top - bottom

Also, the phenomenon of which water freezes faster - hot or cold, is explained as follows. Following certain laws, cold water starts to freeze from the upper layers, when hot water does it the other way around - it starts to freeze from the bottom up. It turns out that cold water, having a cold layer on top with ice already formed in some places, thus worsens the processes of convection and thermal radiation, thereby explaining which water freezes faster - cold or hot. A photo from amateur experiments is attached, and here it is clearly visible.

The heat goes out, tending upwards, and there it meets a very cool layer. There is no free path for heat radiation, so the cooling process becomes difficult. Hot water has absolutely no such barriers in its path. Which freezes faster - cold or hot, on which the probable outcome depends, you can expand the answer by saying that any water has certain substances dissolved in it.

Impurities in the composition of water as a factor influencing the outcome

If you do not cheat and use water with the same composition, where the concentrations of certain substances are identical, then cold water should freeze faster. But if a situation occurs when dissolved chemical elements are present only in hot water, while cold water does not possess them, then hot water has the opportunity to freeze earlier. This is explained by the fact that the dissolved substances in water create centers of crystallization, and with a small number of these centers, the transformation of water into a solid state is difficult. Even supercooling of water is possible, in the sense that at sub-zero temperatures it will be in a liquid state.

But all these versions, apparently, did not suit the scientists to the end, and they continued to work on this issue. In 2013, a team of researchers in Singapore said they had solved the age-old mystery.

A group of Chinese scientists claim that the secret of this effect lies in the amount of energy that is stored between water molecules in its bonds, called hydrogen bonds.

The answer from Chinese scientists

Further information will follow, for the understanding of which it is necessary to have some knowledge in chemistry in order to figure out which water freezes faster - hot or cold. As you know, it consists of two H (hydrogen) atoms and one O (oxygen) atom held together by covalent bonds.

But hydrogen atoms of one molecule are also attracted to neighboring molecules, to their oxygen component. These bonds are called hydrogen bonds.

At the same time, it is worth remembering that at the same time, water molecules act repulsively on each other. Scientists noted that when water is heated, the distance between its molecules increases, and this is facilitated by repulsive forces. It turns out that occupying one distance between molecules in a cold state, one can say that they stretch, and they have a greater supply of energy. It is this energy reserve that is released when water molecules begin to approach each other, that is, cooling occurs. It turns out that a larger supply of energy in hot water, and its greater release when cooled to sub-zero temperatures, occurs faster than in cold water, which has a smaller supply of such energy. So which water freezes faster - cold or hot? On the street and in the laboratory, the Mpemba paradox should occur, and hot water should turn into ice faster.

But the question is still open

There is only theoretical confirmation of this clue - all this is written in beautiful formulas and seems plausible. But when the experimental data, which water freezes faster - hot or cold, will be put in a practical sense, and their results will be presented, then it will be possible to consider the question of the Mpemba paradox closed.

"We have already come across some interesting properties of water that allow us to live in particular, and living beings in general. Let's continue the topic and bring to your attention one more interesting property (though it is not clear whether true or fictional).

Interesting about water - the Mpemba effect: did you know that there are rumors on the Internet that hot water freezes faster than cold water? You may not know, but these rumors are circulating. And very tenacious. So what are we talking about - an experimental error or a new, interesting property of water that has not yet been studied?

Let's figure it out. The legend, repeated from site to site, is this: let's take two containers of 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. Why is this happening?

In 1963, a Tanzanian student named Erasto B. Mpemba, while freezing a prepared 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 a 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 really freezes faster than cold water.

The second version of the legend - Mpemba turned to the great scientist, who, fortunately, was near the African school of Mpemba. And the scientist believed the boy and double-checked what was what. Well, off we go ... 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 do not fully understand the nature of this phenomenon, explaining it either by the difference in hypothermia, evaporation, ice formation, convection, or the effect of liquefied gases on hot and cold water.

So, we have the Mpemba effect (Mpemba Paradox) - a paradox that says that hot water (under certain conditions) can freeze faster than cold water. Although at the same time it must pass the temperature of cold water in the process of freezing.

Accordingly, in order to deal with the paradox, there are two ways. The first is to start explaining this phenomenon, coming up with theories and rejoicing that water is a mysterious liquid. Or you can go the other way - independently conduct this experiment. And draw the appropriate conclusions.

Let's turn to people who have actually had this experience trying to replicate the Mpemba effect. And at the same time, let's look at a small study that determines "where the legs grow from."

In Russian, a message about the Mpemba effect first appeared 42 years ago, as reported by the journal "Chemistry and Life" (1970, No. 1, p. 89). Being conscientious, the employees of "Chemistry and Life" decided to conduct experiments themselves and made sure: "hot milk stubbornly did not want to freeze first." This result was given a natural explanation: “A hot liquid should not freeze earlier. After all, its temperature must first be equal to the temperature of the cold liquid.

One of the readers of "Chemistry and Life" reported the following about his experiments (1970, No. 9, p. 81). He brought the milk to a boil, cooled it to room temperature, and put it in the refrigerator at the same time as the unboiled milk, which was also at room temperature. Boiled milk froze faster. The same effect, but weaker, was achieved by heating milk to 60°C rather than boiling. Boiling could be of fundamental importance: this will evaporate part of the water and evaporate the lighter part of the fat. As a result, the freezing point may change. In addition, when heated, and even more so when boiled, some chemical transformations of the organic part of milk are possible.

But the “broken telephone” was already working, and after more than 25 years, this story was described as follows: “A portion of ice cream becomes cold faster if it is put in the refrigerator, after warming it up well, than if it is first left at a cold temperature” (“Knowledge is power “, 1997, No. 10, p. 100). They gradually began to forget about milk, and it was mainly about water.

After 13 years, in the same "Chemistry and Life" the following dialogue appeared: "If you take two cups out into the cold - with cold and hot water, then which water will freeze faster? .. Wait for winter and check: hot water will freeze faster" ( 1993, No. 9, p. 79). A year later, a letter from one conscientious reader followed, who diligently took cups of cold and hot water out into the cold in winter and became convinced that cold water freezes faster (1994, No. 11, p. 62).

A similar experiment was carried out using a refrigerator, in which the freezer is covered with a thick layer of frost. When I put cups of hot and cold water on this freezer, the frost under the cups of hot water thawed, they sank and the water in them froze faster. When I put glasses on the frost, the effect was not observed, since the frost under the glasses did not melt. The effect was not observed even when, after defrosting the refrigerator, I put the cups on a frost-free freezer. This proves that the cause of the effect is the thawing of frost under cups of hot water (Chemistry and Life, 2000, no. 2, p. 55).

The story about the paradox noticed by the Tanzanian boy was repeatedly accompanied by a significant remark - they say, one should not neglect any, even very strange, information. The wish is good, but unrealizable. If we do not pre-screen unreliable information, then we will drown in it. And the wrong information is often wrong. In addition, it often happens (as in the case of the Mpemba effect) that the improbability is a consequence of the distortion of information in the transmission process.

Thus, it is interesting about water in general, and the Mpemba effect in particular, is not always true 🙂

More details - on the page http://wsyachina.narod.ru/physics/mpemba.html

At what temperature does water freeze? It would seem - the simplest question that even a child can answer: the freezing point of water at normal atmospheric pressure of 760 mm Hg is zero degrees Celsius.

However, water (despite its extremely wide distribution on our planet) is the most mysterious and not fully understood substance, so the answer to this question requires a detailed and reasoned conversation.

In Russia and Europe, the temperature is measured on the Celsius scale, the highest value of which is 100 degrees.
The American scientist Fahrenheit developed his own scale with 180 divisions.
There is another unit of temperature measurement - kelvin, named after the English physicist Thomson, who received the title of Lord Kelvin.

States and types of water

Water on planet Earth can take three main states of aggregation: liquid, solid and gaseous, which can transform into different forms that simultaneously coexist with each other (icebergs in sea water, water vapor and ice crystals in clouds in the sky, glaciers and free-flowing rivers ).

Depending on the characteristics of the origin, purpose and composition, water can be:

fresh;
mineral;
nautical;
drinking (here we include tap water);
rain;
thawed;
brackish;
structured;
distilled;
deionized.

The presence of hydrogen isotopes makes water:

light;
heavy (deuterium);
superheavy (tritium).

We all know that water can be soft and hard: this indicator is determined by the content of magnesium and calcium cations.

Each of the types and aggregate states of water we have listed has its own freezing and melting point.

Freezing point of water

Why does water freeze? Ordinary water always contains some amount of suspended particles of mineral or organic origin. It can be the smallest particles of clay, sand or house dust.

When the ambient temperature drops to certain values, these particles take on the role of centers around which ice crystals begin to form.

Air bubbles, as well as cracks and damage on the walls of the vessel in which water is located, can also become crystallization nuclei. The rate of water crystallization is largely determined by the number of these centers: the more of them, the faster the liquid freezes.

Under normal conditions (at normal atmospheric pressure), the temperature of the phase transition of water from a liquid to a solid state is 0 degrees Celsius. It is at this temperature that water freezes on the street.

Why does hot water freeze faster than cold water?

Hot water freezes faster than cold water - this phenomenon was noticed by Erasto Mpemba, a schoolboy from Tanganyika. His experiments with mass for making ice cream showed that the rate of freezing of the heated mass is much higher than the cold one.

One of the reasons for this interesting phenomenon, called the "Mpemba paradox", is the higher heat transfer of a hot liquid, as well as the presence in it of a larger number of crystallization nuclei compared to cold water.

Are the freezing point of water and altitude related?

With a change in pressure, often associated with being at different heights, the freezing point of water begins to radically differ from the standard, characteristic of normal conditions.
Crystallization of water at a height occurs at the following temperature values:

paradoxically, at an altitude of 1000 m, water freezes at 2 degrees Celsius;
at an altitude of 2000 meters, this happens already at 4 degrees Celsius.

The highest freezing temperature of water in the mountains is observed at an altitude of over 5,000 thousand meters (for example, in the Fann Mountains or the Pamirs).

How does pressure affect the process of water crystallization?

Let's try to link the dynamics of changes in the freezing point of water with changes in pressure.

At a pressure of 2 atm, water will freeze at a temperature of -2 degrees.
At a pressure of 3 atm, the temperature of -4 degrees Celsius will begin to freeze water.

With increased pressure, the temperature of the beginning of the water crystallization process decreases, and the boiling point increases. At low pressure, a diametrically opposite picture is obtained.

That is why in conditions of high mountains and a rarefied atmosphere it is very difficult to cook even eggs, since the water in the pot boils already at 80 degrees. It is clear that at this temperature it is simply impossible to cook food.

At high pressure, the process of ice melting under the blades of the skates occurs even at very low temperatures, but it is thanks to him that the skates glide on the ice surface.

The freezing of skids of heavily loaded sleds in the stories of Jack London is explained in a similar way. Heavy sleds that put pressure on the snow cause it to melt. The resulting water facilitates their sliding. But as soon as the sleds stop and linger for a long time in one place, the displaced water, freezing, freezes the skids to the road.

Crystallization temperature of aqueous solutions

Being an excellent solvent, water easily reacts with various organic and inorganic substances, forming a mass of sometimes unexpected chemical compounds. Of course, each of them will freeze at different temperatures. Let's put this in a visual list.

The freezing point of a mixture of alcohol and water depends on the percentage of both components in it. The more water added to the solution, the closer to zero its freezing point. If there is more alcohol in the solution, the crystallization process will begin at values close to -114 degrees.
It is important to know that water-alcohol solutions do not have a fixed freezing point. Usually they talk about the temperature of the beginning of the crystallization process and the temperature of the final transition to the solid state.
Between the beginning of the formation of the first crystals and the complete solidification of the alcohol solution lies a temperature interval of 7 degrees. So, the freezing point of water with alcohol of 40% concentration at the initial stage is -22.5 degrees, and the final transition of the solution to the solid phase will occur at -29.5 degrees.

The freezing point of water with salt is closely related to the degree of its salinity: the more salt in the solution, the lower the position of the mercury column it will freeze.

To measure the salinity of water, a special unit is used - "ppm". So, we have found that the freezing point of water decreases with increasing salt concentration. Let's explain this with an example:

The salinity level of ocean water is 35 ppm, while the average value of its freezing is 1.9 degrees. The degree of salinity of the Black Sea waters is 18-20 ppm, so they freeze at a higher temperature in the range from -0.9 to -1.1 degrees Celsius.

The freezing point of water with sugar (for a solution whose molality is 0.8) is -1.6 degrees.
The freezing point of water with impurities largely depends on their amount and the nature of the impurities that make up the aqueous solution.
The freezing point of water with glycerin depends on the concentration of the solution. A solution containing 80 ml of glycerin will freeze at -20 degrees, when the glycerol content is reduced to 60 ml, the crystallization process will begin at -34 degrees, and the beginning of freezing of a 20% solution will be minus five degrees. As you can see, there is no linear relationship in this case. To freeze a 10% solution of glycerin, a temperature of -2 degrees will suffice.
The freezing point of water with soda (meaning caustic alkali or caustic soda) presents an even more mysterious picture: a 44% caustic solution freezes at +7 degrees Celsius, and 80% at + 130.

Freezing of fresh water

The process of ice formation in freshwater reservoirs occurs in a slightly different temperature regime.

The freezing point of water in a lake, just like the freezing point of water in a river, is zero degrees Celsius. The freezing of the cleanest rivers and streams does not start from the surface, but from the bottom, on which there are crystallization nuclei in the form of bottom silt particles. At first, snags and aquatic plants are covered with a crust of ice. As soon as the bottom ice rises to the surface, the river instantly freezes through.
Frozen water on Lake Baikal can sometimes cool down to negative temperatures. This happens only in shallow water; the water temperature in this case can be thousandths, and sometimes hundredths of one degree below zero.
The temperature of the Baikal water under the very crust of the ice cover, as a rule, does not exceed +0.2 degrees. In the lower layers, it gradually rises to +3.2 at the bottom of the deepest basin.

Freezing point of distilled water

Does distilled water freeze? Recall that for water to freeze, it is necessary to have some crystallization centers in it, which can be air bubbles, suspended particles, as well as damage to the walls of the container in which it is located.

Distilled water, completely devoid of any impurities, does not have crystallization nuclei, and therefore its freezing begins at very low temperatures. The initial freezing point of distilled water is -42 degrees. Scientists managed to achieve supercooling of distilled water to -70 degrees.

Water that has been exposed to very low temperatures but has not crystallized is called "supercooled". You can place a bottle of distilled water in the freezer, achieve hypothermia, and then demonstrate a very effective trick - see the video:

By gently tapping on a bottle removed from the refrigerator, or by throwing a small piece of ice into it, you can show how instantly it turns into ice, which looks like elongated crystals.

Distilled water: does this purified substance freeze or not under pressure? Such a process is possible only in specially created laboratory conditions.