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 in order to cool to the temperature of the compared cold water in order to turn into ice. In cold water, this stage can be skipped, and, accordingly, in time it wins.
But the answer to the question of which water freezes faster — cold or hot — is cold outside, any resident of the northern latitudes knows. In fact, scientifically, it turns out that in any case, cold water simply must freeze faster.
The physics teacher, who was approached by a schoolboy Erasto Mpemba in 1963, asked to explain why the cold mixture of the future ice cream freezes longer than a similar but hot one.
"This is not world physics, but some kind of physics Mpemba"
At that time, the teacher only laughed at it, but Deniss Osbourne, a professor of physics, who once drove into the same school where Erasto studied, experimentally confirmed the existence of such an effect, although there was no explanation then. In 1969, in a popular scientific journal, a joint article was published by these two people 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 the paradox, Mpemba.
The question arose a long time ago
Naturally, such a phenomenon had taken place before, and it was mentioned in the works of other scientists. Not only the student was interested in this issue, but Francis Bacon, Rene Descartes and even Aristotle thought about it in their time.
Here are just approaches to solving this paradox began to search only at the end of the twentieth century.
Conditions for a paradox to occur
As in the case of ice cream, not just ordinary water freezes during the experiment. Certain conditions must be present in order to start arguing which water freezes faster - cold or hot. What influences the course of 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 hot water has a higher evaporation rate than cold. Thus, its volume decreases, and with a decrease in volume, the freezing time also becomes shorter than if we take the same initial volume of cold water.
It has long been defrosting the freezer
What kind of water freezes faster, and why this happens, can be affected by the snow lining, which can occur 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, the container with hot water will melt the snow underneath, thereby improving the contact of the heat level with the wall of the refrigerator. A cold water container cannot do this. 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 begins to freeze from the upper layers, when hot water does the opposite - it starts to freeze from the bottom up. Moreover, it turns out that cold water, having a cold layer on top with ice already formed in places, 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 comes out, tending upward, and there it meets with a very chilled layer. There is no free path for heat radiation, so the cooling process becomes difficult. Hot water absolutely has no such obstacles in its path. Which freezes faster - cold or hot, on which the likely outcome depends, you can expand the answer by the fact that any water has certain substances dissolved in it.
Impurities in the composition of water as a factor affecting 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 available only in hot water, but cold water does not possess them, then there is a possibility for hot water to freeze earlier. This is explained by the fact that dissolved substances in water create crystallization centers, and with a small number of these centers, the transformation of water into a solid state is difficult. It is even possible to supercool water, in the sense that at minus temperature it will be in a liquid state.
But all these versions, apparently, were not completely satisfied with the scientists and they continued to work on this issue. In 2013, a team of researchers in Singapore said they were able to solve a century-old riddle.
A group of Chinese scientists argues that the secret to this effect is the amount of energy that is stored between water molecules in its bonds, called hydrogen.
The answer from Chinese scientists
This is followed by information, for the understanding of which it is necessary to have some knowledge in chemistry in order to understand what water freezes faster - hot or cold. As you know, a water molecule consists of two H (hydrogen) atoms and one O (oxygen) atom, held together by covalent bonds.
But also the hydrogen atoms of one molecule are attracted to neighboring molecules, to their oxygen component. It is these bonds that are called hydrogen bonds.
It is worth remembering that at the same time, water molecules act repulsively on each other. Scientists noted that when water is heated between its molecules, the distance increases, and this is facilitated by just repulsive forces. It turns out that hydrogen bonds, occupying the same distance between molecules in the cold state, can be said to stretch, and they have a larger supply of energy. It is this supply of energy that is released when water molecules begin to come closer to each other, that is, cooling occurs. It turns out that a larger supply of energy in hot water, and its greater release upon cooling to sub-zero temperatures, occurs faster than in cold water, in which the supply of such energy is less. So which water freezes faster - cold or hot? On the street and in the laboratory, the Mpemba paradox must 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 the question of the Mpemba paradox can be considered closed.