If you put hot water in a refrigerator, you can observe the effect of its accelerated freezing. From two containers with cold and hot water in the freezer, hot water will freeze faster.
This fact is confirmed, for example, by the fact that in severe frost, open pipes with hot water freeze faster than cold ones.
Aristotle, Francis Bacon, and Rene Descartes mentioned this phenomenon as early as the fourth century BC.
Aristotle described how some inhabitants of present-day Turkey sprinkled the stakes of their palisades with hot water to secure them because, in this way, they froze earlier.
And this physical curiosity also attracted later in history such famous figures as the so-called father of scientific empiricism, Francis Bacon, or the French philosopher Rene Descartes.
This phenomenon was explained not until 1969, and today, this phenomenon is called the Mpemba Effect.
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In 1963, an ordinary schoolboy named Erasto Mpemba from Africa noticed that it would freeze faster if you heat ice cream and put it in the refrigerator.
With this, he turned to his school teacher but did not receive an answer. But physics professor Dennis Osborne happily helped the boy understand this phenomenon.
For six years, a student and a professor conducted experiments, and in 1969, the Physics Education journal published their results.
To do this, they studied the freezing time of different water samples in a freezer. They observed that this time was longer when the water was initially around 25 degrees and minimum when it was around 90 degrees.
The Result of the Experiment Explained
Since cooling begins predominantly from the upper surface of the liquid, the cooling rate depends on the temperature of this particular surface and not on the average temperature of the liquid, and convection processes maintain this temperature.
Due to this, the rate of heat loss for a system with a higher initial temperature will also be higher than for an initially cooled system.
This statement is controversial since water must pass intermediate temperatures before freezing, but given the influence of the temperature gradient, the authors overlooked this statement.
After that, the phenomenon began to be actively discussed by researchers and acquired the name Mpemba effect.
Explanation of Mpemba Effect
People have been looking for the answer to the question of why hot water freezes faster for more than half a century.
Hundreds of scientific and research papers have been published on this subject, but it was only 54 years later that we had received the definitive answer.
In 2013, the UK’s Royal Chemical Society pledged to award a £ 1,000 prize to anyone explaining the Mpemba effect.
Nikola Bregoviča from the University of Zagreb in Croatia gave the best answer. He summarized the main theories studied earlier and described them.
In 2016, a group of scientists published research materials that denied the existence of this effect. The explanation of the impact itself was based on the research error.
In 2017, a group of scientists from China and the USA explained the phenomenon of hydrogen bonds in the cluster structure of water.
1. Evaporation of water
Some scientists explained that heated water evaporates faster and either freezes in the air and forms an ice crust or is removed from the system.
In all experiments where the researchers weighed the mass of water before and after freezing, the maximum weight loss reached no more than 3%. Such an insignificant change cannot cause a significant acceleration of freezing.
Another difficulty of this experiment was that it was practically impossible to prove this moment since when sealing a container with frozen water, evaporation and the movement of heat flows would change.
2. Dissolved gases
The solubility of water gases decreases with an increase in its temperature. Some of the researchers assumed that the rapid freezing of water is associated with this fact.
Thomas’s research showed that the difference in freezing temperatures slightly deviates from zero, and Auerbach proved that the concentration of gases in water does not affect the supercooling of water.
Convection happens when a fluid moves while being cooled or heated. What happens is that hot water is less dense than cold water (up to 4ºC)?
So when the container with hot water is being cooled, the cold water, which in general will be closer to the walls of the container, will tend to go to the bottom of it.
This process will generate a circulation of water that will be responsible for the Mpemba effect. From 4ºC, colder water is less dense than hot water.
So, the cold water in the container reaches the freezing point; an ice layer forms on the surface, making it difficult for the water that remained below to freeze.
In the container with initially hot water, the convection does not allow this layer of ice to form that prevents the total solidification of the water, and therefore, it freezes first.
4. Environmental effect
The difference between the starting temperatures of two water containers can affect the environment that can influence the cooling rate.
An example would be hot water melting a pre-existing ice sheet, allowing for a better cooling rate.
Hot water tends to have a more negligible effect of supercooling the cold water. When supercooled, it can remain liquid until disturbed, even well below its average freezing temperature.
Water that is not supercooled is more likely to become solid when it reaches the freezing point of water.
6. Hydrogen Bonds- the best explanation
In 2017, Zi Chang of Nanyang Technological University in Singapore and several colleagues provided one better explanation.
These guys argue that the Mpemba effect results from the unique properties of different bonds that hold water molecules together.
Each water molecule is composed of a relatively large oxygen atom. The conventional covalent bond bonds the oxygen atom to two tiny hydrogen atoms.
But if you put multiple water molecules next to each other, then hydrogen bonds will also begin to play an important role.
This bond happens because the hydrogen atoms of one molecule are located near the oxygen of another molecule and interact with it. Chemists have known the importance of these connections for a very long time.
For example, the boiling point of water is much higher than other liquids with similar molecules because hydrogen bonds hold the molecules together.
Today, chemists have become increasingly interested in other roles that hydrogen bonds can play. For example, water molecules in thin capillaries form long chains held by hydrogen bonds.
This chain is essential for plants in which evaporation of water through leaf membranes pulls a chain of water molecules up from the roots effectively.
Now Zee and co-authors argue that hydrogen bonds also explain the Mpemba effect. Their essential idea is that hydrogen bonds lead to tighter contact with water molecules.
When this happens, the molecules contract the covalent bonds due to the natural repulsion and energy accumulation.
However, the distance between the molecules increases when the liquid heats and the hydrogen bonds stretch. It also allows you to increase the length of covalent bonds and thus give back the energy stored in them.
An essential element of the theory is that the process in which covalent bonds give up the energy stored in them is equivalent to cooling.
The typical cooling process amplifies this effect. Thus, hot water-cooled faster than cold water, the authors argue. And this is what we see in the Mpemba effect.
Still, today, it is not explained correctly why hot water freezes faster than cold water. But it certainly is an exciting phenomenon that has puzzled scientists for many years and has also been one of the trending videos where many people can be seen repeating this magic.