We call this Mpemba Effect. A long time ago, Aristotle was the first man who noticed how hot water froze faster than cold. Even though water is one of the most abundant compounds on this planet, it is also one of the more mysterious compounds. Most liquids it becomes denser as it cools. On the other hands, it reaches a state of maximum density at 4 degrees C and becomes less dense before it freezes. When it becomes solid, it is less dense. That is the reason why standard ice floats on water. Also, that is the reason life on Earth has flourished. If the ice were denser than water, of course, the lakes and oceans would freeze from the bottom up.
Additionally, there is a strange Mpemba effect. It was when a Tanzanian student discovered that a hot ice cream mix freezes faster than a cold mix in his cookery classes at the beginning of the 1960s. Many scientists have noticed this effect, including Aristotle, Francis Bacon, and René Descartes.
It is called the Mpemba effect, the observation that warm water freezes faster than cold water. No one could provide convenient explanations. Therefore, the explanation of this observation is still needed. Today, Xi Zhang at the Nanyang Technology University in Singapore with a few pals provides a nice explanation. These people explain that Mpemba paradox is the result of the unique properties in different bonds. These properties work together to hold water.
Is there something wrong with the bonds in water? A single water molecule has a large oxygen atom along with hydrogen bonds. They have an important role. It happens when hydrogen in one molecule is getting close to the oxygen in another and bond to it. So, hydrogen bonds are weaker compared to covalent bonds. On the other hands, hydrogen bonds are stronger compared to van der Wall forces used by geckos to climb walls.
For chemists, they have long known that these particles are important. They have known that water’s boiling point is higher than other liquids of similar molecules since hydrogen bonds can hold it. Recently, chemists become more aware of the subtle roles of hydrogen bonds. Hydrogen bonds hold water molecules together inside narrow capillaries to form into chains. This is important in trees and plants when water evaporation across a leaf membrane and pulls a chain of water molecules up from the roots.
So, Xi and the team say that hydrogen bonds can explain the Mpemba effect. What they try to say about their idea is that hydrogen bonds can bring water molecules into the close contact. At this point, the natural repulsion between the molecules can cause covalent O-H bonds. It works to stretch and then store energy.
As the liquid warms up, then the hydrogen bonds stretch and the water molecules sit further apart. The covalent molecules then shrink again and give up their energy. This process will create the covalent bonds to give energy that is the same for cooling.
Eventually, the effect will add a more conventional process of cooling. In short, they say that warm water can cool faster than cold water and that is exactly what is in the Mpemba effect. Additionally, these people have calculated the magnitude of the extra cooling effect. They showed accurately in their experiments and they measured the different cooling rates of cold and hot water. This is an interesting insight to answer the complex and mysterious properties of water.
So far, Xi and the team have a nice and convincing idea but many physicists still need to settle the question. It is because the new theory needs more predictive paper. Xi and the team need their theory to predict the new property of water that conventional thinking about water does not. For instance, the shortened covalent bonds may rise to some measurable property of the water that will not be present.
So, these guys have solved the puzzle about the Mpemba effect, but they have to work more to convince everyone. Still, it is interesting to know their work!