why this liquid does not follow the rules of chemistry



If there is a chemical formula that everyone knows, it is the Water (H2O), a molecule essential for life on Earth, a name not without a certain sarcasm, since 70 percent of its surface is covered by water.

Water is the only substance that under environmental conditions can be found in solid, liquid and gaseous state. As a solid it is much lighter than you might expect, as a liquid it is much denser, and as a gas it is one of the lightest known. A triple list of singularities that make the water have little ‘ordinary’ and that are due to hydrogen bonds, the bonds that join its molecules.

Anomalous dilation of water

If an ice cube were to sink into a glass of water, instead of floating, it would leave us speechless, however, that is what should be produced. We have all been taught that substances when heated expand and when they cool down they contract. However, water is an exception. When it is hot it reduces its volume as it cools, but when it reaches the 4ºC -the temperature with the highest density- it begins to dilate again. This allows us to explain that ice has a lower density than liquid water and that, therefore, it floats in it.

This property is known as the anomalous dilation of water and is responsible for the fact that life can continue to exist in the depth of a lake after its surface has frozen.

Hot freezes before cold

If we pull the periodic table it would seem reasonable, at least in theory, that water (H2O) behaves similarly to hydrogen sulfide (H2S), hydrogen selenide (H2Se) or hydrogen tellide (H2Te), other elements that follow oxygen. If so, the water should boil at -80ºC and freeze at -100ºC, instead of at 100ºC and 0ºC, respectively.

This rise in latent heat, understood as the heat that is needed for a substance to change state, makes the strip in the liquid state of water very wide, with all that that entails.

Precisely in the change of state lies another of its peculiarities. Surely if we asked an audience what freezes cold water or hot water before, the majority would be inclined towards cold water. However, the answer would be wrong.

It has been almost 2,500 years since the philosopher Aristotle observed that water that has been previously heated freezes faster than when cold, a phenomenon we now know as Mpemba effect. His name refers to a question he posed Erasto Mpemba, a Tanzanian student, in 1969.

This effect is due to the fact that the cooling process in cold water is slower and that when the temperature is higher the particles interact much more with each other and lose part of their kinetic energy. This explains why in some countries with very cold climates, if, on a day with sub-zero temperatures, boiling water is thrown into the air, it quickly turns into snow.

It is possible to slide on its surface

It is said that water is the universal solvent since it is the liquid that is capable of dissolving a greater number of substances (solutes). This is due to the fact that being a polar substance –neutral charge- when the molecules of another substance come into contact with it, their positive charges (hydrogen) surround the negative ones and their negative charges (oxygen) do the same with the positive ones.

On the other hand, the molecules inside it are attracted in a similar way by the molecules in their environment, while the molecules on the surface are only attracted by those on the sides and bottom. This generates a layer on the surface of the water that exerts a certain resistance to any object that wants to penetrate inside. This is what is known as surface tension.

After mercury, water is the liquid that has the highest surface tension, which allows, for example, that some insects – shoemakers or water scribes – can slide on its surface.

Pedro Gargantilla is an internist at the Hospital de El Escorial (Madrid) and the author of several popular books.

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