Solubility is what?

Chemistry is an interesting and rather complicated science. Its terms and concepts come across to us in everyday life, and it's not always intuitively clear what they mean and what their meaning is. One such concept is solubility. This term is widely used in the theory of solutions, and in everyday life we come across its application because they are surrounded by these very solutions. But it is not so much the very use of this concept as the physical phenomena that it means. But before proceeding to the main part of our narrative, we move on to the nineteenth century, when Svante Arrhenius and Wilhelm Ostwald formulated the theory of electrolytic dissociation.

History

The investigation of solutions and solubility begins with the physical theory of dissociation. It is the simplest to understand, but too primitive and only in moments coincides with reality. The essence of this theory is that the dissolved substance, falling into the solution, decomposes into charged particles, called ions. It is these particles that determine the chemical properties of the solution and some of its physical characteristics, including conductivity and boiling point, melting and crystallization.

However, there are more complex theories that consider the solution as a system, the particles in which interact with each other and form the so-called solvates - ions surrounded by dipoles. The dipole is, as a whole, a neutral molecule, the poles of which are variously charged. Dipolem is most often a solvent molecule. Getting into the solution, the dissolved substance breaks up into ions, and the dipoles are attracted to one ion by a different end loaded with respect to them, and to other ions - respectively, by the other end of the charge, charged with respect to them. Thus, solvates are obtained - molecules with a shell of other neutral molecules.

Now let's talk a little about the essence of the theories themselves and look at them carefully.

Theory of solutions

The formation of such particles can explain a variety of phenomena that can not be described by the classical theory of solutions. For example, the thermal effect of the dissolution reaction . From the position of the Arrhenius theory, it is difficult to say why, when dissolving one substance in another, heat can be absorbed and released. Yes, the crystal lattice is destroyed, and therefore the energy is either expended and the solution is cooled, or is released in the decay due to the excess energy of the chemical bonds. But to explain this from the standpoint of the classical theory is impossible, since the very mechanism of destruction remains incomprehensible. And if you apply the chemical theory of solutions, it becomes clear that the molecules of the solvent, wedging into the cavities of the lattice, destroy it from the inside, as if "shielding" the ions from each other with a solvate shell.

In the next section, we will consider what is solubility and everything related to this seemingly simple and intuitive figure.

The concept of solubility

It is purely intuitively clear that solubility shows how well a particular substance dissolves in a given particular solvent. However, we know very little about the nature of the dissolution of substances. Why, for example, does the chalk not dissolve in water, and table salt - on the contrary? It's all about the strength of the bonds inside the molecule. If the bonds are strong, then because of this, these particles can not dissociate into ions, thereby destroying the crystal. Therefore, it remains insoluble.

Solubility is a quantitative characteristic showing how much of the dissolved substance is in the form of solvated particles. Its value depends on the nature of the dissolved substance and the solvent. The solubility in water for different substances is different, depending on the bonds between the atoms in the molecule. Substances with covalent bonds have the lowest solubility, whereas those with ionic bonds have the highest solubility.

But it is not always possible to understand which solubility is large and which is small. Therefore, in the next section we will discuss what is the solubility of various substances in water.

Comparison

In nature, there are a lot of liquid solvents. Even more there are alternative substances that can serve as the last ones when certain conditions are reached, for example, a certain aggregate state. It becomes clear that if we collect data on the solubility in each other of the "dissolved substance-solvent" pair, there will not be enough for an eternity, because the combinations produce a huge number. Therefore, it so happened that on our planet the universal solvent and standard is water. They did it because it is the most common on Earth.

Thus, a table of water solubility was prepared for many hundreds and thousands of substances. We all saw it, but in a shorter and understandable version. In the cells of the table inscribed letters denoting a soluble substance, insoluble or slightly soluble. But there are more narrowly specialized tables for those who are serious about chemistry. There is indicated the exact numerical value of solubility in grams per liter of solution.

Now let us turn to the theory of such a concept as solubility.

Chemistry of solubility

How the dissolution process itself is going on, we have already disassembled the previous sections. But how, for example, to write this all in the form of a reaction? Here everything is not so simple. For example, when the acid dissolves, the hydrogen ion interacts with water to form the hydroxonium ion H ₃ O ⁺ . Thus, for HCl, the reaction equation will look like this:

HCl + H ₂ O = H ₃ O ⁺ + Cl ^-

The solubility of salts, depending on their structure, is also determined by their chemical reaction. The appearance of the latter depends on the structure of the salt and the bonds within its molecules.

We have figured out how to write graphically the solubility of salts in water. Now it's time for practical application.

Application

If we list the cases when this quantity is necessary, there will not be enough time. Indirectly, it can be used to calculate other quantities that are very important for the study of any solution. Without it, we could not know the exact concentration of the substance, its activity, could not assess whether the medicine cures a person or kills (in fact, in large quantities, even water is dangerous for life).

In addition to chemical industry and scientific purposes, understanding the essence of solubility is also necessary in everyday life. After all, sometimes it is required to prepare, say, a supersaturated solution of a substance. For example, it is necessary to obtain salt crystals for a child's homework. Knowing the solubility of salt in water, we can easily determine how much it needs to fall asleep in a vessel, so that it begins to precipitate and form crystals from an overabundance.

Before completing our brief excursion into chemistry, let's talk about several concepts that are adjacent to solubility.

What else is interesting?

In our opinion, if you have reached this section, you probably already understood that solubility is not just a strange chemical quantity. It is the basis for other quantities. And among them: concentration, activity, dissociation constant, pH. And this is not a complete list. You must have heard at least one of these words. Without this knowledge of the nature of solutions, the study of which began with solubility, we can no longer imagine modern chemistry and physics. What does physics mean? Sometimes physicists also deal with solutions, measure their conductivity, and use their other properties for their own needs.

Conclusion

In this article we have become acquainted with such a chemical concept as solubility. This, probably, was quite useful information, since most of us hardly imagine the deep essence of the theory of solutions, without having the desire to plunge in detail into its study. In any case, it is very useful to train your brain, learning something new. After all, a person must "study, learn and study again".