Chemistry is an interesting and rather complex science. Its terms and concepts come across to us in everyday life, and it is 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 encounter its use because we are surrounded by these very solutions. But what is important is not so much the use of this concept as the physical phenomena that it means. But before moving on to the main part of our story, we will be transported to the nineteenth century, when Svante Arrhenius and Wilhelm Ostwald formulated the theory of electrolytic dissociation.
History
The study of solutions and solubility begins with the physical theory of dissociation. It is the easiest to understand, but too primitive and only in terms of moments coincides with reality. The essence of this theory is that a dissolved substance, falling into a 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 a solution as a system in which particles interact with each other and form the so-called solvates - ions surrounded by dipoles. A dipole is, on the whole, a neutral molecule, the poles of which are oppositely charged. A dipole is most often a solvent molecule. Once in solution, the dissolved substance decomposes into ions, and dipoles are attracted to one end of an oppositely charged end, and to other ions to an end of an opposite oppositely charged end. 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 more closely.
Theories of solutions
The formation of such particles can explain many phenomena that cannot be described using the classical theory of solutions. For example, the thermal effect of a dissolution reaction . From the perspective of the Arrhenius theory, it is difficult to say why, when one substance is dissolved in another, heat can be absorbed and released. Yes, there is a destruction of the crystal lattice, and therefore the energy is either expended and the solution is cooled, or released during decay due to the excess energy of chemical bonds. But it is impossible to explain this from the standpoint of classical theory, since the very mechanism of destruction remains incomprehensible. And if you apply the chemical theory of solutions, it becomes clear that the solvent molecules, wedging into the voids of the lattice, destroy it from the inside, as if “protecting” ions from each other with a solvation shell.
In the next section, we consider what solubility is and everything related to this seemingly simple and intuitive value.
The concept of solubility
It is purely intuitive that solubility shows how well a substance dissolves in a given particular solvent. However, we usually know very little about the nature of the dissolution of substances. Why, for example, does chalk not dissolve in water, and table salt - on the contrary? It's all about the strength of the bonds within the molecule. If the bonds are strong, then because of this, these particles cannot dissociate into ions, thereby destroying the crystal. Therefore, it remains insoluble.
Solubility is a quantitative characteristic showing how much of the solute is in the form of solvated particles. Its value depends on the nature of the solute and solvent. The solubility in water for different substances is different, depending on the bonds between atoms in the molecule. Substances with covalent bonds have the lowest solubility, while with ionic ones the highest.
But it is not always possible to understand which solubility is large and which is small. Therefore, in the next section we discuss what the solubility of various substances in water is equal to.
Comparison
In nature, there are many liquid solvents. There are even more alternative substances that can serve as the last when certain conditions are reached, for example, a certain state of aggregation. It becomes clear that if you collect data on the solubility of each pair of “dissolved substance - solvent” in each other, it will not be enough for ages, because a huge number of combinations are obtained. Therefore, it so happened that on our planet a universal solvent and standard is water. They did this because it is most common on Earth.
Thus, a table of solubility in water was compiled for many hundreds and thousands of substances. We all saw her, but in a shorter and more understandable way. The cells of the table contain letters indicating a soluble substance, insoluble or sparingly soluble. But there are more highly specialized tables for those who are seriously versed in chemistry. It indicates the exact numerical value of solubility in grams per liter of solution.
Now we turn to the theory of such a thing as solubility.
Chemistry of solubility
How the dissolution process itself occurs, we have already examined in the previous sections. But how, for example, to write it all in the form of a reaction? It's not that simple. For example, when acid is dissolved, a hydrogen ion reacts with water to form a 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 its chemical reaction. The form of the latter depends on the structure of the salt and the bonds inside its molecules.
We figured out how to record graphically the solubility of salts in water. Now it's time for practical application.
Application
If we list those cases when this value is necessary, not a century is enough. Indirectly using it, you can calculate other quantities that are very important for the study of any solution. Without it, we would not be able to find out the exact concentration of a substance, its activity, and we would not be able to assess whether a medicine would cure a person or kill (because in large quantities even water is life-threatening).
In addition to the chemical industry and scientific purposes, an understanding of the essence of solubility is also necessary in everyday life. Indeed, sometimes it is required to prepare, say, a supersaturated solution of a substance. For example, this 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 be poured into a vessel so that it begins to precipitate and form crystals from an excess.
Before completing our brief excursion into chemistry, let’s talk about a few concepts related to solubility.
What else is interesting?
In our opinion, if you get to this section, then you probably already realized 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 about the nature of solutions, the study of which began with solubility, we can no longer imagine modern chemistry and physics. What does physics have to do with it? Sometimes physicists also deal with solutions, measure their conductivity, and use their other properties for their needs.
Conclusion
In this article, we met with a chemical concept such as solubility. This was probably quite useful information, since most of us are unlikely to imagine the deep essence of the theory of solutions without having the desire to dive into its study in detail. In any case, it is very useful to train your brain, learning something new. After all, a person’s life must “study, study and study again”.