To characterize a sufficiently large number of both chemical and physical processes and systems, the category "phase" is used.
In the simplest sense, the category “phase” is called an independent, part of a certain system, which is characterized by the relative uniformity of structure and composition. The phase, as a rule, is quite autonomous and is separated from other phases of the system by a certain boundary - the interphase section.
From the point of view of chemical systems, which include a homogeneous system, a phase can be a substance in any state of aggregation: gas, solid, liquid, as well as their mixtures and solutions. A common property is also the fact that any non-miscible part of the gas, liquid, or any of the solids constitutes a separate phase. For example, if we consider as a sample a system that includes ice, which melts and water, then in this system two phases can be distinguished - water and ice. The fact is that although they have the same composition, they are different in the structure of the substance, and therefore they are autonomous phases. In the same way, we can give examples of the existence of substances in which we can fix the presence of only one phase. These include air, an aqueous solution of potassium permanganate, hydrochloric acid. In these compounds there are no visible interphase sections, and at each point in the system its composition is homogeneous.
At the same time, the definition of the concept indicated at the very beginning of the article is not sufficient and exhaustive. It includes the requirements of uniformity and identity of the composition, but this approach is only relevant for equilibrium systems. In the same case, when the system is in a state of a chemical reaction, the phase can also be inhomogeneous. And the volumes of the homogeneous phase will be incommensurable with the dimensions of the elements making up this phase.
The category “phase” also serves as a criterion for the classification of physicochemical systems. By the number of them, all systems are divided into species such as a homogeneous system and a heterogeneous system. Accordingly, one can also formulate and define these categories. A heterogeneous system is one in which the number of phases is not equal to one. And, accordingly, a homogeneous system is a system in which only one phase is released.
The phases themselves are also classified into types. A continuous phase is one in which a hit from one point to another occurs without crossing the interphase. It is the presence of such and only such a phase that a homogeneous system is characterized.
In contrast, a heterogeneous system is created both by a continuous phase and a dispersed one, that is, fragmented into a number of separate components.
True solutions are an example of homogeneous systems , because the particles that make them up represent a mixture that exists at the atomic level.
The most significant physical meaning of this concept is characterized by such a category as the equilibrium constant of a homogeneous system. This constant, in turn, characterizes another important property of homogeneous systems - equilibrium. Reactions that occur within the same phase are called homogeneous, which means that all the elements involved in it are in the same state of aggregation. Equilibrium in the course of such reactions characterizes such a thing as equilibrium in a homogeneous system. This means that the course of such reactions occurs to such an established concentration level, at which both direct and reverse reactions are possible. This state is also called the state of chemical equilibrium and it, by and large, acts as an indicator at which the reaction stops as such.
The equilibrium constant makes it possible to determine the ability of the full course of a chemical reaction under certain conditions. It is a universal value and applies to any systems.