A heterogeneous system, in contrast to a homogeneous system, is such a physicochemical system in which there are phases differing in physical properties. That is, there are parts in it that have different aggregate states and composition of elements. One phase of such a system is separated from another by a boundary, the transition through which causes a qualitative leap in the change in the physicochemical properties of the system (substance). Such variable parameters can be the shape and structure of the crystal lattice, the density of a substance, its composition, electromagnetic field and others.
Thus, in a heterogeneous system, at least mechanical phase separation exists. Examples of such systems are water and steam, which have different aggregate states, oil and water, which, being in the same vessel, have a different composition and others. Quite often it is impossible to draw a clear line between homogeneous and heterogeneous systems, because it is difficult to establish the very boundary of the transition between phases. For example, in mechanical suspensions, this boundary is occupied by colloids, and in them there are tiny particles of solute. On the one hand, it is a homogeneous system, because the particle sizes are so small that their size can be neglected. And on the other hand, although at the atomic level, but the substance is still present in them, in this case, such a system can be considered heterogeneous.
The most important parameter characterizing the principle of heterogeneity is heterogeneous equilibrium. Consider the meaning of this phenomenon on the example of aqueous solutions. In them, heterogeneous equilibria have such properties that are the transfer of particles across the boundary of the phase limit in at least two neighboring phases. In this description we touch only a small circle of these phenomena, which are most significant from the point of view of their practical application. According to their properties, heterogeneous equilibria are quite diverse, and therefore they also occur in chemical technological processes, and are of interest from the point of view of theoretical analysis in physical chemistry.
On a practical plane, of interest is, first of all, systems such as solid - liquid phase (sediment - saturated solution). Such a system is important because in practice many chemical technologies are based on the separation of one substance from another. In addition, reverse processes are also important, where heterogeneous equilibria play a large role. These include reactions of the translation of poorly soluble compounds into solutions.
Another system - the solid phase - a liquid solution is based on the phenomenon of ion exchange. This chemical phenomenon is common in technologies for purifying aqueous solutions from various kinds of impurities and is used when it is necessary to separate substances from each other. In this case, there are heterogeneous equilibria in electrolyte solutions, they are important because conventional chemical methods of separation of compounds are ineffective. The practical implementation of such transitions is provided by ion exchange, which occurs on the surface of ion exchangers in the course of an electrolytic reaction.
The third common of the systems under consideration is the liquid - liquid phase system, we are talking about solvents with different compositions of the substance. This case is applied when two solutions contain substances that are immiscible in each other with different solubility indices, that is, isolated heterogeneous equilibria take place. The practical use of this phenomenon is called extraction. In industrial production and in chemical processes, extraction acts as the most effective way of separating substances.