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The heterogeneous system, in contrast to the homogeneous system, is a physicochemical system in which the phases differing in physical properties exist. 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 the other by a boundary, the passage through which causes a qualitative jump in the change in the physico-chemical properties of the system (substance). Such variable parameters can be the shape and structure of the crystal lattice, the density of matter, its composition, electromagnetic field, and others.
Thus, at least a mechanical phase separation exists in the heterogeneous system. Examples of such systems are water and steam, which have different aggregate states, oil and water, which are in the same vessel, have a different composition, and others. Quite often it is impossible to draw a clear boundary 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 minute particles of the dissolved substance. On the one hand, it is a homogeneous system, because the particle sizes are so small that their magnitude can be neglected. And on the other hand, although at the atomic level, but the substance in them is still present, in this case, such a system can be regarded as heterogeneous.
The most important parameter characterizing the principle of heterogeneity is heterogeneous equilibria. Consider the meaning of this phenomenon using water solutions as an example. In them, heterogeneous equilibria have such properties, which consist in the transfer of particles through the boundary of the phase limit, at least in two neighboring phases. We will touch on this description only a small circle of these phenomena, which are most significant from the point of view of their practical application. By their properties, heterogeneous equilibria are quite diverse, and therefore take place in chemical technological processes, and are of interest from the point of view of theoretical analysis in physical chemistry.
In the practical plane of interest are, above all, such systems as solid - liquid phase (precipitate - 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 important, where heterogeneous equilibria play a large role. These include the reactions of translating poorly soluble compounds into solutions.
Another system - the solid phase - the liquid solution is based on the phenomenon of ion exchange. This chemical phenomenon is widespread in the technology of cleaning 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 for separating compounds are ineffective. The practical realization of such transitions is provided by ion exchange, which proceeds on the surface of ion exchangers during the electrolytic reaction.
The third system that is common among the systems under consideration is a liquid-liquid system, it is a question of solvents with different compositions of matter. This case is used when two substances contain substances immiscible to each other with different solubility indices, that is, isolated heterogeneous equilibria take place. The practical use of this phenomenon has been called extraction. In industrial production and in chemical processes, extraction is the most effective way of separating substances.