Homogeneous and Heterogeneous Systems

Homogeneous and Heterogeneous Systems. Homogeneous systems are those that possess identical physical and chemical properties in all their mechanically separable parts. In this case, the molecules from which the system is constructed may be different. Examples of homogeneous systems can be a salt solution in water, a mixture of alcohol with water, or air, which represents a mixture of various gases. Heterogeneous systems are considered to be those consisting of several homogeneous parts separated from each other by phase boundaries. Each of such uniform parts, making up a heterogeneous system, is called its "phase," and the heterogeneous system itself is called multiphase (depending on the number of phases - two-phase, three-phase, etc.). In many processes occurring in a heterogeneous system, the magnitude of the surface separating its phases is of great importance. The larger this surface, the faster, for example, chemical interactions between both phases can occur. At the phase boundary, the properties of matter change sharply. However, the boundaries of phases are, strictly speaking, not surfaces in the strictly mathematical sense of the word, but very thin boundary layers or films, within which the properties of one phase pass into the properties of another. These boundary films play a major role in capillarity and surface tension phenomena. The phases of a heterogeneous system may be identical in chemical relationship and differ only in their aggregate state (e.g., the ice-water-steam system). The reverse relationship is also possible: the phases of a heterogeneous system may represent the same aggregate state and differ in their chemical composition. For example: when ether is mixed with water, two layers are formed, two liquid phases (below - a solution of ether in water, above - a solution of water in ether). In the simplest case, each phase is separated from the others by a single continuous, unbroken surface. The latter, however, may break up into a number of separate, isolated surfaces. Thus, when water and oil are shaken, the latter breaks up into many separate droplets, forming an oil-in-water emulsion. Such a phase, consisting of identical but spatially isolated parts, is called a dispersed or disperse phase, and the heterogeneous system in this case is called a disperse system (see). Colloids also belong to such disperse heterogeneous systems. The heterogeneity of disperse systems is less obvious and can be optically detected only with the help of a microscope (in emulsions and suspensions) or an ultramicroscope (in colloids); therefore, we can speak here of microheterogeneous systems. The existence of such microheterogeneous systems, representing all successive transitions from coarse heterogeneous to homogeneous systems, clearly shows the conventional nature of the concepts "homogeneous and heterogeneous systems." Only the imperfection of our research methods does not allow us to see (or mechanically separate from each other) the individual molecules of a homogeneous system and thus notice its spatial heterogeneity. Heterogeneous systems can differ both in the number of phases and in the number of chemical individuals, chemical components, making up the system. Heterogeneous systems built from one chemical component (e.g., water-ice; water-steam-ice) are called unary; systems from two chemical components (e.g., an emulsion of water with oil) are called binary, from three - ternary, etc. With an increase in the number of components, the number of possible phases obviously increases greatly. There can of course be only one gaseous phase, due to the mixability of gases. There can be several solid phases even in a system of one chemical component; e.g., in the presence of octahedral and rhombic sulfur in the system, both these crystal forms should be considered as separate phases. When studying heterogeneous systems, the question arises about the conditions for the possible coexistence of individual phases, or in other words, about the conditions for equilibrium of the system. According to the "phase rule," which was thermodynamically derived by Gibbs, for heterogeneous systems there is a certain relationship between the number of chemical components (n), the number of phases (r), and the number of degrees of freedom (f), i.e., the number of those conditions determining the state of the system (temperature, pressure) that can be changed without disturbing the equilibrium. This relationship is expressed by the ratio f = n + 2 - r. For a system in which the number of phases is 2 more than the number of components, f equals 0, i.e., there is not one condition that could be changed without disturbing the equilibrium. Any change in conditions will lead to a disturbance of equilibrium, to the disappearance of one phase. Such systems are called nonvariant (or invariant). A system of n chemical components forming n+1 phases has one degree of freedom, i.e., one of the conditions determining the system can be arbitrarily changed - this is a monovariant system. Systems of n chemical components and n phases have two degrees of freedom and are called divariant, etc. Thus, in the simplest case for a system consisting of one component, for example, water molecules, as shown in the figure, only at one point (at temperature -0.0076 and pressure 4.57 mm) can all three phases coexist: ice, water, and steam. The system is invariant (f = 1+2-3 = 0): the constancy of temperature and pressure cannot be violated without the disappearance of at least one phase of the system. Conversely, in a two-phase system - water-steam - one of the physical conditions can be arbitrarily changed, but the other then receives a strictly fixed value; e.g., each temperature corresponds to a certain vapor pressure. Finally, water alone (or steam alone) has 2 degrees of freedom, i.e., it allows arbitrary change of both temperature and pressure.