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a nonuniform physicochemical system comprising parts (phases) that differ in physical properties or chemical composition.
One phase of a heterogeneous system is separated from an adjacent phase by an interface where one or more properties of the system (for example, composition, density, crystal lattice parameters, and electrical or magnetic field) change abruptly. The differences in properties of the individual phases of a heterogeneous system make possible, at least theoretically, their mechanical separation. Examples of heterogeneous systems are water and the water vapor above it (difference in the state of aggregation), a mixture of two different crystalline variants of sulfur (rhombic and monoclinic), and two immiscible liquids, such as oil and water (difference in composition). It is often impossible to draw a sharp line between a heterogeneous system and a homogeneous (uniform) one. Thus the so-called colloidal solutions, where the particles of dissolved substance are so small that the concept of phase cannot be applied to them, form an intermediate range between mechanical mixtures (suspensions) and true (molecular) solutions. The term “heterogeneous system” is widely used in physics and chemistry, especially in chemical thermodynamics.