A system is any part of the universe under consideration or that is the focus of thermodynamic analysis. For example, the dashed line shown in Figure 6.10 represents the boundary of a system. The system itself is composed of all the units enclosed within the boundary. The part of the universe that is excluded from the system or is outside the system boundary is termed as the surroundings [5]. A system may or may not exchange mass and energy with the surroundings; an open system is one where both mass and energy are exchanged with the surroundings, whereas a closed system, by definition, does not exchange any mass but is able to exchange energy with the surroundings. A system that exchanges neither mass nor energy with the surroundings is termed an isolated system [5].
It is clear that each system will exhibit certain characteristics that distinguish it from other systems and surroundings. For example, a system will have certain temperature, pressure, volume, as well as constituents present in certain proportions. The characteristics or properties allow us to quantify the system. Certain properties of the system, such as the temperature or pressure, do not depend on the size of the system and are termed intensive properties, as stated in the previous chapter. On the other hand, properties such as mass or volume are dependent on the size of the system and are called extensive properties [4]. Extensive properties are additive in nature; the magnitude of an extensive property is the sum of the magnitudes of that property for various parts of the system [5].
A state of a system refers simply to the conditions present in the system, such that the properties are invariant with respect to time [4]. For a pure component present in a single phase, the state of the system can be specified simply by stating its pressure and temperature. This fixes all of its intensive properties, whereas the extensive properties require specification of the quantity present. In other words, specifying the pressure and temperature of that pure substance automatically fixes the specific volume (volume per mole) of the substance. The total volume of the system can be determined if the total number of moles is specified.
The relationship among the pressure, volume, and temperature describes the volumetric behavior for a substance. This behavior, or the volumetric properties of the substance, is used in thermodynamics to determine its thermodynamic properties [6]. The following sections describe some of these thermodynamic properties.
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