A substance that has a fixed chemical composition throughout is called a pure substance. A pure substance exists in different phases depending on its energy level. In the liquid phase, a substance that is not about to vaporize is called a compressed or subcooled liquid. In the gas phase, a substance that is not about to condense is called a superheated vapor. During a phase-change process, the temperature and pressure of a pure substance are dependent properties. At a given pressure, a substance changes phase at a fixed temperature, called the saturation temperature. Likewise, at a given temperature, the pressure at which a substance changes phase is called the saturation pressure. During a boiling process, both the liquid and the vapor phases coexist in equilibrium, and under this condition the liquid is called saturated liquid and the vapor saturated vapor.
In a saturated liquid–vapor mixture, the mass fraction of vapor is called the quality and is expressed as
Quality may have values between 0 (saturated liquid) and 1 (saturated vapor). It has no meaning in the compressed liquid or superheated vapor regions. In the saturated mixture region, the average value of any intensive property y is determined from
where f stands for saturated liquid and g for saturated vapor. In the absence of compressed liquid data, a general approximation is to treat a compressed liquid as a saturated liquid at the given temperature,
where y stands for u, u, or h.
The state beyond which there is no distinct vaporization process is called the critical point. At supercritical pressures, a substance gradually and uniformly expands from the liquid to vapor phase. All three phases of a substance coexist in equilibrium at states along the triple line characterized by triple-line temperature and pressure. The compressed liquid has lower u, u, and h values than the saturated liquid at the same T or P. Likewise, superheated vapor has higher u, u, and h values than the saturated vapor at the same T or P.
Any relation among the pressure, temperature, and specific volume of a substance is called an equation of state. The simplest and best-known equation of state is the ideal-gas equation of state, given as
where R is the gas constant. Caution should be exercised in using this relation since an ideal gas is a fictitious substance.
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