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Thermodynamics: An Engineering Approach, 4/e

Yunus A. Çengel, University of Nevada, Reno
Michael A. Boles, North Carolina State University

Properties of Pure Substances

Chapter Glossary

Beattie-Bridgeman equation of state	is one of the best known and is a reasonably accurate equation of state.



Benedict-Webb-Rubin equation of state	is one of the more recent and very accurate equations of state.



Compressed liquid	has a pressure greater than the saturation pressure corresponding to the temperature.



Compressed liquid region	is all compressed liquid states located in the region to the left of the saturated liquid line and below the critical temperature line. In the absence of compressed liquid data, a general approximation is to treat compressed liquid as saturated liquid at the given temperature.



Compressibility factor	Z is a correction factor to account for deviation from ideal-gas behavior at a given temperature and pressure. Z = Pv/RT.



Critical point	is defined as the point at which the saturated liquid and saturated vapor states are identical.



Critical pressure	Pcr is the pressure of a substance at the critical point.



Critical temperature	Tcr is the temperature of a substance at the critical point.



Critical volume	vcr is the volume of a substance at the critical point.



Dome	is the saturation states located beneath the joined saturated liquid line and saturated vapor line.



Enthalpy	H (from the Greek word enthalpien, which means to heat) is a property and is defined as the sum of the internal energy U and the PV product.



Enthalpy change of an ideal gas	is given as



Enthalpy of vaporization	(or latent heat of vaporization) is the quantity hfg listed in the saturation tables.



Equation of state	is any equation that relates the pressure, temperature, and specific volume of a substance. Property relations that involve other properties of a substance at equilibrium states are also referred to as equations of state.



Gas constant	R is different for each gas and is determined from R = Ru/M.



Gas phase of a substance	has molecules that are far apart from each other, and a molecular order is nonexistent. Gas molecules move about at random, continually colliding with each other and the walls of the container they are in.



Generalized compressibility chart	shows that by curve-fitting all the data, gases seem to obey the principle of corresponding states reasonably well.



Ideal gas	is a gas that obeys the ideal-gas equation of state.



Ideal-gas equation of state	(or ideal-gas relation) predicts the P-v-T behavior of a gas quite accurately within some properly selected region where Pv = RT.



Ideal gas specific heat relation	is Cp = Cv + R.



Internal energy change of an ideal gas	is given as .



Latent heat	is the amount of energy absorbed or released during a phase-change process.



Latent heat of fusion	is the amount of energy absorbed during melting and is equivalent to the amount of energy released during freezing.



Latent heat of vaporization	is the amount of energy absorbed during vaporization and is equivalent to the energy released during condensation.



Liquid	phase has a molecular spacing not much different from that of the solid phase, except the molecules are no longer at fixed positions relative to each other. In a liquid, chunks of molecules float about each other; however, the molecules maintain an orderly structure within each chunk and retain their original positions with respect to one another. The distances between molecules generally experience a slight increase as a solid turns liquid, with water being a rare exception.



Liquid-vapor saturation curve	is a plot of saturation temperature Tsat versus saturation pressure Psat.



Mass of a system	is equal to the product of its molar mass M and the mole number N.



Melting line	separates the solid and liquid regions on the phase diagram.



Molar mass	M can simply be defined as the mass of one mole (also called a gram-mole, abbreviated gmol) of a substance in grams, or the mass of one kmol (also called a kilogram-mole, abbreviated kgmol) in kilograms. In English units, it is the mass of 1 lbmol in lbm. Notice that the molar mass of a substance has the same numerical value in both unit systems because of the way it is defined.



Phase diagram	is the P-T diagram of a pure substance and shows all three phases separated from each other by the sublimation line, vaporization line, and melting line.



Principle of corresponding states	is the fact that compressibility factor Z for all gases is approximately the same at the same reduced pressure and temperature.



Pseudo-reduced specific volume	vR is used with the generalized compressibility chart to determine the third property when P and v, or T and v, are given instead of P and T.



P-v-T surface	is a three-dimensional surface in space which represents the P-v-T behavior of a substance. All states along the path of a quasi-equilibrium process lie on the P-v-T surface since such a process must pass through equilibrium states. The single-phase regions appear as curved surfaces on the P-v-T surface, and the two-phase regions as surfaces perpendicular to the P-T plane.



Pure substance	is a substance that has a fixed chemical composition throughout.



Quality	x is the ratio of the mass of vapor to the total mass of a saturated mixture. The quality lies in the range .



Reduced pressure	PR is the ratio of the pressure to the critical pressure.



Reduced temperature	TR is the ratio of the temperature to the critical temperature



Reference state	is chosen to assign a value of zero for a convenient property or properties at that state.



Saturated liquid	is a liquid that is about to vaporize.



Saturated liquid line	is the saturated liquid states connected by a line that meets the saturated vapor line at the critical point, forming a dome.



Saturated liquid-vapor mixture	is a mixture of the liquid and vapor phases that coexist in equilibrium.



Saturated liquid-vapor mixture region, or the wet region	is all the states that involve both the liquid and vapor phases in equilibrium and are located under the dome.



Saturated vapor	is a vapor that is about to condense.



Saturated vapor line	is the saturated vapor states connected by a line that meets the saturated liquid line at the critical point, forming a dome.



Saturation pressure	Psat is called the pressure at which a pure substance changes phase at a given temperature.



Saturation temperature	Tsat is the temperature at which a pure substance changes phase at a given pressure.



Solid phase	has molecules arranged in a three-dimensional pattern (lattice) that is repeated throughout. Because of the small distances between molecules in a solid, the attractive forces of molecules on each other are large and keep the molecules at fixed positions.



Specific heat	is defined as the energy required to raise the temperature of a unit mass of a substance by one degree. In general, this energy will depend on how the process is executed.



Specific heat at constant pressure	Cp as the energy required to raise the temperature of the unit mass of a substance by one degree as the pressure is maintained constant. Cp is a measure of the variation of enthalpy of a substance with temperature. Cp can be defined as the change in the enthalpy of a substance per unit change in temperature at constant pressure.



Specific heat at constant volume	Cv is the energy required to raise the temperature of the unit mass of a substance by one degree as the volume is maintained constant. Cv is related to the changes in internal energy. It would be more proper to define Cv as the change in the internal energy of a substance per unit change in temperature at constant volume.



Specific heats for solids and liquids	or incompressible substances, are equal.



Subcooled liquid	has a temperature less than the saturation temperature corresponding to the pressure.



Specific heat ratio	k, is defined as the ratio Cp/Cv.



Sublimation	is the process of passing from the solid phase directly into the vapor phase.



Sublimation line	separates the solid and vapor regions on the phase diagram.



Superheated vapor	is a vapor that is not about to condense (not a saturated vapor). A superheated vapor has a temperature greater than the saturation temperature for the pressure.



Superheated vapor region	is all the superheated states located to the right of the saturated vapor line and above the critical temperature line.



Triple line	is the locus of the conditions where all three phases of a pure substance coexist in equilibrium. The states on the triple line of a substance have the same pressure and temperature but different specific volumes.



Triple point	is a point on the P-T diagram that represents the triple line.



Universal gas constant	Ru is the same for all substances and its value is 8.314 kJ/kmol·K and 1.986 Btu/lbmol·R.



Vacuum cooling	is a way to cool a substance by reducing the pressure of the sealed cooling chamber to the saturation pressure at the desired low temperature and evaporating some water from the products to be cooled. The heat of vaporization during evaporation is absorbed from the products, which lowers the product temperature.



Vacuum freezing	is the application of vacuum cooling when the pressure (actually, the vapor pressure) in the vacuum chamber is dropped below 0.6 kPa, the saturation pressure of water at 0°C.



van der Waals equation of state	is one of the earliest attempts to correct the ideal gas equation for real gas behavior.



Vapor	implies a gas that is not far from a state of condensation.



Vaporization line	separates the liquid and vapor regions on the phase diagram.



Virial equations of state	is an equation of state of a substance expressed in a series form as P = RT/v + a(T)/v2 + b(T)/v3 + c(T)/v4 + d(T)/v5 +...where the coefficients a(T ), b(T ), c(T ), and so on, are functions of temperature alone and are called virial coefficients.

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