Chapter 3 Properties of a Pure Substance

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Chapter 3 Properties of a Pure Substance

• Three familiar properties of a substance in the
  previous chapter
• specific volume,
• pressure, and
• temperature.

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3.1 THE PURE SUBSTANCE

• has a homogeneous and invariable chemical
  composition,
• exist in more than one phase, and
• exist with no change of phase.
  
• Examples
• liquid water,
• a mixture of ice and liquid water,
• a mixture of gases, such as air
• A mixture of liquid air and gaseous air ( X )
• Because the chemical composition of the liquid
  phase is different from that of the vapor phase. )

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Simple Compressible Substances (system)

• Those whose surface effects, magnetic effects,
  and electrical effects are insignificant when
  dealing with the substances.
• But changes in volume, such as those associated
  with the expansion of a gas in a cylinder, are
  very important.

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3.2 VAPORLIQUIDSOLID-PHASE EQUILIBRIUM IN A
PURE SUBSTANCE
0.1MPa 20 0C,1kg
Heat ,?
Heat, ? 99.6 0C
Fig.3.1
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• Saturation Temperature
• The temperature at which vaporization takes place
  at a given pressure.
• And this given pressure is called the Saturation
  Pressure for the given temperature.

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Fig. 3.2 A vapor-pressure curve for a
pure substance
Sub-cooled liquid
Compressed liquid
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• Saturated liquid (state)
• A substance exists as liquid (state) at the
  saturation temperature and pressure.
• Subcooled liquid (Compressed liquid)
• If the temperature of the liquid is lower than
  the saturation temperature for the existing
  pressure, it is called either a subcooled liquid
  (implying that the temperature is lower than the
  saturation temperature for the given pressure) or
  a compressed liquid (implying that the pressure
  is greater than the saturation pressure for the
  given temperature).

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• Quality of substance
• When a substance exists as part liquid and part
  vapor at the saturation temperature,its quality
  is defined as the ratio of the mass of vapor to
  the total mass.
• Quality has meaning only when the substance is in
  a saturated state.

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• Saturated vapor
• A substance exists as vapor at the saturation
  temperature.
• The quality of dry saturated vapor is 100.

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• Superheated vaporis the vapor at a temperature
  greater than the saturation temperature.
• Actually, the substances we call gases are highly
  superheated vapors.

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Fig. 3.3 Temperaturevolume diagram for water
showing liquid and vapor phases.
Supercritical fluid
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Table 3.1
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FIGURE 3.4 T v diagram for the two-phase
liquidvapor region to show the quality specific
volume relation.
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To Derivative the Quality, x

• V Vliq Vvap mliq v fmvap v gthen divide
  the above equation by total mass m,

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Table 3.2
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FIGURE 3.5 Pressure temperature diagram for a
substance such as water.
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FIGURE 3.6 Carbon dioxide phase diagram.
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Fig. 3.7 Water phase diagram.
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3.3 INDEPENDENT PROPERTIES OF A PURE
SUBSTANCE

• The state of a simple compressible pure
  substance is defined by two independent
  properties.
• For example, if the specific volume and
  temperature of superheated steam are
  specified, the state of the steam is determined.

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A exception, in a saturation state, should be
noted.

• Consider the saturated-liquid and saturated-vapor
  states of a pure substance. These two states have
  the same pressure and the same temperature, but
  they are definitely not the same state.
  Therefore, in a saturation state, pressure and
  temperature are not independent properties.
• Two independent properties such as pressure and
  specific volume or pressure and quality are
  required to specify a saturation state of a pure
  substance.

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• A mixture of gases, such as air, has the same
  characteristics as a pure substance as long as
  only one phase is present, concerns precisely
  this point.
• The state of air, which is a mixture of gases of
  definite composition, is determined by specifying
  two properties as long as it remains in the
  gaseous phase.

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3.4 TABLES OF THERMODYNAMIC PROPERTIES
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• Example Let us calculate the specific volume of
  saturated steam at 200oC having a quality of 70.
  
• ltSolutiongtUsing Eq. 3.1, and looking up Table
  B.1.3 givesv 0.3 (0.001 156) 0.7 (0.127 36)
  0.0895 m 3 /kg

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Example. 3.1
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Example 3.2
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continued
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Example 3.3
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Example 3.4
(p.412)
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3.5 THERMODYNAMIC SURFACES
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3.6 THE PVT BEHAVIOR OF LOW- AND
MODERATE-DENSITY GASES

• At very low densities the average distances
  between molecules is so large that the
  intermolecular ( IM ) potential energy may
  effectively be neglected.
• In such a case, the particles would be
  independent of one another, and the situation
  is referred to as an ideal gas.
• Therefore, a very low density gas behaves
  according to the ideal gas equation of state.

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• R is a different constant for each particular
  gas. The value of R for a number of substances is
  given in Table A.5 of Appendix A.

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Example 3.5
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Example 3.6
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• Over what range of density will the idealgas
  equation of state hold with accuracy?
• How much does an actual gas at a given pressure
  and temperature deviate from ideal gas behavior?

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• As would be expected, at very low pressure or
  high temperature the error is small and the gas
  behavior becomes closer to the ideal gas model.
• But this error becomes severe as the density
  increases (specific volume decreases).

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FIGURE 3.14 Temperature-specific volume diagram
for water that indicates the error in assuming
ideal gas for saturated vapor and for
superheated vapor.
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Compressibility factor, Z

• A more quantitative study of the question of the
  ideal-gas approximation
• Z 1, for an ideal gas
• The deviation of Z from unity is a measure of the
  deviation of the actual relation from the
  ideal-gas equation of state.

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Fig.3.15 Compressibility of nitrogen
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Is there a way in which we can put all of the
substances on a commonbasis? To do so, we
reduce the properties with respect to the
values at the critical point.
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Example 3.7
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Example 3.8