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Lecture 2

Chapter 3

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Pure Substance

a substance that has a homogeneous and invariable chemicalcomposition

may exist in more than one phase (i.e., mixture of phases) butchemical composition is similar in all phases

Example: A mixture of ice and liquid water

A mixture of liquid water and vapor (steam)

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Phase and State Changes of a

Substance A pure substance can exist in different phases (solid,

liquid, or gas)

It can exist in equilibrium as a mixture of differentphases

When a pure substance exists as a mixture of two ormore phases, it is in phase equilibrium when there isno tendency for the mass of each phase to change

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Phase and State Changes of a

Substance

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T-v diagram for the heating process of water at

constant pressure.

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Definitions:

Saturation Temperature

- the temperature at which vaporization takes placefor a given pressure

Saturation Pressure

- the pressure at which vaporization takes place for agiven temperature

For a pure substance, there is a definite relation

between the saturation temperature and saturationpressure which can be shown by the vapor-pressurecurve.

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A substance can exist in any of the

following phases

Compressed Liquid

Subcooled Liquid

Saturated Liquid

Saturated Vapor

Saturated Mixture

Superheated Vapor

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Compressed Liquid

a liquid whose pressure is higher than the saturation pressure

for the existing liquid temperature (sat. temp.)Subcooled Liquid

another term for compressed liquid; a liquid whosetemperature is lower than the saturation temperature for theexisting liquid pressure (the saturation pressure)

Saturated Liquidsubstance that exists as liquid only at saturation temperatureAND saturation pressure

Saturated Vapor

substance that exists as vapor only at saturation temperature

AND saturation pressure

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Saturated Mixture

substance that exists as a mixture of liquid and vapor for a

given temperature (saturation temperature) AND pressure(saturation pressure)

The liquid component in the mixture has the same propertiesas saturated liquid for the given temperature or pressure

The vapor component in the mixture has the same propertiesas saturated vapor for the given temperature or pressure

For a liquid-vapor mixture, a property that relates the mass ofvapor to the mass of the mixture is defined:

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Superheated Vapor

substance in vapor phase whose temperature is higherthan the saturation temperature for the existing

pressure

OR

substance in vapor phase whose pressure is lower than

the saturation pressure for the existing temperature

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Quality - the ratio of the mass of vapor component to thetotal mass of the liquid-vapor mixture

x = mg / m = mass of vapor / total mass of liquid-vapormixture

where

mf = mass of liquid component

mg = mass of vapor component

m = mf + mg

Note that

x = 0 for saturated liquid

x = 1 for saturated vapor

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Property Relationships

*note

liq gV V V

liq f g gmv m v m v

fg g f v v v

(1 ) f g f fgv x v x v v x v

specific volume

=specific volume of a saturated liquid

=specific volume of a saturated gas

fg

f

g

v average

v

v

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Critical Point

The state wherein the saturated liquid and saturated

vapor states are identical

It is specified by the critical temperature Tc , critical

pressure Pc , or critical specific volumeAt pressures above the critical pressure, the liquid

and vapor phases do not exist in equilibrium

At supercritical pressures, the substance is simply

called a fluid

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T-v Diagram

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P-V Diagram

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Processes that involve phase change

Sublimation direct phase change from solid to vapor along the sublimation line, the solid and vapor phases (solid-

vapor mixture) exist in equilibriumFusion direct phase change from solid to liquid along the fusion line, the solid and liquid phases (solid-liquid

mixture) exist in equilibriumVaporization direct phase change from liquid to vapor along the vaporization line, the liquid and vapor phases

(liquid-vapor mixture) exist in equilibrium

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Triple Point

the state in which three phases are present in

equilibrium

for a pure substance, there may be many

triple points but only one triple point where

the solid, liquid, and vapor phases exist in

equilibrium

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Tables of Thermodynamic Properties

The thermodynamic tables lists other properties of asubstance (e.g., v, u, h, s) as a function of two independentproperties, commonly T and P

1. Saturated Liquid

Saturated Vapor Region Tabulates properties of saturated liquid-vapor mixtures for a

given temperature ( which is also equal to Tsat ) or a givenpressure ( which is also equal to Psat )

Quality - An independent property of the liquid-vapor mixture

which can be used to specify its state, defined as

vapor vapor g

mixture vapor liquid g f

m m mx

m m m m m

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Property Relationships

liq vapV V V

liq f vap gmv m v m v

fg g f v v v

(1 ) f g f fgv x v x v v x v

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3. Compressed or Subcooled Liquid Region

Tabulates properties of compressed or subcooledliquid

Tests

1. For a given T and P, see if P > Psat) T or T 1 actual density is less than predicted from

ideal gas equation due to greater intermolecular

repulsive force caused by much reduced

intermolecular distances

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GENERALIZED COMPRESSIBILITY CHART

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Pr = P / Pc = reduced pressure

Tr = T / Tc = reduced temperature

Pc = critical pressure

Tc = critical temperature

See Table A.2 for Pc and Tc ofvarious substances

This plot of the generalizedcompressibility factor Z in termsof the reduced pressure Pr andreduced temperature Tr is

applicable to various simplesubstances.

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The generalized compressibility chart indicates that

ideal gas behavior can be assumed when P 2Tc up to P 4 to 5 times Pc

For other conditions, especially in the superheated

vapor region, it is more accurate to use thethermodynamic tables

May be used to estimate P-v-T data of substances ifno tables are available