Properties of a Pure Substance

3-1 The Pure Substance

A pure substance is one that has a homogeneous and invariable chemical composition. It may exist in more than one phase, but the chemical composition is the same in all phases.

3-2 Vapor-Liquid-Solid-Phase Equilibrium in a Pure Substance

Saturation Temperature and Saturation Pressure

The term saturation temperature designates the temperature at which vaporization takes place at a given temperature. This pressure is called the saturation pressure for the given temperature.

Saturated liquid or saturated vapor, subcooled liquid or compressed liquid, and superheated vapor.

Vapor-Pressure Curve of a Pure Substance

Temperature-Volume Diagram

Some Critical-Point Data

T-v Diagram

The Quality Specific Volume Relation (1/2)

gf xvvx 1

g

vap

f

liq

xv

m

mv

m

m

m

Vvv

fgf xvv

fgf vvxv

The Quality Specific Volume Relation (2/2)

m

m

mm

m liq

liqvap

liq

where

liqvap

vapvap

mm

m

m

mx

fgfg vvv

: quality or dryness

liqvap

vapvap

mm

m

m

mx

111

Some Triple-Point Data

P-T Diagram of Water

CO2 Phase Diagram

H2O Phase Diagram

3-3 Independent Properties of a Substance

The state of a simple pure substance is defined by two independent properties.

In a saturation state, pressure and temperature are not independent properties.

3-4 Tables of Thermodynamic Properties

3-5 Thermodynamic Surfaces

Expand on freezing Contract on freezing

P-V-T Surface for a Substance that Expands on Freezing (1/2)

P-V-T Surface for a Substance that Expands on Freezing (2/2)

P-V-T Surface for a Substance that Contracts on Freezing (1/2)

P-V-T Surface for a Substance that Contracts on Freezing (2/2)

3-6 The P-V-T Behavior of Low-and Moderate –Density Gases

At very low densities the average distances between molecules is so large that the intermolecular potential energy may effectively be neglected. In such a case, the particles would be independent of one another, a situation referred to as an ideal gas.

The Ideal Gas Equation of State (1/2)

TnRPV u

TRvP u

kmolkg

kg

M

mn

Kkmol

kJ

Kkmol

mkNR

u

3145.83145.8

uR : the universal gas constant

The Ideal Gas Equation of State (2/2)

mRTPV

RTPv

M

RR u

R : the gas constant

T-v Diagram for Water

Compressibility Factor Z

zRTpv

TR

vpz

u

TzRvp u

1z for ideal gas

Compressibility of N2

Observations from Figure 3.15

At all temperatures as . At the temperatures of 300K and above the

compressibility factor is near unity up to pressure of about 10MPa.

At lower temperatures or at very high pressure, the compressibility factor deviates significantly from the ideal-gas value.

1z 0P

Reduced Properties

cr P

PP

cr T

TT

rP : reduced pressure

rT : reduced temperature

cP : critical pressure

cT : critical temperature

The Benedict-Webb-Rubin Equation of State

2

223632

2000 1 ve

vTv

c

v

a

v

aRTb

v

TCARTB

v

RTP

Lee-Kesler Simple Fluid Compressibility Factor