CHAP. 7: PHASE EQUILIBRIA [CONTENTS] 184
Solution
If the system shown in Figure7.1is kept under constant pressurepst, then for temperatures
T < TNFPit is in the solid phase (sα).TNFPdenotes the melting temperature (ifpst= 101 325
Pa, it is the normal melting temperature). Although there is one degree of freedom on the curves
in Figure7.1, this number drops to zero when the pressure is chosen (another fixed condition)
[see7.3]. If heat is supplied to the system from its surroundings, the temperature does not
change until all of the solid phase has melted. In the temperature regionTNFP< T < TNBP, only
the liquid phase exists in the system. At the boiling temperatureTNBP, the second phase, gas,
occurs. At the given temperature both phases are in equilibrium, with neither their temperature
nor their pressure changing while heat is being supplied to the system, and with only the amount
of the gaseous phase increasing. After all of the liquid has evaporated, isobaric heating of the
gas occurs.7.4.3 Phase diagrams of two-component (binary) mixtures
The phase diagram of a two-component mixture [see Figure7.2] delimits the homogeneous
and heterogeneous [see1.1.4] regions at different compositions of the mixture for different
temperatures (isobaric diagram) or pressures (isothermal diagram).
- In the regions denoted (α), (β) and (γ) in Figure7.2, the system is homogeneous (one-
phase) - In the regions denoted (α) + (β), (α) + (γ) and (β) + (γ) in Figure7.2, the system
is heterogeneous, i.e. it is formed by two phases whose composition can be found on
the binodal lines [see7.4.1]. The conodes (tie lines) are identical with the isotherms or
isobars and they are usually not drawn.
Example
Describe what is going on in the system shown in Figure7.2during isobaric heating of a mixture
of global compositionZ 1 from the initial temperatureT 1.