CHAP. 7: PHASE EQUILIBRIA [CONTENTS] 185
rqq(^0) x( 1 β) Z 1 x( 1 γ) 1
T 1
TH
T
^/(α) + (β) (α) + (γ)(α)(β)(γ)
(β) + (γ)A B CObr. 7.2: Isobaric phase diagram of a two-component system. The meaning of the symbols is
explained in the text.
Solution
At the initial temperatureT 1 , the system of global compositionZ 1 is in the heterogeneous region.
It splits into two phases whose composition is given by the intersections of the isothermT 1 with
the binodal lines (x( 1 β) andx( 1 γ)in the diagram). The relative amount of both phases is given
by the lever rule (i.e. the material balance, see7.4.5). With increasing temperature the system
continues to be composed of two phases, with only the composition of both phases changing. At
temperatureTH, three phases are in equilibrium in the system, with phase (α) added to phases
(β) and (γ); their composition is again given by the intersections of the given isotherm with
the respective binodal lines. The number of degrees of freedom drops to zero (three coexisting
phases in a two-component system at constant pressure), and the system stays at temperature
THuntil one of the phases disappears. When a mixture of global compositionZ 1 is heated, it is
phase (β) that ceases to exist.