Page 121 - Instant notes

D5
PHASE DIAGRAMS OF MIXTURES

Key Notes
The phase rule, F=C−P+2, is used to determine the number of
degrees of freedom in any system. P is the number of phases
present, each of which must be uniform chemically and
physically throughout. A pure solid, a pure liquid and a pure gas
are each separate phases. A mixture of gases is one phase. A
mixture of two miscible liquids is one phase and of two
immiscible liquids is two phases. C is the number of components,
which is usually equal to the number of different chemical
species in the system.
For a two component system consisting of a mixture of A and B,
the maximum value of F, when P=1, is F=3. Thus, three
variables would need to be plotted in the phase diagram. Instead,
the pressure is fixed at 1 atmosphere and the remaining two
variables are plotted. These are usually chosen to be temperature
and the mole fraction of A, x A , plotted in a temperature-
composition phase diagram.
Two partially miscible liquids, A and B, have a temperature-
composition phase diagram which either displays an upper
consolute temperature, above which the two liquids are
completely miscible for all compositions, or a lower consolute
temperature below which they are completely miscible, or both.
In the two-phase region, specifying the temperature is sufficient
to calculate the composition of each phase and specifying the
overall composition allows determination of the amount of each
phase.
A mixture of two miscible liquids, A and B, has its lowest
freezing point at the eutectic temperature and composition. At
this point, the liquid freezes to form both solid A and solid B. At
all other temperatures the liquid freezes to form first solid A or
solid B, before the other solid forms at the eutectic temperature.
The temperature-composition phase diagram can be used to
predict the shape of the cooling curves for any mixture. In
practice, cooling curves are obtained experimentally across the
composition range and used to construct phase diagrams.
For an ideal miscible liquid mixture of A and B, the liquid-vapor
temperature-composition phase diagram can be used to determine
the compositions of both vapor and liquid at any temperature in
the two-phase region. The relative amounts of each phase can