Page 116 - Instant notes

D4
PHASE EQUILIBRIA

Key Notes
The melting point and the freezing point are identical for a pure
substance, but not necessarily for a mixture. The melting
temperature of a pure substance increases with increasing
pressure, except for water, where the melting temperature
decreases with increasing pressure.
The boiling point of a pure substance increases with increasing
pressure. This increase is relatively small compared to the
increase in melting temperature with pressure. As the pressure is
increased, the density of the liquid and the vapor become
increasingly similar. Eventually, at the critical point, denoted by
the critical temperature and pressure, the densities become equal,
it is impossible to distinguish between the liquid and vapor
phases, and there is no longer a phase transition.
The variation of the boiling point and freezing (melting) points
can be plotted as lines on a diagram of p versus T. This produces
a phase diagram that shows the phase(s) present at any state point
p, T. At one point on the diagram where the two lines meet,
called the triple point, all three phases (solid, liquid and gas)
coexist. There are three variables, p, the molar volume, V m , and T
for the system, and any two can form the axes of a phase
diagram. The number of degrees of freedom, F, is the minimum
number of independent variables that can be varied without
changing the number of phases in the system. F=2 for a single-
phase region, which means that a single phase can be maintained
whilst moving in two dimensions. When two phases are in
equilibrium, F=1 and two phases can only be maintained by
moving along the two-phase line in one dimension. At the triple
point, there are three phases, F=0 and this only occurs at this
fixed point on the diagram.
Related topic Phase diagrams of mixtures (D5)

Melting and freezing points

When a pure solid species is heated through its melting point (melting temperature),
the solid changes to a liquid. At the melting point, an equilibrium is established between
the solid and liquid phases. Conversely, when a pure liquid is cooled through its freezing
point or freezing temperature, the liquid is transformed to a solid, and at the freezing