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180 PHASE EQUILIBRIA

We can predict whether an ice cube will melt just by looking
carefully at the phase diagram. As an example, suppose we take
When labelling a phase an ice cube from a freezer at −5 C and put it straightaway in
◦
diagram, recall how the ◦
only stable phase at our mouth at a temperature of 37 C (see the inset to Figure 5.1).
high pressure and low The temperature of the ice cube is initially cooler than that of the
temperature is a solid; mouth. The ice cube, therefore, will warm up as a consequence
a gas is most stable of the zeroth law of thermodynamics (see p. 8) until it reaches the
at low pressure and temperature of the mouth. Only then will it attain equilibrium. But,
high temperature. The as the temperature of the ice cube rises, it crosses the phase bound-
phase within the crook ary, as represented by the bold horizontal arrow, and undergoes a
of the ‘Y’ is therefore phase transition from solid to liquid.
aliquid. We know from Hess’s law (see p. 98) that it is often useful to
consider (mentally) a physical or chemical change by dissecting it
into its component parts. Accordingly, we will consider the melting of the ice cube
◦
◦
as comprising two processes: warming from −5 Cto37 C, and subsequent melting
◦
at 37 C. During warming, the water crosses the phase boundary, implying that it
◦
changes from being a stable solid (when below 0 C) to being an unstable solid
◦
◦
(above 0 C). Having reached the temperature of the mouth at 37 C, the solid ice
converts to its stable phase (water) in order to regain stability, i.e. the ice cube melts
in the mouth. (It would be more realistic to consider three pro-
◦
cesses: warming to 0 C, melting at constant temperature, then
The Greek root meta warming from 0 to 37 C.)
◦
means ‘adjacent to’ or Although the situation with melting in two stages appears a little
‘near to’. Something artificial, we ought to remind ourselves that the phase diagram is
metastable is almost made up of thermodynamic data alone. In other words, it is possible
stable ... but not quite.
◦
to see liquid water at 105 C, but it would be a metastable phase,
i.e. it would not last long!

Aside

The arguments in this example are somewhat simplified.
Remember that the phase diagram’s y-axisisthe applied pressure. At room tempera-
ture and pressure, liquid water evaporates as a consequence of entropy (e.g. see p. 134).
For this reason, both liquid and vapour are apparent even at s.t.p. The pressure of the
vapour is known as the saturated vapour pressure (s.v.p.), and can be quite high.
The s.v.p. is not an applied pressure, so its magnitude is generally quite low. The
s.v.p. of water will certainly be lower than atmospheric pressure. The s.v.p. increases
with temperature until, at the boiling temperature, it equals the atmospheric pressure.
One definition of boiling says that the s.v.p. equals the applied pressure.
The arguments in this section ignore the saturated vapour pressure.