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178 PHASE EQUILIBRIA

Further thermodynamic background: terminology

In the thermodynamic sense, a phase is defined as part of a chemical system in
which all the material has the same composition and state. Appropriately, the word
comes from the Greek phasis, meaning ‘appearance’. Ice, water and steam are the
three simple phases of H 2 O. Indeed, for almost all matter, the three
A phase is a compo- simple phases are solid, liquid and gas, although we must note that
nent within a system, there may be many different solid phases possible since H 2 O (s)
existing in a precisely can adopt several different crystallographic forms. As a related
defined physical state,
example, the two stable phases of solid sulphur are its monoclinic
e.g. gas, liquid, or a and orthorhombic crystal forms.
solid that has a single
Ice is a solid form of water, and is its only stable form below
crystallographic form. ◦
0 C. The liquid form of H 2 O is the only stable form in the tem-
◦
◦
perature range 0 <T < 100 C. Above 100 C, the normal, stable
◦
phase is gaseous water, ‘steam’. Water’s normal melting temperature T (melt) is 0 C
O
(273.15 K). The word ‘normal’ in this context implies ‘at standard pressure p ’. The
5
O
pressure p has a value of 10 Pa. This temperature T (melt) is often called the melting
point because water and ice coexist indefinitely at this temperature and pressure, but
at no other temperature can they coexist. We say they reside together at equilibrium.
To melt the ice, an amount of energy equal to H O must be
(melt)
Concerning transi- added to overcome those forces that promote the water adopting
tions between the a solid-state structure. Such forces will include hydrogen bonds.
two phases ‘1’ and Re-cooling the melted water to re-solidify it back to ice involves
‘2’, Hess’s Law states the same amount of energy, but this time energy is liberated,so
O
O
that H (1→2) =−1 × H (melt) =− H (freeze) . The freezing process is often called fusion.
H (2→1) . (Strictly, we ought to define the energy by saying that no pres-
sure–volume work is performed during the melting and freezing
processes, and that the melting and freezing processes occur without any changes in
temperature.)
Table 5.1 gives a few everyday examples of phase changes, together with some
useful vocabulary.
Two or more phases can coexist indefinitely provided that we maintain certain
conditions of temperature T and pressure p. The normal boiling temperature of water
◦
O
is 100 C, because this is the only temperature (at p = p ) at which both liquid and
Table 5.1 Summary of terms used to describe phase changes

Phase transition Name of transition Everyday examples
Solid → gas Sublimation ‘Smoke’ formed from dry ice
Solid → liquid Melting Melting of snow or ice
Liquid → gas Boiling or vaporization Steam formed by a kettle
Liquid → solid Freezing, solidification or fusion Ice cubes formed in a fridge; hail
Gas → liquid Condensation or liquification Formation of dew or rain
Gas → solid Condensation Formation of frost