Now we consider something that is usually so obvious to us that we do not
really think about it. The stable phase of a single-component system depends
on the conditions of the system. Let us use water as an example. When it is cold
outside, it might snow (we see solid H 2 O), but when it’s warmer it rains (we
see liquid H 2 O). To make spaghetti, we have to boil water (make gaseous H 2 O).
The temperature of the system determines the stable phase of the H 2 O. This
idea is obvious to most of us. What might not be so obvious is that the phase
of any single-component system depends on allof the conditions of the sys-
tem. Those conditions are the pressure, temperature, volume, and amount of
material in the system.
A phase transitionoccurs when a pure component changes from one phase
to another. Table 6.1 lists the different types of phase transitions, most of which
should already be familiar to you. There are also phase transitions between dif-
ferent solid forms of a chemical component, which is a characteristic called
polymorphism. For example, elemental carbon exists as graphite or diamond,
and the conditions for phase transitions between the two forms are well known.
Solid H 2 O can actually exist as at least six structurally different solids, de-
pending on the temperature and pressure. We say that water has at least six
polymorphs.(In application to elements, we use the word allotropeinstead of
polymorph. Graphite and diamond are two allotropes of the element carbon.)
In mineral form, calcium carbonate exists either as aragonite or calcite, de-
pending on the crystalline form of the solid.
Under most conditions of constant volume, amount, pressure, and temper-
ature, a single-component system has a unique stable phase. For example, a
liter of H 2 O at atmospheric pressure and 25°C is normally in the liquid phase.
However, under the same conditions of pressure but at 125°C, a liter of H 2 O
would exist as a gas. These are the phases that are thermodynamically stable
under these conditions.
For an isolated single-component system having fixed volume and amount,
at certain values of pressure and temperature, more than one phase can exist
simultaneously in the system. If the state variables of the system are constant,
then the system is at equilibrium. Therefore,it is possible for two or more phases
to exist in a system at equilibrium.
If the system is not isolated but simply closed, then heat can enter or leave
the system. In that case, the relative amounts of each phase will change. For ex-
ample, in a system containing solid dimethyl sulfoxide (DMSO) and liquid
DMSO at 18.4°C and atmospheric pressure, when heat is added to the system,
some of the solid phase will melt to become part of the liquid phase. The sys-
tem is still at chemicalequilibrium, even though the relative amounts of phases
are changing (which is a physical change). This is true of other phase transi-
tions as well. At atmospheric pressure and 189°C, liquid DMSO can exist in
equilibriumwith gaseous DMSO. Add or remove heat, and DMSO will go from
liquid to gas phase or from gas to liquid phase, respectively, while maintaining
a chemical equilibrium.
For a given volume and amount, the temperature at which these equilibria
can occur varies with pressure, and vice versa. It is therefore convenient to
identify certain benchmark conditions. Thenormal melting point is that tem-
perature at which a solid can exist in equilibrium with its liquid phase at 1 atm
pressure.* Because the solid and liquid phases are so condensed, the melting
point of single components are affected only by large pressure changes. The6.2 A Single-Component System 143Table 6.1 Phase transitionsa
Term Transition
Melting (or fusion) Solid →liquid
Boiling (or vaporization) Liquid →gas
Sublimation Solid →gas
Condensation Gas →liquid
Condensation (or deposition) Gas →solid
Solidification (or freezing) Liquid →solid
aThere is no specific term for a solid phase →solid phase
transition between two solid forms of the same component.
*We note the disparity that “normal” boiling and melting points are defined in terms of
a non-SI unit.