On the other hand, a mixture of iron filings and sulfur powder is composed
of the two components iron and sulfur. The mixture may look like a single
component, but a close enough inspection reveals two distinct materials in the
system that have their own unique properties. This Fe/S mixture is therefore a
two-component system. The phrase “chemically inhomogeneous” is used to de-
scribe multicomponent systems.
A solutionis a homogeneous mixture. Examples of solutions include salt
water [NaCl (s) dissolved in H 2 O] and the alloy brass, which is a solid solution
of copper and zinc. Solutions are a little more difficult to consider, because the
isolated components might not have the same chemical identity when in solu-
tion. For example, NaCl (s) and H 2 O () are two chemical components, but
NaCl (aq) consists of Na(aq) and Cl(aq) ions as well as excess H 2 O sol-
vent. When we use solutions as an example of a system, we will be explicit in
defining the components of the system. Even though they are homogeneous,
properties of solutions will not be considered in this chapter.
In this chapter, we are considering single-component systems—that is, sys-
tems that have the same chemical composition throughout. However, there is
another way to describe the state of the system in addition to its chemical com-
position. We recognize that matter can exist in different physical forms. A phase
is a portion of matter that has a uniform physical state and is distinctly sepa-
rated from other phases. Chemically, we recognize the solid, liquid, and gas
phases. We also recognize that one chemical substance may have more than
one solid form, and that each form is a different solid phase. Single-component
systems can exist in one or more phases simultaneously, and we will apply the
concepts of equilibrium from the last chapter to understanding the phase tran-
sitions in these systems.Example 6.1
Identify the number of components and phases that exist in each system.
Assume no component other than the ones given exists in each system.
a.A system containing ice and water
b.A 5050 solution of water and ethanol, C 2 H 5 OH
c.A pressurized tank of carbon dioxide that contains both liquid and gas
d.A bomb calorimeter containing a pellet of benzoic acid, C 6 H 5 COOH (s),
and 25.0 bar of O 2 gas
e.The same bomb calorimeter after the explosion, in which the benzoic acid
is converted to CO 2 (g) and H 2 O (), and assuming excess oxygenSolution
a.Ice water contains H 2 O in both solid and liquid forms, so there is a single
component and two phases.
b.Both water and ethanol are liquids, so there is one phase of two com-
ponents.
c.Just like the ice water, the pressurized carbon dioxide with liquid and gas
in a tank consists of a single chemical component in two phases.
d.In an unexploded bomb calorimeter, the solid pellet and oxygen gas are
two components and two phases.
e.After the explosion, the benzoic acid combusts to make carbon dioxide gas
and liquid water. In the presence of excess O 2 , there are therefore three com-
ponents in two phases.142 CHAPTER 6 Equilibria in Single-Component Systems