A substance that has a fixed chemical composition throughout is called a
. Water, nitrogen, helium, and carbon dioxide, for example,
are all pure substances.
A pure substance does not have to be of a single chemical element or
compound, however. A mixture of various chemical elements or compounds
also qualifies as a pure substance as long as the mixture is homogeneous.
Air, for example, is a mixture of several gases, but it is often considered to
be a pure substance because it has a uniform chemical composition
(Fig. 3–1). However, a mixture of oil and water is not a pure substance.
Since oil is not soluble in water, it will collect on top of the water, forming
two chemically dissimilar regions.
A mixture of two or more phases of a pure substance is still a pure sub-
stance as long as the chemical composition of all phases is the same
(Fig. 3–2). A mixture of ice and liquid water, for example, is a pure sub-
stance because both phases have the same chemical composition. A mixture
of liquid air and gaseous air, however, is not a pure substance since the
composition of liquid air is different from the composition of gaseous air,
and thus the mixture is no longer chemically homogeneous. This is due
to different components in air condensing at different temperatures at a
PHASES OF A PURE SUBSTANCE
We all know from experience that substances exist in different phases. At
room temperature and pressure, copper is a solid, mercury is a liquid, and
nitrogen is a gas. Under different conditions, each may appear in a different
phase. Even though there are three principal phases—solid, liquid, and