Modern Engineering Thermodynamics

36 CHAPTER 2: Thermodynamic Concepts
PURE
SUBSTANCE
ELEMENTS
Elements contain atoms of a
single atomic number and have
well-defined physical
properties.
COMPOUNDS
Compounds contain atoms of
two or more elements
chemically combined and have
well-defined physical properties.
MIXTURE
HOMOGENEOUS MIXTURES
Homogeneous mixtures have
the same composition
throughout. The components
in the mixture are NOT
distinguishable from each
other, and different mixtures
have widely different physical
properties.
HETEROGENEOUS MIXTURES
Heterogeneous mixtures do
NOT have the same composition
throughout. The components
in the mixture ARE
distinguishable, and different
heterogeneous mixtures may
have widely different physical
properties.
FIGURE 2.2
Pure substances and mixtures.
CRITICAL THINKING
From the previous discussion as to whether or not air is a pure substance, do you think that a more practical definition of
a pure substance should require only that the substance not change chemical composition under the conditions of a given
engineering process? For example, if air at atmospheric pressure is being heated from say 20°C to 200°C, then it would not
change chemical composition and therefore could be considered as a pure substance. If, on the other hand, this air is being
cooled from 20°C to–200°C, then it could not be considered a pure substance because the oxygen gas condenses into a
liquid at –183°C and this would change the chemical composition of the remaining gas.
We further define a homogeneous substance as anything that contains a single physical phase. Air at normal
atmospheric conditions is a homogeneous substance, but being a mixture of various gases, it is not a pure
substance. A mixture of liquid water and ice on the other hand is not a homogeneous substance, but it is a pure
substance. A pure substance that is also a homogeneous substance is called a simple substance. Liquid water is an
example of a simple substance.
Pure substances must be chemically uniform but need not consist of a single chemical species. For example,
a homogeneous mixture of uniform chemical composition can often be treated as if it were a single phase of a
pure substance. Air in its gaseous state is usually treated as a pure substance, even though it does not satisfy the
general definition of a pure substance.
2.4 SYSTEM STATES AND THERMODYNAMIC PROPERTIES
The thermodynamic state of a system can be either an equilibrium state or a nonequilibrium state. A thermodynamic
equilibrium state is defined by the values of its thermodynamic properties. A nonequilibrium state is
much more difficult to define and generally requires the existence of a condition called local thermodynamic
equilibrium, which exists when thermodynamic equilibrium occurs locally within a series of small volumes that
make up the system. Conversely, a thermodynamic property is any characteristic of a system whose numerical
value depends only on the (local) thermodynamic equilibrium state of the system and is independent of how