The principal goal of physical chemistry is to understand the properties
and behavior of material systems and to apply this understanding in
useful ways.
The state of a system is specified by giving the values of a certain number
of independent variables (state variables).
In an equilibrium one-phase fluid system of one substance, three
macroscopic variables such as temperature, volume, and amount of
substance can be independent variables and can be used to specify the
macroscopic equilibrium state of the system. At least one of the variables
used to specify the state of the system must be proportional to the size of
the system (be extensive). Other macroscopic variables are mathematical
functions of the independent variables.
The intensive state, which includes only intensive variables (variables
that are independent of the size of the system), is specified by only two
variables in the case of an equilibrium one-phase fluid system of one
substance.
Nonideal gases and liquids are described mathematically by various
equations of state.
The coexistence of phases can be described mathematically.
The liquid–gas coexistence curve terminates at the critical point, beyond
which there is no distinction between liquid and gas phases.
The law of corresponding states asserts that in terms of reduced variables,
all substances obey the same equation of state.
