### Intensive versus Extensive Variables

Thus far we have discussed a number of "process variables"
(variables that may be measured and used to characterize the
operation of a process). It is helpful, prior to discussing state
variables to group these variables into two categories:

DEFINITION:

An **extensive variable** is one which
depends on system size (like mass or volume).

While extensive variables are useful for characterizing the
*specific* system being analyzed, they are not general and
(as we will see later in the course) do not help us to determine
the "state" of the system.

DEFINITION:

An **intensive variable** is one which
does not depend on system size (like temperature, pressure, or
density).

While it may not be immediately obvious, intensive variables
tell us *much more* about the system than extensive
variables. In particular, the temperature and pressure are two of
the most critical intensive variables.

OUTCOME:

Explain the difference between intensive and
extensive variables.

NOTE:

A **ratio** of extensive variables will yield an
intensive variable! (For example, mass/volume -- two extensive
variables -- gives density, an intensive variable) This is one way
to understand why intensive variables "tell you more".