In the Soave–Redlich–Kwong (SRK) equation of state (EOS), the compressibility factor occurs as a solution of the following cubic equation:
,
where
and
with
,
, 
, and
. Here
is the acentric factor,
and
are the critical temperature and pressure, and
=
is the reduced pressure.
When
and
are greater than one, the supercritical behavior is observed with
varying continuously between low (close to zero, liquid-like behavior) and high values (near one, vapor-like behavior).
For specific values of
and
(when
, the saturation pressure, and
), one gets three roots with the smallest and largest corresponding to the liquid and vapor phases. The intermediate value of
has no physical significance.
The Demonstration plots
for ethane (
,
, and
) and displays the location and values of its real roots.
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