The animation shows a series of p-T diagrams of the
old good van der Waals equation of state for
b1/b2 = 7/13,
a11/a22 = 3/2, and k in the range [1,1.3]
(a12 = k sqrt(a11a22)).
The animation starts with a diagram of type III. Then, a tricritical
point appears on a critical line and splits into a pair of critical
end-points connected by a short segment of a three-phase line; this
is called type IV behavior (more exactly, IV*).
For even larger values of k, two
critical end-points merge (in the so called double critical end-point)
and disappear. The final type is called II.