ABSTRACT
In 1873, a little over two decades after the formulation of the first and second laws of thermodynamics, van der Waals in Leiden published a thesis entitled The Continuity of the Gaseous and Liquid States. He introduced the famous van der Waals equation in that work. This chapter addresses the central issues concerning the vdW equation, namely, the multivalued solutions to the equation of state, and the unphysical behaviour of the isotherms. If more than one solution to the equation of state is thermodynamically stable, it is tempting to interpret them as different phases of a single component system. The chapter discusses a very elegant construction discussed by Fermi which requires only the existence of the coexistence part and does not require any calculations at all. It utilizes the fact that the Gibbs potential per unit mass of the two phases must be equal to determine the flat part, also called the Maxwell line.
