Van der waals & Andrews curve: discrepancy

1. Mar 28, 2010

samreen

1. The problem statement, all variables and given/known data
the Pressure (P) versus Volume (V) isotherms of a real gas theoretically obtained form van der waals equation of state approximately overlap with the experimentally obtained isotherms of Andrews for carbon dioxide gas. However, for those isotherms below the critical isotherm,in the region of phase transition, Andrews curve gives a straight line, while van der waals gives a maxima and minima. i need a proper and substantial explanation for the discrepancy

2. Relevant equations
(P + a/V2)(V – b) = RT

since its a cubic eqn, either all 3 roots will be real, or only one. but i dont see how thats relevant here.

3. The attempt at a solution

the maxima and minima represent metastable states that arent realisable under experimental conditions, and were thus not obtained by Andrews