(P + a/V²)(V − b) = RT — isotherms, Maxwell construction, phase diagram
The van der Waals equation (P+a/V²)(V−b)=RT models real gases with attractive forces (a) and finite molecular volume (b). Critical point: Tc=8a/(27Rb), Pc=a/(27b²), Vc=3b. Below Tc the isotherm has an S-shape; the Maxwell equal-area construction (∮VdP=0) gives the physical phase boundary. The spinodal region (∂P/∂V>0) is mechanically unstable.