The van der Waals equation (P + a/V²)(V − b) = RT accounts for molecular attraction (a) and finite volume (b). Below the critical temperature T_c = 8a/27Rb, isotherms become non-monotone — physically realized through the Maxwell equal-area construction, which gives the liquid-vapor coexistence pressure and defines the phase boundary. The critical point is P_c = a/27b², V_c = 3b.