When attraction between the solute and the solvent is stronger than the attraction of either substance for itself, the principle applies only imperfectly.  
For strong solutions, it is generalized as Duhring's Rule, which uses water as a model -- but other electrolytes do not obey it.  For it to apply, the activity coefficient must be 1.0, making the solution ideal.  When attraction between the solute and the solvent is stronger than the attraction of either substance for itself, the principle applies only imperfectly.  FTP, name this principle of chemistry, which states that the mole fraction of a solvent times its vapor pressure in a pure state equals the vapor pressure of the solvent.
For strong solutions, it is generalized as Duhring's Rule, which uses water as a model -- but other electrolytes do not obey it.  
Raoult's law ("Rah-OOH")