A thermodynamic argument is used to show that all colligative properties of a substance 2 (solute) in a solution with a liquid substance 1 (solvent) differ from those of the pure condensed phase of solute precisely as if the internal tension between molecules of solute in the solution was increased by π2, the osmotic pressure of solute in the solution. It follows from this argument that molecules of solvent serve both to couple the molecules of solute and to enhance the tension in the coupling force between them by an amount π2. It may be inferred that pressure dependent chemical properties, like all partial molar quantities of the solute, are altered by this enhanced internal tension. A plausible mechanism is proposed by which the internal solute and solvent tensions are enhanced, as required by thermodynamic argument.