Colligative Properties

Adding a solute to a pure solvent, A, decreases A's mole fraction, which in turn reduces A's chemical potential, μ_A.

The changes in μ_A influence the solution's colligative properties, which depend only on the number of solute molecules.

Generally, μ_A reflects the tendency of solvent A to escape from a solution. A decrease in μ_A shows that the solution's vapor pressure is lower than the vapor pressure of pure solvent A.

The introduction of a nonvolatile solid solute barely impacts the overall vapor pressure of the solution. As such, the solution's vapor pressure is expressed as the product of A's mole fraction, its vapor pressure, and the activity coefficient, ��_A.

The final expression for the change in vapor pressure, which is the difference between the vapor pressure of the solution and pure A, comes from substituting the equation for solution vapor pressure and assuming ��_A to be unity in very dilute solutions.

This equation can also be expressed using the mole fraction of a non-dissociating solute.