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Addition of a Non-volatile Solute to a Volatile Solvent

When a non-volatile solute is added to a liquid (solvent), it is found that the vapor pressure of the solution is less than the vapor pressure of the pure liquid. This is because of the following two reasons:
  1. Attraction between the solute and solvent molecules.
  2. Reduction in the number of solvent molecules per unit area of the surface.
Let us understand these two reasons.
 
Solutesolvent attraction: When the solute is non-volatile, it is always invariably a solid. The reason a solid would dissolve in a liquid is the attraction that the solvent has for the molecules/ions of the solute. Therefore, there is a strong solute–solvent attraction present in the solution. Moreover, this attraction would be dependent on the nature of solute, as different solutes will have different extent of attraction with the same solvent. To do away with the dependency on the nature of solute, we would like this attraction to have negligible impact on the vapor pressure of a solution.
 
Reduction in the number of solvent molecules: When a non-volatile solute is dissolved in a liquid, it distributes itself homogenously throughout the solution. Therefore, it occupies the surface of the liquid also. This reduces the number of solvent molecules present per unit area on the surface. This further diminishes the number of vapor molecules that are in equilibrium with the molecules of the liquid present per unit area on the surface. Thus, the vapor pressure gets reduced.




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