This question concerns vapor pressure lowering.

Question

Why is the vapor pressure of an aqueous solution of calcium chloride lower than the vapor pressure of water?

Answer 1

When a solvent contains a solute, the vapor pressure is lowered. The simple explanation, on a macroscopic level, is that the particles of solute (calcium chloride, in this case), interfere with the ability of the solvent (water in this case) to evaporate or to enter the vapor phase. This lowers the vapor pressure of the pure solute. This is a colligative property, and the magnitude of the vapor pressure lowering will depend on the number of particles (van't Hoff factor) and on the concentration (molarity)