The colligative properties are important for explaining a vareity of natural phenomena and for controlling processes in practice. The effects of the collgiative properties are freezing point depression, boiling point elevation and osmotic pressure. Although most books do not discuss in this way, the vapor pressure lowering is the key feature that unifies these three effects. Since the lowering of the vapor pressure upon addition of a non-volarile solute is a property of the solvent (and not the solute) the colligative ffects are properties of the solvent that dependon the addition of solute.
Vapor pressure lowering
One could make the case that the essence of the colligative properties is the lowering of the vapor pressure of a solvent when a non-volatile solute is added. In reality this too is a colligative property since it occurs irrespective of the identity of the solute. The vapor pressure lowering can be seen as a consequence of Raoult's law, P1 = x1P*1. The vapor pressure is lowered proportional to the mole fraction of the solute. The effect of this lowering is easiest to see for boiling. If water is boiling at 1 atm of pressure and one adds 50 grams of salt to the water it will immediately cease boiling. The reason can be understaood from Raoult's law. The vapore pressure has been reduced by the addition of a non-volatile solute. Therefore, we will need to raise the temperature in order to reestablish equilibrium and start the water boiling again. The boiling point has increased.
The colligative properties depend only on the number of solute moleucules (their concentration) and not on their kind. It does not matter whether the solute atoms or molecules are charged. Therefore, when we consider salts we need to take into account the fact that salts dissolve with a stoichiometry determined by the charges of the cation and anion. A single salt mat dissolve to give 2, 3, 4 or 5 equivalents of ions that will affect the colligative properties. Therefore, we can speak of a colligative molality of a salt as the actual molality of ions (postivie and negative) once it has dissolved. This is a useful concept since we need to be careful to differentiate the concentration in the species we see in the jar (before the solution is made) to the dissociated form after the solution has been made.
