Intensive, Extensive, and Colligative Properties
Most physical properties can be classified as intensive or extensive. Intensive properties are characteristic of the substance and do not depend on the size of the material being studied; an example of this is density. Extensive properties are properties that are directly linked to the size of the material being studied; an example of this is mass. A third category of properties can only be applied to solutions; these are known as colligative properties. Properties can be considered colligative only if their properties are dependent on the amount of solute present in the solution, disregarding the identity of the solute itself.
The boiling point of a solution in the presence of a solute increases. This is referred to as boiling point elevation. Because the elevation of the boiling point is directly dependent on the amount of solute present in the solution but not based on the identity of the solute, it is considered a colligative property. This phenomena happens because of the displacement of solvent molecules by the solute; it can occur in both electrolyte and non-electrolyte solutions.
The Relationship Between Boiling Point Elevation and Vapor Pressure
The boiling point elevation can be explained in terms of vapor pressure. Vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases at a given temperature. In layman's terms it is simply a measure of the tendency of the solution molecules to escape by entering the gas phase. A liquid boils when its vapor pressure is equal to the surrounding pressure. The addition of a solute to a solvent dilutes the solution, and decreases the vapor pressure: the molecules escape into the environment more slowly. A non-volatile solute has zero vapor pressure, so the solution consisting of both the solute and solvent has lower pressure than the pure solvent. For the vapor pressure to exceed the external pressure, a higher temperature is required; thus, a higher boiling point is observed.
Calculating Boiling Point Elevation
The extent of the boiling point elevation can be calculated. It is directly proportional to the molal concentration of the solution. The amount the boiling point is elevated determined using the equation: