Boiling point elevation, freezing point depression, vapor pressure lowering, and osmotic pressure are examples of colligative properties. These are properties of matter that are affected by the number of particles in a sample.
Boiling Point Elevation Definition
Boiling point elevation is the phenomenon that occurs when the boiling point of a liquid (a solvent) is increased when another compound is added, such that the solution has a higher boiling point than the pure solvent. Boiling point elevation occurs whenever a non-volatile solute is added to a pure solvent.
While boiling point elevation depends on the number of dissolved particles in a solution, their identity is not a factor. Solvent-solute interactions also do not affect boiling point elevation.
An instrument called an ebullioscope is used to accurately measure boiling point and thus detect whether boiling point elevation has occurred and how much the boiling point has changed.
Boiling Point Elevation Examples
The boiling point of salted water is higher than the boiling point of pure water. Salt is an electrolyte that dissociates into ions in solution, so it has a relatively large affect on boiling point. Note nonelectrolytes, such as sugar, also increase boiling point. However, because a nonelectrolyte does not dissociate to form multiple particles, it has less of an effect, per mass, than a soluble electrolyte.
Boiling Point Elevation Equation
The formula used to calculate boiling point elevation is a combination of the Clausius-Clapeyron equation and Raoult's law. It is assumed the solute is non-volatile.
ΔTb = Kb · bB
- ΔTb is the boiling point elevation
- Kb is the ebullioscopic constant, which depends on the solvent
- bB is the molality of the solution (typically found in a table)
Thus, boiling point elevation is directly proportional to the molal concentration of a chemical solution.