Boiling Point Elevation
Boiling point elevation is one of four colligative properties (the others being freezing point depression, vapor pressure lowering, and osmotic pressure). It occurs when adding a solute to a solvent lowers the resulting solution’s vapor pressure. As a result, the solution will boil at a higher temperature than the pure solvent. This effect depends on the number of solute particles, not their identity, making it a colligative property. [1-4]
Why does Boiling Point Elevation Occur
When a nonvolatile solute, such as sugar or salt, is dissolved in a solvent, such as water or milk, it reduces the solvent molecules’ ability to escape into the gas phase. It occurs because solute particles occupy some of the surface area of the liquid, leaving fewer solvent molecules available to evaporate. Vapor pressure, which is the pressure exerted by the vapor molecules above a liquid, decreases. As a result, the solution requires a higher temperature to reach the vapor pressure needed for boiling. [1-4]
Equation
The boiling point elevation (ΔTb) equation predicts how much the boiling point will increase when a solute is dissolved in a solvent. It is calculated from the equation: [1-4]
ΔTb = i x Kb x m
Where:
– m is the molality
– Kb is the boiling point elevation constant
– i is the van’t Hoff factor
Let us look at the individual terms in the equation.
Molality
Molality (m) measures the concentration of the solution. It is defined as the number of moles of solute per kilogram of solvent. Molality is independent of the solution’s temperature or volume, making it ideal for boiling point calculations.
Boiling Point Elevation Constant
The boiling point elevation constant (Kb) varies for different solvents, depending on their chemical properties. This constant determines how much the boiling point of a solvent increases per molal concentration of solute particles. For example, water has a Kb value of 0.512 °C·kg/mol, meaning that for every 1 mol of solute particles per kilogram of water, the boiling point increases by about 0.512 °C.
The values of Kb for some common solvents are tabulated below.
| Solvent | Kb value (in oC.kg.mol-1) |
|---|---|
| Water | 0.512 |
| Phenol | 3.04 |
| Acetic Acid | 3.07 |
| Chloroform | 3.63 |
| Benzene | 2.53 |
Van’t Hoff Factor
The van’t Hoff factor accounts for the number of particles the solute produces in the solution. For non-electrolytes like sugar, i is 1 because it does not break into smaller particles. For electrolytes like sodium chloride (NaCl), i is greater than 1 because the solute dissociates into ions (Na+ and Cl−), so i = 2. Therefore, substances that dissociate into multiple particles cause a greater elevation of the boiling point than those that do not break apart in solution.
Applications [1-4]
- Automotive Coolants: Antifreeze solutions use boiling point elevation to prevent engines from overheating in high-temperature conditions.
- Cooking: Adding salt to water increases its boiling point, aiding in efficient cooking, especially at high altitudes.
- Food Preservation: Concentrated sugar solutions in jams and syrups leverage boiling point elevation to inhibit microbial growth.
- Industrial Processes: Boiling point elevation is used in processes like distillation and chemical separations to modify solvent boiling points.
- Cryoprotectants: Certain solutions designed to protect biological tissues in freezing conditions also benefit from raising the boiling point.
- Pharmaceutical Formulations: Boiling point elevation helps in creating stable solutions for drug delivery systems.
- Desalination: In some thermal desalination processes, boiling point elevation influences the evaporation stage by altering solvent behavior.
