Boiling Point Elevation
Boiling point elevation is the phenomenon by which the boiling point of a solution is higher than that of the pure solvent, due to the presence of dissolved solute particles lowering the vapour pressure of the solvent. The increase in boiling point is directly proportional to the molal concentration of solute particles. This principle is exploited in automotive antifreeze formulations and in certain cooking techniques to raise the boiling temperature of water.
Key Formula
ΔTb = i × Kb × m
LaTeX: \Delta T_b = i \cdot K_b \cdot m
| Symbol | Meaning | Unit |
|---|---|---|
| ΔTb | Elevation in boiling point | °C or K |
| i | van't Hoff factor (number of particles per formula unit) | dimensionless |
| Kb | Ebullioscopic constant of the solvent | K·kg/mol |
| m | Molality of the solution | mol/kg |
Worked Example
Problem
Calculate the boiling point of a solution prepared by dissolving 18.0 g of glucose (M = 180 g/mol) in 200 g of water. Kb for water = 0.512 K·kg/mol.
Solution
Step 1 – Find moles of glucose: n = 18.0 g ÷ 180 g/mol = 0.100 mol. Step 2 – Find molality: m = 0.100 mol ÷ 0.200 kg = 0.500 mol/kg. Step 3 – Glucose is a non-electrolyte so i = 1. Step 4 – Apply formula: ΔTb = 1 × 0.512 × 0.500 = 0.256 °C. Step 5 – New boiling point = 100.000 + 0.256 = 100.256 °C.
Answer
Boiling point of solution = 100.256 °C
Ebullioscopic Constants (Kb) for Common Solvents
| Solvent | Normal BP (°C) | Kb (K·kg/mol) | Common Use |
|---|---|---|---|
| Water | 100.00 | 0.512 | Aqueous solutions |
| Ethanol | 78.37 | 1.22 | Organic reactions |
| Benzene | 80.10 | 2.53 | Non-polar solutes |
| Chloroform | 61.20 | 3.63 | Organic chemistry |
| Acetic acid | 117.90 | 3.07 | Acid solutions |
Interactive Tools
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Related Terms
Colligative Properties
Colligative properties are physical properties of solutions that depend only on the number of solute particles dissolved, not on the chemical identity of those particles. These properties include boiling point elevation, freezing point depression, vapour pressure lowering, and osmotic pressure. They are widely used in industries such as food preservation, antifreeze formulation, and clinical medicine to control solution behaviour.
Freezing Point Depression
Freezing point depression is the decrease in the freezing (solidification) point of a solvent caused by dissolving a solute, because the solute particles disrupt the formation of the ordered solid lattice. The magnitude of depression depends on the number of solute particles per unit mass of solvent, not their chemical nature. Practical applications include road de-icing with salt, antifreeze in vehicle radiators, and the preservation of biological samples in cryoprotective solutions.
Raoult's Law
Raoult's Law states that the partial vapour pressure of each volatile component in an ideal solution is equal to the vapour pressure of the pure component multiplied by its mole fraction in the solution. This law quantifies vapour pressure lowering as a colligative property and is the foundation for understanding distillation and solution thermodynamics. Solutions that obey Raoult's Law perfectly (ideal solutions) have similar intermolecular forces between all components; deviations occur in real solutions due to unlike-molecule interactions.
From Old English "boiling" (to bubble with heat) and Latin "elevatio" (a lifting up). The quantitative relationship was established by François-Marie Raoult in the 1880s and further developed by Jacobus van't Hoff.