Boiling point elevation is the physical phenomenon where the temperature required for a liquid to boil increases when a non-volatile substance is dissolved in it. A solution, which is a mixture of a solvent and a dissolved solute, will thus have a higher boiling temperature than the pure solvent alone. The solute must be non-volatile, meaning it does not readily evaporate and stays in the liquid phase as the temperature rises. For example, water, a common solvent, will boil above 100°C at standard atmospheric pressure if substances like salt or sugar have been dissolved.
The Physics Behind the Increase
A liquid begins to boil when its vapor pressure equals the surrounding atmospheric pressure. Vapor pressure is the pressure exerted by the gas phase of a substance in equilibrium with its liquid phase. In a pure solvent, molecules at the surface easily escape into the gas phase once they gain sufficient energy.
When a non-volatile solute is introduced, its particles disperse throughout the liquid, physically blocking some solvent molecules from reaching the surface and escaping. This interference lowers the vapor pressure of the solution compared to the pure solvent at any given temperature. Because the solution’s vapor pressure is lower, it requires more energy to reach the atmospheric pressure threshold needed for boiling.
Supplying this additional energy necessitates heating the solution to a higher temperature than the pure solvent. This higher temperature provides the kinetic energy needed for the solvent molecules to overcome the solute’s interference. The degree of temperature increase is the boiling point elevation, which represents the extra thermal energy required to match the external pressure.
Colligative Properties and Quantification
Boiling point elevation belongs to colligative properties, which are characteristics of solutions that depend solely on the number of solute particles present, not their chemical identity. Different substances can produce different numbers of particles when dissolved, which affects the magnitude of the elevation.
The quantitative relationship for boiling point elevation (\(\Delta T_b\)) is calculated using a formula linking the temperature change directly to the concentration of particles. This concentration is measured in molality (\(m\)), which is the amount of solute in moles per kilogram of solvent.
The formula incorporates the ebullioscopic constant (\(K_b\)), a unique value specific to the solvent, such as water or benzene. For water, \(K_b\) is approximately \(0.512^{\circ}\text{C}\cdot\text{kg}/\text{mol}\). The final variable is the van’t Hoff factor (\(i\)), which accounts for the dissociation of the solute.
Substances that do not dissociate, like sugar, have a van’t Hoff factor of one because one molecule yields only one particle. In contrast, an ionic compound like sodium chloride (NaCl) dissociates into two ions, giving it a factor close to two. The elevation is greater for ionic compounds at the same molality because they generate more particles in the solution.
Where Boiling Point Elevation Matters
The most widespread application of boiling point elevation is in the cooling systems of automobiles. Engine coolant, commonly a mixture of water and ethylene glycol (antifreeze), relies on this principle to prevent overheating. Ethylene glycol is a non-volatile solute that raises the boiling point of the water in the radiator well above \(100^{\circ}\text{C}\). This allows the engine to operate at a higher, more efficient temperature.
This elevated boiling point ensures the liquid coolant remains in the engine block even when temperatures exceed the boiling point of pure water. A typical 50/50 mixture of water and ethylene glycol can raise the boiling point to around \(106^{\circ}\text{C}\) at standard pressure. This effect is further enhanced by the pressurized nature of a car’s cooling system, which independently raises the boiling point.
Boiling point elevation is also observed when salt is added to water for cooking. While the effect is chemically measurable, it is negligible for practical cooking purposes. Adding a single tablespoon of salt to a gallon of water, for instance, only raises the boiling temperature by a fraction of a degree.