What Is Boiling Point Elevation?

Boiling point elevation is the effect that occurs when a material is dissolved in a pure solution, causing an increase in the mixture's boiling point. The solute, the material that is to be dissolved, is added to the pure solution, called the solvent, which reduces the mixture's vapor pressure. Reducing the mixture's vapor pressure means that it takes more energy for the mixture to boil, which results in a higher boiling point of the mixture.

Every chemical has a measurable boiling point change for mixtures with different solvents. This measurable quantity is known as the molal boiling point elevation constant, or molal elevation constant. If the concentration of the chemical in the mixture is known or measured, this concentration can be multiplied by the molal elevation constant, and one can calculate the resulting boiling point elevation and compare it to measured values. The molal elevation constant also can be used to determine the concentration of solute in a mixture by measuring the mixture's boiling point and dividing the solvent's boiling point elevation by the molal elevation constant.

A common and useful application of boiling point elevation is adding antifreeze, typically ethylene glycol, to automobile cooling systems. Ethylene glycol is added in a 50 percent by volume concentration to water in the automobile’s radiator to prevent freezing, but the elevation in the boiling point of the resulting solution is a benefit. Water boils at 212° Fahrenheit (100° Celsius); the ethylene-glycol-and-water mixture boils at 225° Fahrenheit (107.2° Celsius) and even higher when the cooling system is pressurized, which is normal for automotive cooling systems.

Cooks have been taking advantage of boiling point elevation for centuries. Adding salt to water raises the boiling point of the mixture, which results in faster cooking times. Ocean sea water, which contains about 3.5 percent total salts, boils at 216.5° Fahrenheit (102.5° Celsius). This is perhaps not a great difference from plain water, but faster cooking usually is preferred by cooks.

The boiling point elevation resulting from a mixture is a factor of the molal boiling point constant, so the boiling point of a mixture will continue to rise as more of the solute is added to the mixture. This results from the reduction of the solvent's vapor pressure as its molecules are trapped by the solute. There are practical limits to boiling point elevation in consumer and industrial applications. In automotive cooling, for example, the boiling point of pure ethylene glycol is 386° Fahrenheit (197° Celsius) which could be seen as an advantage. The viscosity, or thickness, of pure ethylene glycol at colder temperatures makes its use impractical, however, because at 40° Fahrenheit (4.4° Celsius) pure ethylene glycol has a measured thickness that is seven times greater than the 50 percent ethylene-glycol-and-water solution.