Examples of Colligative Property

By Dan Antony
Salt lowers the freezing point of water, which is one of the reasons arctic water does not freeze completely.

Automotive antifreeze, kidney dialysis and using rock salt to make ice cream all depend upon the colligative properties of solutions, which are the physical properties of solutions, ones that depend only on the ratio of the number of particles of solute and solvent (e.g., salt in water) in solution and not on the identity of the solute.

Vapor Pressure Depression

A solvent (such as water) has a vapor pressure denoted by p1. At equilibrium, the gas phase (such as water vapor) above the solvent has a partial pressure equal to p1, but when you add a solute (like table salt, NaCl), the partial pressure of the solvent in the gas phase decreases. The cause is simply that solvent molecules on the surface of the solution have been replaced by solute molecules, which “crowd out” vaporization.

Boiling Point Elevation

Because boiling is essentially vaporizing the solvent, boiling point elevation occurs for the same reason as does vapor pressure depression; the solute inhibits vaporization of the solvent at the surface.

This presumes the solute is non-volatile, that is, it has a low vapor pressure at room temperature. A volatile solute with a lower boiling point than the solvent may actually depress the boiling point. Benzene is an example of a volatile organic compound (VOC).

Freezing Point Depression

The freezing point of a solution will be lower than that of the pure solvent. A practical application of this colligative property is automotive antifreeze; the freezing point of a 50/50 solution of ethylene glycol (CH2(OH)CH2(OH)) is -33 degrees Celsius (-27.4 degrees Fahrenheit), compared with 0 degrees Celsius (32 degrees Fahrenheit).

Osmotic Pressure

Osmosis occurs when solvent molecules move through a semipermeable membrane and from an area of higher concentration to an area of lower concentration, or from higher chemical potential to lower chemical potential. The pressure that would stop that flow is the osmotic pressure, which generally increases for solutions. The semipermeable membrane acts as a sieve which filters solute particles.

Molality

Colligative properties are all dependent upon the molality (m) of a solution, defined as moles of solute/kg of solvent. In our example of antifreeze, the molality of the water-antifreeze mixture is about 117 moles/6.6 kg or 18 m. That assumes 7 quarts of water and 7 quarts of antifreeze, which is fairly typical of cars.

About the Author

Dan Antony began his career in the sciences (biotech and materials science) before moving on to business and technology, including a stint as the international marketing manager of an ERP provider. His writing experience includes books on project management, engineering and construction, and the "Internet of Things."