Chapter 8 Laboratory: Colligative Properties of Solutions 147

The colligative properties of solutions are those affected by the number of solute particles

(ions, atoms, molecules, or accretions of multiple molecules) in a solution rather than by the

type of solute particles present. There are three colligative properties, as described here.

Laboratory: 8

Colligative Properties of Solutions

vpor pressure reductiona

Vapor pressure reduction occurs as the number of solute

particles in a solution increases. In effect, some solute

particles displace solvent particles at the surface of the

solution (the liquid-gas boundary), making fewer solvent

particles available for vaporization and thereby reducing the

vapor pressure.

Boiling point elevation is sometimes considered a separate

colligative property, but is in fact a special case of vapor

pressure reduction. The boiling point is reached at the

temperature where the vapor pressure of the liquid matches

the pressure of the gas at the liquid-gas boundary. Because

the presence of solute particles reduces the vapor pressure of

the solution, it also increases the boiling point of the solution.

Boiling point elevation (ΔTb or ΔTbp) is the product of the

van’t Hoff factor (i) of the solute, the boiling point elevation

constant (also called the ebullioscopic constant) of the

solvent (Kb or Kbp), and the molality (m) of the solution, and

can be expressed as the formula ΔT = iKbm.

One familiar application of vapor pressure reduction is the use

of antifreeze (which could just as easily be called anti-boil) in

automobile radiators. The addition of high-boiling ethylene

glycol and similar chemicals to the water in the radiator raises

the boiling point of the solution above that of pure water,

reducing the likelihood of the radiator boiling over.

freezing point depression

Freezing point depression occurs as the number of solute

particles in a solution increases. One familiar application

of freezing point depression is the application of road salt

(usually sodium chloride or calcium chloride) to melt ice on

streets and sidewalks. As the ice dissolves the salt (yes, one

solid can dissolve another solid) the solid ice is converted

to a liquid solution of the salt, because the solution has a

lower freezing point than the essentially pure water that

makes up the ice. Freezing point depression is calculated

in the same way as boiling point elevation, but substituting

the freezing point depression constant (Kf or Kfp) for the

boiling point elevation constant. Because the freezing point

depression constant is conventionally expressed as an

unsigned value, the formula for calculating freezing point

depression, ΔT = –(iKfm), adds a negative sign to indicate

the reduction in freezing point.

osmotic pressure

Osmotic pressure occurs when a differential concentration

of solute particles causes pressure to be exerted across a

semipermeable membrane. The phenomenon of osmotic

pressure is exploited in many chemical and industrial

processes as well as in medical procedures such as kidney

dialysis, not to mention such routine bodily functions as your

kidneys extracting waste products from your bloodstream.

The behavior of solutions under osmotic pressure resembles

the behavior of gases. In fact, the formula for calculating

osmotic pressure uses the ideal gas constant: π = (niRT)/V,

where π is the osmotic pressure, n the moles of solute, i the

van’t Hoff constant, R the ideal gas constant, T the absolute

temperate in kelvins, and V the volume.