Colligative properties
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In chemistry, colligative properties are the properties of dilute solutions of non-volatile solutes whose values depend only on the concentration of solute particles, not on the type of particles present.
The four colligative properties are:
- Vapor pressure: The change in vapor pressure where the solute is less volatile than the solvent is regulated by Raoult's law, which states that the pressure is equal to the mole fraction of the solvent times the vapor pressure of pure solvent: P = Xsolvent*P°. This holds truest for ideal solutions.
- Freezing-point depression: The presence of a solute decreases the freezing point as compared to a pure solvent. The exact change (ΔT) can be calculated as van 't Hoff factor (i) of the solute multiplied by its molality (m) multiplied by the freezing point depression constant of the solvent (Kf): ΔT = i(−Kf)m. Alternatively, it can be calculated as the total molality of all solutes in solution times the depression constant: ΔT = Kf ∑ m.
- Boiling-point elevation: Because of the lowered vapor pressure, the boiling point of a solution is elevated as compared to the pure solvent. The change in boiling point (ΔT) can be calculated in the same way as the change in freezing point, except a different constant is used: the van 't Hoff factor (i) of the solute multiplied by its molality multiplied by the boiling point elevation constant of the solvent (Kb): ΔT = iKbm. ΔT can also be calculated using the sum of all molalities: ΔT = Kf ∑ m.
- Osmotic pressure: The presence of solute can cause pressure to be exerted across a permeable membrane according to an equation quite similar to the ideal gas law: π is the osmotic pressure, n is the number of moles of solute, R is the ideal gas constant, T is the absolute temperature in kelvins, and V is the volume: π = (nRTi)/V.
In (1), the two columns of pure solvent (blue) under the same pressure. When solute (green) is added to the right column (2), osmotic pressure is exerted and solvent flows through the permeable membrane (red) to the right side.
