Colligative Properties
Definition and meaning of Colligative Properties in chemistry.
Colligative properties are physical properties of a solution, vapor pressure lowering, boiling point elevation, freezing point depression, and osmotic pressure, that depend only on the number (concentration) of dissolved solute particles, not on their chemical identity.
In more detail
These properties arise because solute particles dilute the solvent, lowering its escaping tendency (chemical potential) and thus reducing vapor pressure, which in turn raises the boiling point and lowers the freezing point relative to the pure solvent. Osmotic pressure develops because a semipermeable membrane allows solvent, but not solute, to flow toward the more concentrated side. For dilute solutions, the magnitude of each effect is proportional to solute molality (or molarity) multiplied by the van't Hoff factor, i, which corrects for solutes that dissociate (e.g., NaCl → Na+ + Cl-) or associate in solution.
Key facts
| Field | Physical Chemistry |
|---|---|
| Key equations | ΔTf = iKfm; ΔTb = iKbm; π = iMRT |
| Depends on | Number of solute particles, not their identity |
| Correction factor | van't Hoff factor, i (accounts for ionic dissociation/association) |
Dissolving 1 mol of NaCl in 1 kg of water lowers the freezing point by about 3.72 degC (i ≈ 2, since NaCl dissociates into two ions), roughly twice the 1.86 degC depression produced by 1 mol of a nonelectrolyte like glucose in the same amount of water.
Frequently asked questions
Why is it called 'colligative'?
The term comes from the Latin colligatus, meaning 'bound together,' reflecting that these properties are tied collectively to the total count of solute particles rather than to what the solute is.
Do ionic and molecular solutes affect colligative properties differently?
The underlying relationship is the same, but ionic solutes dissociate into multiple particles per formula unit, so their effect is scaled by the van't Hoff factor i (i approx 2 for NaCl, approx 3 for CaCl2), making them affect colligative properties more strongly per mole than nonelectrolytes.