Freezing Point Depression
Definition and meaning of Freezing Point Depression in chemistry.
Freezing point depression is the lowering of a solvent's freezing point that occurs when a solute is dissolved in it. It is a colligative property, meaning its magnitude depends on the number of dissolved solute particles, not their chemical identity.
In more detail
Solute particles interfere with the solvent molecules' ability to arrange into an ordered crystal lattice, so the solution must be cooled below the pure solvent's normal freezing point before solid begins to form. The size of the effect is given by ΔTf = i·Kf·m, where Kf is the solvent's molal freezing-point-depression constant, m is the solute's molality, and i is the van't Hoff factor, which accounts for solutes that dissociate into multiple ions in solution. This principle explains why antifreeze protects car engines and why salt is spread on icy roads.
Key facts
| Formula | ΔTf = i·Kf·m |
|---|---|
| Kf of water | 1.86 °C·kg/mol |
| Field | Physical Chemistry |
| Property type | Colligative |
Dissolving 1 mole of glucose in 1 kilogram of water lowers the freezing point of the water by 1.86°C, since water's Kf is 1.86 °C·kg/mol and glucose does not dissociate (i = 1), giving a new freezing point of -1.86°C.
Frequently asked questions
Does it matter what kind of solute is dissolved?
Only the total number of dissolved particles matters, not their identity. Ionic solutes like NaCl produce a larger depression per mole than nonelectrolytes like glucose because they dissociate into multiple ions, giving a van't Hoff factor (i) greater than 1.
Why does road salt melt ice?
Salt dissolves into the thin liquid water film on the ice, lowering that water's freezing point below the surrounding temperature so the ice melts and does not easily refreeze.