Freezing Point
Definition and meaning of Freezing Point in chemistry.
Freezing point is the temperature at which a liquid solidifies under a given pressure, corresponding to the temperature at which the liquid and solid phases coexist in equilibrium. For a pure substance at standard atmospheric pressure (1 atm), this temperature equals its melting point.
In more detail
At the freezing point, molecules lose enough kinetic energy for intermolecular attractive forces to lock them into an ordered crystalline lattice, and the rates of freezing and melting become equal. Freezing point depends on pressure and is lowered by dissolved solutes, a colligative property described by freezing point depression (ΔTf = Kf·m). Pure crystalline solids have a sharp, well-defined freezing point, whereas mixtures and amorphous substances freeze over a range of temperatures.
Key facts
| Field | Physical Chemistry |
|---|---|
| Water's Freezing Point | 0 °C (273.15 K) at 1 atm |
| Also Known As | Solidification point |
| Governing Equation | ΔTf = Kf·m (freezing point depression) |
Pure water freezes at 0 °C (32 °F, 273.15 K) at 1 atm pressure, but seawater, containing dissolved salts, freezes at a lower temperature, typically around -2 °C, due to freezing point depression.
Frequently asked questions
Is freezing point the same as melting point?
For a pure substance, yes, both refer to the same temperature at which solid and liquid phases are in equilibrium at a given pressure, though melting point is approached from the solid side and freezing point from the liquid side.
Why does adding salt lower the freezing point of water?
Dissolved salt ions disrupt the formation of the ice crystal lattice and lower the solution's chemical potential, a colligative effect called freezing point depression that requires a lower temperature to reach solid-liquid equilibrium.