Question about supercooling During the cooling of a liquid, if no impurity of site of nuclearation, an appreciable solidification will begin only after the temperature has been lowered to below the equilibrium solidification (or melting) temperature. This phenomenon is termed supercooling. My question is, if supercooling is necessary, what is the meaning of "equilibrium" solidification (or melting) temperature (e.g. 0oC for water)?
 A: If you draw the phase diagram of water, the phase boundaries will be the points at which the free energy change for phase change is zero, that is the two phases are in thermodynamic equilibrium. However thermodynamic equilibrium is the infinite time limit of the system so at the phase boundary the phase change will only take place if you wait long enough.
It isn't the case that supercooling is necessary to freeze water, only that supercooling is necessary to freeze water quickly. Water at $273 - \delta$K will freeze for arbitrarily small values of $\delta$ provided you are willing to wait long enough.
The reason for this is that there is a potential barrier that has to be overcome for the phase change to take place. In the water there will be some water molecules with enough energy to overcome the barrier but the proportion of those molecules varies roughly as $e^{-\Delta E/kT}$, where $\Delta E$ is the barrier height. The barrier doesn't have to be too many multiples of $kT$ before the rate is effectively zero.
Supercooling increases the rate of freezing because it reduces the height of the potential barrier $\Delta E$.
