What Is the Meaning of Equilibrium Solidification Temperature in Supercooling?

[kelvin490]

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. 0^oC for water)?

 

[kelvin490]

Another question is, at precisely equilibrium temperature, will liquid becomes a mixture of solid and liquid or nothing will change? Some may say solid and liquid co-exist but what is the driving force if it is not energetically favorable for either side at exact equilibrium temperature? What determines the portion of liquid and solid at this temperature if they really co-exist?

 

[DrClaude]

My question is, if supercooling is necessary, what is the meaning of "equilibrium" solidification (or melting) temperature (e.g. 0^oC for water)?

At the instant when the solidification starts, the temperature shoots back up to the solidification temperature. For instance, while you can have liquid water below 0°C, it will only make ice while exactly at 0°C.

Another question is, at precisely equilibrium temperature, will liquid becomes a mixture of solid and liquid or nothing will change? Some may say solid and liquid co-exist but what is the driving force if it is not energetically favorable for either side at exact equilibrium temperature? What determines the portion of liquid and solid at this temperature if they really co-exist?

You can have any proportion of liquid and solid, in constant equilibrium, when exactly at the temperature of the phase transition. To actually convert liquid water to ice, additional heat must be extracted from the water. Usually, the liquid will exchanging heat with an environment below the transition temperature (such as a freezer at -18°C).

You could imagine an ideal environment for which you could control the temperature exactly. Setting it to 0°C, you would cool down the water to zero without forming any ice. Then reducing the temperature an infinitessimal degree below 0°C, some ice would start forming (assuming no supercooling), and bringing back the temperature to exactly 0°C would stop the crystallization process and maintain the proportion of solid over liquid.