Super-cooled liquids

I understand that it is possible to cool a liquid past its freezing point, and still maintain its liquid state because crystallisation would not happen in the absence of a seed crystal.


My question, was marked as a duplicate and I was linked to to [Can all substances be supercooled?](https://chemistry.stackexchange.com/questions/32459/can-all-substances-be-supercooled)


One of the answers over there does address my question but only in part.
Here's what it says: 



> 
> "However, when there is only liquid present, you will first have to form a small crystal nucleus which is not as stable as the bulk solid i.e. has a higher free energy. This is basically a kinetic barrier which prevents forming the solid.
>  The formation energy of such a nucleus will depend on the interfacial energy of the crystal-liquid system. In a substance where this interfacial free energy is very little there won't be too much supercooling."
> 
> 
> 


My question still remains. Why is the lack of a nucleus a kinetic barrier to the formation of a bulk solid? If you continue past the glass transition temperature, we form an amorphous solid anyway. How do the mechanics of vitrification and crystallisation differ in this regard?




When a liquid is supercooled its structure doesn't change significantly, only the particles have less kinetic energy available. As you cool even further, it could occur that particles get caught in local minima and eventually you'll end up with a glass, which can be thought of as a frozen liquid structure. As particles are trapped it will resist shear in an elastic fashion so you have a solid. In this regard it is a barrierless transition. There is no interface whatsoever.


Crystal formation is fundamentally different as the local structure changes first to form a nucleus. This has both energetic and entropic consequences and the aforementioned kinetic barrier is observed. I would also like to add that crystal nuclei are formed all the time but they don't reach the critical size if the temperature is too high.


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Supercooling (and superheating) occur mainly due to the difference between surface energy and volume energy. An intersection of two phases/compounds has higher energy than a bulk material.


For example, the interface of ice and water has higher energy because frozen liquid molecules are favored by entropy to mix with the liquid. The only factor keeping the phase boundary in existence is the lack of (or excess of) energy to break the equilibrium between the phases. For any molecule to enter the liquid phase, one must enter the solid phase.


Boiling and freezing come into this when you consider that the temperatures tabulated for boiling and freezing actually measure these phenomena for when a phase boundary already exists. That phase boundary lowers the energy required for phase change, thus lowering the boiling point (or raising the freezing point).


Adding/removing energy when a phase boundary already exists simply shifts the phase equilibrium I already mentioned to the appropriate side. If no boundary exists, however, it becomes more difficult for molecules to escape. When a vessel is heated from the bottom, the hottest portion of the liquid is trapped by other liquid. Any single molecule, even if it attains the required energy, is quickly collided with other molecules and pushed back to the average.


Substances will spontaneously change phase if enough energy exists to favor that change, even without a phase boundary. Going even further, enough energy in the right form can cause [a breakdown in the phase boundary](https://www.youtube.com/watch?v=yBRdBrnIlTQ).


I do not feel I can adequately answer the later part of your question, as I do not have as much background with solid chemistry. However, I will note that solids typically can exist in [different configurations of the molecules](http://en.wikipedia.org/wiki/Microstructure). A common example is the difference between steel that has been quickly cooled and steel that has been slow-cooled: cooling more quickly, in this case, results in a harder, more brittle material.


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