Reaction pathway and localization of the phenomena

We already noted in section 8.2 that nucleation describing the creation of a solid phase starting from an initial solid used two groups of steps  

We already came across reactions of creation of point defects in section 2.5 and we can classify these reactions into two categories  

It is noted thus that the majority of heterogenous reactions lead to surface nucleations indeed, only some polymorphic transformations, that is, those not requiring any other reactant and not producing s phase, which would occur starting from defects of Frenkel or from defects of misplaced atoms, would be likely to lead to nucleation in the bulk. 

Whatever is the case of a nucleation on the surface or in the bulk, the creation of the defects uses various elementary steps that proceed in precise zones either the bulk of the initial sohd or the interface between the initial solid and a surrounding phase, and in this case, if gases are concerned, phenomena of chemisorption or desorption may intervene on the surface of the sohd. 

For example, the nucleation of an anhydrous salt starting from the corresponding hydrated salt needs the creation of water vacancies according to the following 

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