The global model with two processes

It is commonly accepted that a ehemieal reaetion that leads to the creation of a new phase involves two proeesses nneleation and growth. These two processes are actually both identical chemically heterogeneous reactions, as their respective overall balance is the same as the overall reaetion balance. Hence, for a reaction that from solid A gives solid B, the two proeesses have the same chemical equation [14.R1]. These proeesses take plaee in different areas, however, and therefore have different mechanisms. 

Indeed, nucleation is the proeess that allows a small block of solid B to be created on the surface of sohdH (or more rarely within the solid) so that the growth is that of the size of B and requires the presence. 4 and B simultaneously. 

Each process operates as a single reaction with its own kinetic properties rate, fractional extent, surface areas and elementary steps. 

It is clear that these two processes are interdependent, however, since they can be eonsidered as  

Regarding reaction zones, nucleation is a single-area reaction the surface of solid v4 is not yet covered by B whereas growth involves several areas, as discussed in section 14.4.1. 

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