The initial solid reacts with another phase

The initial solid reacts with another phase 

If solid A reacts with another substance present in a third phase (gas, solid, or possibly hquid), the reaction between the reactants or the reaction intermediates resulting from both reactants is necessary for the continuation of the reaction. This thus implies a zone of diffusion, which is the layer of the formed solid B obligatorily placed between the initial phases. 

The intermediate steps imperatively include adsorption of the gaseous reactants and desorption of produced gases, from where a zone of adsorption-desorption on the smface of sohd B is obtained if one gas (or more) is involved. 

If the second reactant is a solid, the diffusion through B can be ensured either by point defects in B or through an external gas phase (see Chapter 14). 

If the second reactant is a gas, then the diffusion through B can be either molecular through pores of B or atomic or ionic via point defects of B (see Chapter 15). 

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