It is similar to decolorising , but infers a high degree of selectivity. The difference between differentiation and decolorising is that decolorising is non-selective and removes almost all of the dye from a section non selectively. Apart from this, the two processes are essentially the same. There are a limited number of ways that differentiation can be accomplished. When we are using a simple solution of a dye to stain the tissue, it is usually feasible to remove excess dye from the tisuue by washing in the solvent .
Solvent differentiation works because of the nature of solvents.
In the case of eosin a polar solvent, water, is used. Polar solvents have an attraction for compounds that can ionise and tends to dissolve them. The charged ions of the dye are attracted to the complementary charge on the appropriate side of the water molecule and are removed from the tissue. Of course, this is far from instantaneous. The dye is bound fairly strongly to the tissue, and this must be overcome before the dye can dissolve in the solvent.