In evaporation the impurities of the solute are separated with it but in case of crystallization only pure crystals are formed and the impurities of solute remains dissolved in it.why?why does it happen so?
First, in evaporation, the solvent is separated into its components, leaving the impurities (solute) behind. Fractional distillation can be used to separate the components of the solvent, though azeotropes may make separation difficult.
Second, in crystallization, those components that fit into a regular structure precipitate out preferentially, since the regular pattern is a lower energy condition. However, "pure crystals" do not always form. For example, various alums co-crystallize without separating, so that a non-stoichiometric mixture may result.