At what point should you stop adding the second solvent during recrystallization?

The appropriate moment to stop adding the second solvent during recrystallization is when the solution becomes cloudy. This cloudiness indicates that the solution is becoming saturated with solute. At this stage, the solvent can no longer dissolve any more of the solid, which is essential for the recrystallization process.

When a solution is saturated, the maximum amount of solute is dissolved at a given temperature, and any excess solute will crystallize out upon cooling. This is the desired outcome, as it allows for the generation of pure crystals as impurities will remain in the mother liquor.

If you were to continue adding solvent until the solid dissolves completely (as suggested by the other option), you might end up diluting the solution too much, which could hinder the crystallization process. Likewise, if you were to stop adding solvent at any other point without observing cloudiness, you might not achieve the concentration gradient needed for effective recrystallization. Thus, recognizing when cloudiness appears is a critical indicator for optimizing purity and yield of the crystalline product.