If the process is conducted carefully enough, the recrystallized solid is morepure than it was prior to recrystallization. This is true even if a particular impurity has the samesolubility in the solvent as the desired solid provided that the impurity is present in a smaller concentration than the desired solid. As the solution cools, it is more likely to start to crystallize than the impurity. The purest crystals result when they are allowed to grow relatively slowly. However, if water is added to an ethanol solution of naphthalene, the solubility of the naphthalene is lowered enough so that much more naphthalene can be recoveredwhen the solution is cooled than would be recovered if no water were added.
Many organic precipitates are quite bulky and filter very slowly by gravity.
Therefore, suction filtration is usually preferred over gravity filtration in organic chemistry. Quite often, colored or odoriferous contaminants are produced during an organic chemical reaction. Removal of these can often be aided by adding a small amount of activated charcoal to the hot solution. The charcoal and other insoluble impurities are then removed by filtration while the solution is still hot. Erlenmeyer flaskand reweigh.