During the recrystallization how do you remove soluble, insoluble and colored impurities from your unknown?
The correct answer and explanation is:
Correct Answer:
During recrystallization, soluble impurities remain in the hot solvent and are removed by filtration if necessary, insoluble impurities are removed by hot filtration through filter paper or a fluted filter, and colored impurities are often removed by adding a small amount of activated charcoal before hot filtration.
Detailed Explanation (300 words):
Recrystallization is a common laboratory technique used to purify solid compounds. It works on the principle that different substances have different solubilities in a given solvent at various temperatures. The goal is to dissolve the impure solid in a minimum amount of hot solvent and then allow the desired compound to crystallize out as the solution cools, leaving impurities behind.
Soluble impurities are substances that stay dissolved in the solvent even when it cools down. This is because they either do not saturate the solution enough to crystallize or have higher solubility than the target compound. When the solution is cooled, the pure compound crystallizes while soluble impurities stay in the mother liquor. After filtration, these impurities are discarded with the filtrate.
Insoluble impurities include dust, sand, or other solid debris that do not dissolve in the hot solvent. They are removed by gravity or vacuum filtration while the solution is still hot. This step is crucial because if the solution cools during filtration, premature crystallization may occur in the filter funnel, trapping some of the product. Using a stemless funnel with fluted filter paper helps maintain a steady flow and prevents early cooling.
Colored impurities are often organic contaminants that impart a tint to the solution. A small amount of activated charcoal can be added to the hot solution to adsorb these colored substances. The charcoal, along with the bound impurities, is then removed during hot filtration. This leaves a colorless or less-colored solution, which can then cool and yield pure crystals. Each step ensures the final crystals are as pure as possible by targeting different types of impurities.