How are soluble impurities removed during a recrystallization?
The correct answer and explanation is:
Correct Answer:
Soluble impurities are removed during recrystallization by selective crystallization of the pure compound while the impurities remain dissolved in the solvent and are separated when the solution is filtered.
300-word Explanation:
Recrystallization is a widely used technique in chemistry for purifying solid compounds. The fundamental idea is based on the difference in solubility between the desired compound and the impurities. During recrystallization, a solid is first dissolved in a suitable hot solvent to form a saturated solution. The goal is to find a solvent in which the compound of interest is highly soluble at high temperatures but sparingly soluble at low temperatures, while the impurities either stay dissolved at all temperatures or do not dissolve at all.
When the hot solution cools slowly, the solubility of the pure compound decreases, and it begins to form crystals. Crystals form an ordered lattice structure that tends to exclude foreign particles, so the pure compound separates from the solution. Soluble impurities stay dissolved in the solvent because they do not fit into the crystal lattice of the compound being purified or because their concentration remains too low to crystallize.
After crystals form, the mixture is typically filtered using vacuum filtration. The pure crystals are trapped on the filter paper while the remaining solvent containing the soluble impurities, called the mother liquor, passes through and is discarded. Washing the crystals with a small amount of cold solvent helps remove any residual soluble impurities clinging to the surface of the crystals.
The success of recrystallization depends on careful selection of the solvent, controlled cooling, and proper filtration techniques. If done correctly, recrystallization can produce a highly pure sample even if the original material contains significant amounts of soluble impurities. The process is common in both laboratory and industrial settings to purify chemicals, pharmaceuticals, and natural products.