How are insoluble impurities removed during recrystallization?
The correct answer and explanation is:
Correct Answer:
Insoluble impurities are removed during recrystallization by hot gravity filtration.
Explanation:
Recrystallization is a purification method used to separate a pure solid compound from its impurities. The process relies on the differences in solubility between the desired compound and the impurities in a given solvent. Insoluble impurities are those that do not dissolve in the hot solvent and therefore must be removed before the solution cools down and the purified compound begins to crystallize.
To remove these insoluble impurities, the solution containing the dissolved compound is subjected to hot gravity filtration. In this process, the solution is passed through a filter paper in a funnel while it is still hot. This ensures that only the insoluble particles, such as dust, dirt, or other solid contaminants, are retained on the filter paper. The compound of interest remains in the filtrate, which is the clear solution that passes through the filter. The temperature is kept high during this step to prevent the premature crystallization of the desired compound, which could trap the impurities within the crystals.
A stemless funnel is often used during hot filtration to minimize the cooling of the solution, which might otherwise cause the compound to crystallize within the funnel and clog it. If crystallization occurs too early, the separation process becomes less effective.
After the insoluble impurities are removed, the hot, clear filtrate is allowed to cool slowly. As the temperature decreases, the solubility of the compound also decreases, and the pure substance begins to form crystals. These crystals can then be collected by vacuum filtration.
Thus, hot gravity filtration is essential to ensure that insoluble impurities are not incorporated into the final purified product.