Question

Recrystallization and Distillation Chemistry and Context

1. A sample of naphthalene, a common component of coal tar and bitumen, is pure white. However, when a Middlebury College student recrystallizes it, the bulk material has a gray cast. The melting point matches the literature value and the melting point range is small. Explain the cause of the gray cast of the recrystallized naphthalene.

2. What is the difference between a recrystallization and a precipitation?

3. Why is the product allowed to dry for one day before it is analyzed?

Answer 1

1. The cause of the gray cast of the recrystallized napthalene is the loss of compound during recrystallization due to inherent solubility as non-polar interaction takes place between naphthalene and ethanol (solvent used in recrystallization process) therefore, naphthalene is partially soluble in ethanol which usually increases the solubility with increase in temperature and thus make naphthalene gray and reduces its melting range.

2. Recrystallization is the purification technique which used to purify solid organic compound by removing impurities and dissolving in appropriate solvent and formed pure crystalline solid on the other hand precipitation is the technique of rapid formation of solid from an oversaturated solution and product formed will be crystalline or amorphous solid.

3. The product is allowed to dry for one day before it is analyzed in order to remove solvent from the solid organic compound so that pure crystalline product left for analyzed.