Question

Impure compounds tend to have lower melting points than pure samples. Explain why this occurs, using biphenyl as an example. Make sure you consider what happens upon melting in molecular terms. (b) Is recrystallization a suitable method for purifying all types of organic compounds? If not, why not?

Answer 1

Explanation:

(a)

Substances with impurity melts at lower temperature and this phenomenon is called depression in melting point.  

Pure solid organic compounds or inorganic compounds tend to have highly packed structure. When a substance melts, these arrangements are disturbed. Impurities disrupt the regular arrangement of the crystals. Therefore, If impurities are present within the crystals, less energy are required to disturb the crystal, therefore, melts at lower temperature.

(b)

Recrystallization is not a suitable method for the purification of all organic compounds.

In recrystallization, organic compounds are dissolved in a suitable solvent at high temperature. After that solution is cooled slowly. During cooling process, the crystals of organic compound separate from the solution and impurities are left in the solution.

So, finding a suitable solvent for a solid organic compound is difficult. Moreover, liquid organic compounds cannot be purified by this method.