Q.Give the basic requirement for vapour phase refining.

For vapour-phase refining to work at all, the impure metal must be convertible into a compound that is volatile, that can later be broken back down to the pure metal, and whose formation and decomposition happen at two conveniently different, practically achievable temperatures.

Vapour-phase refining is a method of purifying a metal by converting it into a volatile compound, which is then decomposed elsewhere to deposit the pure metal, leaving non-volatile impurities behind. For this method to be applicable to a given metal, two basic requirements must be met:

1. The metal must form a volatile compound with an available reagent. The impure metal is reacted with a suitable reagent to form a compound that readily vaporises, leaving behind the solid, non-volatile impurities.
2. The volatile compound must be easily decomposable, to give back the pure metal. The compound formed must not be so stable that it cannot be readily broken down again — it should decompose cleanly, at an accessible temperature different from its formation temperature, depositing the pure metal and regenerating the reagent (which can then be recycled). …