Question

Naphthalene is a volatile solid. It is best purified by-
(A) Crystallisation
(B) Distillation
(C) steam distillation
(D) sublimation

Answer 1

Hint : A volatile solid is one that evaporates rapidly; passing off readily in the form of vapour. There is no liquid phase in between which would allow precipitation.

Complete step by step answer
Naphthalene is an organic compound with the simplest polycyclic aromatic hydrocarbon structure, and is a white crystalline solid with a characteristic odour that is detectable at concentrations as low as $ 0.08ppm $ by mass.
Sublimation is the transition of a substance directly from the solid to the gas state, without passing through the liquid state. It is an endothermic process that occurs at temperatures and pressures below a substance's triple point in its phase diagram, which also corresponds to the lowest pressure at which the substance can exist as a liquid.
Sublimation has also been used as a generic term to describe a solid-to-gas transition (sublimation) followed by a gas-to-solid transition (deposition). While vaporization from liquid to gas occurs as evaporation from the surface if it occurs below the boiling point of the liquid, and as boiling with formation of bubbles in the interior of the liquid if it occurs at the boiling point, there is no such distinction for the solid-to-gas transition which always occurs as sublimation from the surface.
The reverse process of sublimation is deposition or de-sublimation, in which a substance passes directly from a gas to a solid phase.
Naphthalene sublimes easily because it is made of non-polar molecules, that are held together only by van der Waals intermolecular forces. Naphthalene is a solid that sublimes at standard atmospheric temperature with the sublimation point at around $ {80^ \circ }C $ or $ {176^ \circ }F $ . At low temperature, its vapour pressure is high enough, $ 1mmHg $ at $ {53^ \circ }C $ , to make the solid form of naphthalene evaporate into gas. On cool surfaces, the naphthalene vapours will solidify to form needle-like crystals.
Hence the correct answer is option D.

Note
As an aromatic hydrocarbon, naphthalene's structure consists of a fused pair of benzene rings. It is best known as the main ingredient of traditional mothballs.