JAMB Chemistry 2015 Paper

Concept: Sublimation is the direct change of a solid into a gas on heating and back to a solid on cooling, without passing through the liquid state.Explanation: When ammonium chloride ($\mathrm{NH}_4\mathrm{Cl}$) is heated gently, it decomposes into ammonia gas ($\mathrm{NH}_3$) and hydrogen chloride gas ($\mathrm{HCl}$).These gases move to the cooler upper part of the test tube, where they recombine to form a white cloudy deposit of solid ammonium chloride again.This shows the solid changes directly to the gaseous state and then returns to the solid state, bypassing the liquid state.Distillation involves boiling and condensation of a liquid, precipitation forms a solid from a solution, and evaporation changes a liquid to a gas.Therefore, the process described is sublimation.Answer: B. sublimation.