Question

State your observations when: Ammonium chloride is heated in a hard glass test tube.

Answer 1

Ammonium chloride, when heated, undergoes thermal dissociation, to form ammonia and hydrogen chloride. Ammonia and hydrogen chloride recombine at the cooler upper parts of the test tube, and settle down as a white layer. Since the same ammonium chloride is re-obtained, this change is sometimes referred to as "sublimation". In actual sublimation, there is only a change of state and no decomposition. In the case of ammonium chloride, decomposition does take place, though the products, when cooled, recombine to from ammonium chloride. 
NH4Cl(s) = NH3 (g) +HCl(g)
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