Ammonium chloride, a sublimable solid undergoes__________ (synthesis/dissociation) on heating.

Hint: A change occurs in which the products are formed after heating. The change is reversible in nature. This means the products formed, on cooling return back to the starting material. The reaction is generally endothermic, meaning it absorbs energy in the form of heat during the change.

Complete step by step answer:
Ammonium chloride, a sublimable solid undergoes thermal dissociation on heating.
A substance that undergoes thermal dissociation forms several products.
When white solid ammonium chloride is heated strongly above 340\[^{\circ }C\], it thermally dissociates into a mixture of two colourless gases. Those are ammonia and hydrogen chloride. After cooling, the reaction is reversed and solid ammonium chloride forms again.
\[N{{H}_{4}}Cl\,(s)\rightleftharpoons N{{H}_{3}}(g)+HCl(g)\text{ }\]
 It is an example of sublimation. This change consists of both physical and chemical changes.
The forward reaction is the thermal decomposition of ammonium chloride into ammonia and hydrogen chloride and the backward reaction is the formation of solid ammonium chloride from its products.
In the same way, ammonium chloride, ammonium bromide and ammonium iodide sublime with both physical and chemical changes like that of ammonium chloride.
So, the correct option is dissociation.

Note: When the reaction conditions have reversed the direction of chemical change also gets reversed. Dissociation means heat is used to break down molecules into other molecules or ions. The decomposition of ammonium chloride from ammonia and hydrogen is endothermic and the formation of ammonium chloride from ammonia and hydrogen chloride is exothermic.
Exothermic means release of heat in the form of energy during a reaction.