Question

Why are any large clumps of ammonium salts broken down into a powder?

Answer 1

The breaking down of ammonium salt occurs at relatively wider temperature range. Say for example we can talk about here Ammonium chloride, since there is a very real possibility that NH4Cl is more like H3N+HCl where ammonia and hydrogen chloride are held together by hydrogen bonding, a strong form of intermolecular bonding. Hydrogen bonds are weaker than intramolecular bonds and so the “ammonium chloride” easily separates to form ammonia and hydrogen chloride. It is less likely that there is strong “ionic” bonding between NH4^+ and Cl-.

Same like case or more often this occurs with the other ammonium salts.