Question

Why is a cotton plug inserted at the mouth of a flask during the process of sublimation of ammonium chloride and salt?

Answer 1

During sublimation process, the solid material is directly converted into gas. The gases are condensed and crystals of the salts are collected. So cotton plug is inserted at the mouth of the funnel so that the gases do not get escaped.
NH₄Cl_(s) + NaCl_(s) --------> NH₄Cl_(g) + NaCl_(s)