Ammonia can be dried by using

Ammonia can be dried by using CaO because ammonia does not react with CaO.

${\mathrm{CaCl}}_2$ reacts with ammonia and forms an adduct.

${\mathrm{CaCl}}_2+{\mathrm{NH}}_3\to {\mathrm{CaCl}}_2·8{\mathrm{NH}}_3$

${\mathrm{H}}_2{\mathrm{SO}}_4$ and ${\mathrm{P}}_4{\mathrm{O}}_{10}$ forms acid-base reaction with ammonia.

$\begin{array}{ccc}2{\mathrm{NH}}_3+{\mathrm{H}}_2{\mathrm{SO}}_4& \to & {\left({\mathrm{NH}}_4\right)}_2{\mathrm{SO}}_4\\ {\mathrm{P}}_4{\mathrm{O}}_{10}+12{\mathrm{NH}}_3+6{\mathrm{H}}_2\mathrm{O}& \to & 4\left[{\left({\mathrm{NH}}_4\right)}_3{\mathrm{PO}}_4\right]\end{array}$