`Mg_3 N_2` on reaction with water gives off `NH_3`, but `MgCl_2` on reaction with water does not give `HCl` at room temperature.

Explanation

`Mg_3 N_2` is a salt of strong base, `Mg(OH)_2 `and a weak acid, `NH_3`, so it gets hydroysis and gives off `NH_3`.
`Mg_3 N_2 6 H_2 O rarr 3 Mg (OH)_2 + 2 NH_3`
On the other hand, `MgCl_2` is a salt of strong base, `Mg(OH)_2` and strong acid `HCl`, hence it does not undergo hydrolysis and hence `HCl` is not obtained.