Question

what type of chemical reaction takes place when ammonia and hydrogen chloride gases are mixed?

Answer 1

They neutralise each other, if the quantities are right you end up with a neutral ionic solution of ammonium chloride. If one or the other is in excess then you will have a solution of ammonium chloride with excess hydrochloric acid or excess ammonia.

NH3+HCl=NH+4Cl−NH3+HCl=NH4+Cl−

The fumes from ammonia solution and hydrochloric acid  also react in the gas phase to produce a white cloud of solid ammonium chloride.

Answer 2

When it comes to ammonia, it is a base because ammonia includes an extensive pair of electron atoms on nitrogen atom that is donated to the acid which is hydrogen chloride.

If you mix Ammonia as well as Hydrogen chloride, you will get ammonium chloride. It is given by NH3 (g) + HCl (g) → NH4Cl(s).