When hydrogen chloride gas (HCl) reacts with ammonia gas (NH₃), the product formed is ammonium chloride (NH₄Cl), a white solid.

The reaction is as follows:

HCl(g) + NH₃(g) → NH₄Cl(s)

This reaction is highly exothermic, meaning it releases heat, and it produces a visible white smoke of ammonium chloride.