Ammonium chloride formation. Cotton wool balls have been soaked in hydrochloric acid (HCl, left) and ammonia (NH3, right). They have been placed at either end of the glass tube. The vapours from the acid and alkaline ammonia diffuse along the tube until they meet and react. The reaction forms the white fumes of ammonium chloride (NH4Cl). The ammonium chloride is not formed at the centre because ammonia diffuses along the tube at a greater rate than the hydrochloric acid.