Take two cotton plugs, one soaked in ammonium hydroxide (liquid) and another in hydrochloric acid (liquid). Place them simultaneously at the two ends A and B of a long glass tube. Close the two ends of the tube immediately.
After some time, a white ring of a solid substance is formed near the end B. the molecules of ammonium hydroxide and hydrochloric acid move in opposite directions and meet to react.
Finally ammonium chloride, a white solid is formed in the form of a ring. The formation of the ring near to the end B shows that the molecules of ammonium hydroxide have covered larger distances.
This means that these molecules move faster than the molecules of hydrochloric acid. This is because the molecules of ammonium hydroxide are lighter.