b Diffusion rates of ammonia and hydrogen chloride gases The long glass tube should be horizontal Corks should fit at both ends Using a pair of tongs or tweezers dip a piece of cotton wool into concentrated hydrochloric acid and another piece into concentrated ammonium hydroxide Drain off excess liquid As nearly as possible at the same time put the ammonia cotton wool at one end of the tube and the acid cotton wool at the other Close the ends of the tube with corks After a while look carefully for a white ring which will form where the ammonia gas and the hydrogen chloride gas meet after diffusing through the air towards each other Ammonia is the less dense gas and the white ring of ammonium chloride should form nearer to the hydrogen chloride end than from the ammonia end of the tube