4.  Diffusion from two drops of conc. solutions: Molecules of ammonia and
     of  "hydrochloric acid gas" travel at different speeds  

* Photo: Place one drop of conc. ammonia (NH₃)  solution on a film canister and (at an distance of 1 cm)  one drop of  conc. hydrochloric acid (HCl).
* Cover the canister by a beaker to prevent it from air motion.
* Observations:  Each of the two liquids has a characteristic sharp smell.
* After a few minutes white crystals have collected around the acid drop.
* Explanations: Molecules of ammonia and of "hydrochloric acid gas" escape from the two drops.
* The ammonia molecules are lighter. So they diffuse much quicker than HCl molecules.
* NH₃ molecules and HCl molecules collide close to the acid drop.
* Their chemical reaction results in white crystals of ammonium chloride: NH₃Cl (g) + HCl(g) -------> NH₄Cl (s).