P204: After the classic ammonium chloride demo: Some really interesting stuff is going on in there
The ammonium chloride demo, where HCl and NH3 are used in a sealed tube to demonstrate both molecular speed in the gas phase and interesting non-aqueous acid/base chemistry, has another aspect that seems to have been rarely noted. In the classic demonstration, the two gases diffuse towards each other and form NH4Cl solid, often seen as a fine cloud. The distance of this cloud from the gas sources is related to the speed of the respective species in the gas phase; the solid forms closer to the HCl source, reflecting the greater speed of NH3 diffusion. At this point the typical demonstration is done and the materials cleaned up. This is unfortunate, for some really interesting dynamic behavior can be observed if the tube is left undisturbed. Under the right circumstances, the NH4Cl cloud can be fascinating to watch, especially when captured on video. The conditions that have led to some of the more interesting observations so far are discussed.