Chemical Concept Demonstrated: Graham's Law of Diffusion
A white ring of NH4Cl forms closer to the HCl end of the tube. If the bottles of the concentrated ammonia and concentrated hydrochloric acid are opened near each other, a white cloud will form above them.
Diffusion of gases results from the kinetic motion of individual molecules. Molecules diffuse by a succession of random collisions with other molecules or the walls of containers. They diffuse from more concentrated regions to more dilute regions. It should be noted that this diffusion from higher concentrations to lower concentrations can be understood as a purely statistical effect.
The heavier HCl molecules have a slower rate of diffusion than the lighter NH3 molecules. The NH3 molecules are able to diffuse farther through the tube than the HCl molecules. The reaction to form NH4Cl, therefore, occurs closer to the swab of HCl than the swab of NH3.
It should be noted that Graham's Law of Diffusion, which deals with the rate at which two gases mix, differs from Graham's Law of Effusion, which deals with the rate at which a gas will escape from a pinhole into a vacuum (see demonstration 4.17).