Although diffusion is the subject of beginning chemistry classes, there are a variety of experiments, such as the spread of perfume in the classroom, that are often "incorrectly" explained by diffusion alone. To eliminate this common misconception, a recently published method is modified to determine the diffusion constant of hydrogen chloride in air with a simple experimental setup. The differences between the particularly slow diffusion progress and the fast particle motion are discussed. The method applied and the equations derived therefrom are then used to re-evaluate the well-known experiment of the counterdiffusion of the gases HCl and NH[subscript 3].