Thomas Graham discovered the law that bears his name while studying gas diffusion into air and other gases. He also found that the same relationship held with gas effusion, the movement of gases through a pinhole into a vacuum. Modern understanding of diffusion and effusion is based on kinetic-molecular theory, and it is generally accepted that Graham's law as originally stated holds for effusion. Diffusion is more complicated, involving intermolecular collisions and composition gradients. This article describes a simple laboratory activity in which diffusion of gases into air though an open syringe is studied over time. The activity could be adapted and modified into an inquiry-based independent study activity or undergraduate research project.
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