It’s hard to make a good demo for diffusion. You calculated in Physics 131 that it should take molecules on average about four hours to diffuse a distance of one meter, and yet when perfume is spilled, you smell it across the room in just a few seconds. That’s because usually gases don’t just diffuse; they also advect, which is bulk movement of the gas, like wind. (You may have heard of convection, which is a special case of advection, driven by temperature differences.) Advection is much faster than diffusion, and it’s hard to eliminate.
There’s a demonstration that has been used for years to show diffusion, in which an ammonia-soaked cotton ball is placed at one end of a glass tube and hydrochloric acid at the other. The vapors meet in less than a minute and form a white ring of ammonium chloride in the tube. Since it happens so quickly, can those gases really be diffusing, or are they carried by advection, like the perfume? Surprisingly, this question is unanswered. You’ll work to answer it by quantifying the amount of gas that reaches the other end of the tube, as a function of time, and comparing it to diffusion.
This project is open to all physics majors. Some knowledge of chemistry is also helpful.
Number of Student Researchers
Applications open on 01/03/2020 and close on 03/11/2020