Project Overview

Testing the diffusion demonstration

Faculty Sponsor

Beth Parks (


Physics and Astronomy


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?  

Like many physics questions, this one turns out to be much more complicated than it would seem at first glance.  For example, the process of forming the ammonium chloride precipitate is not straightforward.  And the models tell us that the timing may be very dependent on the exact temperature and concentration of the ammonia solution.  Fortunately, there's lots of room for clever experimental ideas and careful measurements.  Even as an undergraduate, you can make a big contribution not only to the day-to-day measurements, but to the experimental design and interpretation.  So this is a great project for someone who likes to think about how simple tools can be used creatively to solve complex problems.

Student Qualifications

This project is open to all physics majors.  Some knowledge of chemistry is also helpful.

Number of Student Researchers

1 student

Project Length

8 weeks

Applications open on 01/03/2021 and close on 03/22/2021

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If you have questions, please contact Karyn Belanger (