In this simple demonstration the relative diffusion rates of helium, air, and sulfur hexafluoride are shown with balloons. The demo requires two lectures to see before and after balloon diameters.

Several rubber balloons (10-12 in diameter)
Tape measure (soft, not metal)
Marking pen
Tank of helium with regulator
Lecture bottle of sulfur hexafluoride with regulator

This demonstration would normally come near the end of a set of lectures on the properties of gases. It integrates with the discussion of the kinetic-molecular view of gas behavior involving Graham’s law. The central issue is the inverse (square root) relation between diffusion rate (rms average speed) and molecule weight. The commonly observed phenomenon of the shrinkage of a newly inflated balloon over time is an example of the diffusion of the trapped gas through the rubber membrane. This happens with air-filled balloons, and even faster with helium-filled balloons. What is usually ignored in a superficial discussion is the fact that diffusion through a porous barrier is a two-way process. The balloon shrinks because air (under pressure) and helium diffuse out faster than air from the surrounding atmosphere can diffuse in. To illustrate the two-way nature of diffusion, we use a gas with a slower rate of diffusion than air. Under these conditions, the balloon actually expands over a period of 1-2 days.

Inflate balloons with air, helium, and sulfur hexafluoride and tie them closed. Invite a member of the class to measure the circumference of each with a tape measure. Use a marking pen to record the size on each balloon. To add drama, have the student mark the time and date and his/her name on each balloon. Set aside the marked balloons until the next lecture.

In the next lecture have the same student (warn him/her not to ditch the next class) remeasure the three balloons. The air and helium filled balloons will be smaller, but the one containing sulfur hexafluoride will have a greater circumference. It has been my experience that such balloons regularly double in volume (not circumference) over a 1-2 day period. Use this result to discuss the effect of different diffusion rates on the volume of trapped gas in each balloon.

Don’t drop the gas cylinders on your foot. Don’t let anyone in the class con you into breathing either helium or sulfur hexafluoride.