Purpose:Demonstrate on a macroscopic scale using ping pong balls how random molecular motion causes substances to diffuse.
This model consists of a wooden frame with clear plastic covers, about one ping pong ball in width, ten bins at the top and bottom for setting up initial and analyzing final distributions, with several rows of pegs in between. When the horizontal plastic baffle holding the balls at the top is pulled away, balls will drop through the peg array, become randomly scattered, and drop into bins at the bottom.
• Put four orange balls into bin 5 and four white balls into each of bins 3, 4, 6, and 7. Challenge students to predict whether they will keep this same arrangement as they fall. (When the balls reach the bottom, the four orange balls will have become distributed into the white balls.)
This shows on a larger scale how random molecular motion causes substances to diffuse. The array of fallen balls will approximate a probability curve; this is an opportunity to introduce statistical concepts in a physical, measurable manner.