I apologize for the delay in getting the next lesson to you. I have been trying to improve the Learnalong, while trying to answer new questions brought up by new information.
Thanks to the results you sent in, we are getting closer to determining which chemical(s) in the coloring are causing the motion.
Several students in other countries have found that their coloring does not move around when a dried piece is placed in water. Their coloring uses the same dyes as the colorings I tried (McCormick and Western Family). Since both contain the same dyes, I think we can eliminate the dyes from the list of suspects for the cause of the motion.
The McCormick brand coloring that I am using contains chemicals that the coloring from other countries did not list:
propylene glycol
propylparaben (listed as a preservative)
The Western Family coloring uses the same dyes, but also contains:
glycerine
citric acid
sodium benzoate (listed as a preservative)
Other students tried coloring without propylene glycol or glycerine, but that did contain sodium benzoate. They did not get the motion either, so that would seem to remove it from the list of possible causes.
Both propylene glycol and glycerine are added as humectants to keep the coloring from drying. That interaction with water molecules is my main suspect for causing the motion.
As many of you mentioned, the movement could be the result of something (propylene glycol or glycerine?) changing the surface tension of the water. For an example of motion caused by changes in surface tension, take a look at this video:
Compare what happens with the soap and pepper to what happens with the dried food coloring. Can you think of any ways to test the idea that surface tension is causing the food coloring to move?
Can you think of ways to narrow the list of suspected chemicals causing the movement?
You can email me your thoughts, results, and questions at: happy_scientist@thehappyscientist.com