I give this activity an A. We didn't have Alka Seltzer, so we used denture cleaner. (Don't ask why I have denture cleaner. Okay, it's for my nite-time mouth guard.) We used green dye because the denture cleaner was green, and I figured it would influence the color of the water. This activity uses a lot of oil, be forewarned.
Complete instructions in the link above.
Part two! This time we used Alka Selzter. We made three, and only two worked. Huh.