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In the vinegar and Salt solution, is it possible there was some other compound that formed on the egg shell instead of the food coloring? Based on what you know about the charge on the egg shell and the ions in the solution, what might have reacted to form a compound on the shell instead of the food coloring?
Answer: Most food dyes are acid dyes, so called because they only work in acidic conditions. The vinegar—a solution of 5 percent acetic acid in water—is there to bring the pH low enough that the dye will actually bind.