|MadSci Network: Chemistry|
Easter egg dye is a form of food coloring. Like most dyes, the chemical that does the dyeing is a molecule with two parts. One part gives the dye its particular color - red, violet, green, or any other color. The other part is the part that bonds to whatever the substrate is, in this case, the eggshell. Many dyes suggest adding vinegar because making the dye solution acidic helps the bonding part work better. So what happens when you dye an egg is that some of the dye in the solution attaches itself to the eggshell. This is why a stronger dye solution, or longer time in the dye bath, gives you a deeper color - with either a higher concentration of dye molecules in solution, or more time for the reaction to happen, you get more dye bonded to the egg.
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