|MadSci Network: Chemistry|
Assuming that all other variables were kept constant (that is, concentration of sugar, the amount of liquid exposed at the surface to air were the same between the two groups), I can suggest that the candy grew faster due to the presence of multiple nucleation sites in tap water. In other words, tap water is quite dirty and these little particles of insoluble dirt served as site where the sugar crystals could start growing. I sincerely doubt that chemical impurities (namely salts and other harmless chemicals present in tap water) make a big difference, unless you have boiled your solution for a long time which would reduce the volume of the water and thus concentrate the impurities several fold.
Hope this helps. Please do not hesitate to write me with specific questions.
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