|MadSci Network: Earth Sciences|
Thanks for the question. I think that you can find part of your answer here
in this article in our Mad Scientist Archives.
To sum up, if water has other substances dissolved in it, those substances get in the way of the crystal lattice of the ice as the water freezes. Some extra effort, that is a lower temperature, must be applied to get the extraneous substances out of the way of the water molecules so that the water can freeze. Since tap water has more of these dissolved substances than distilled water does, it is harder to freeze the tap water.
Another effect that occurs which slows up the freezing process is the
concentration of the dissolved substances as the tap water freezes. As water in
a closed container freezes, the dissolved substances try to remain in solution
in the liquid part of the water. Since the liquid part gets smaller as freezing
continues, the concentration of the dissolved substances increases. This
increasing concentration makes it harder to freeze the very last bit of liquid
water in the container.
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