Why does tap water freeze more rapidly than cold water?

All I know is that tap water contains heavy metals that can only be separated by distiling it. I don't know distill tap water and try it then

Eh, by "tap water" do you mean cold tap water, or hot tap water?
And by "cold water", do you mean "tap water" or well-, river-, lake- or sea-water?
Anyway, here it goes:
It is common believe that hot water freezes more quick than cold water. This is not true. Hot water takes longer to freeze, because cooling it down takes additional time.
What is true, is that water, that was first heated and left to cool down, will freeze more quick than fresh cold water because heating it will have chased the solved gases (mainly oxygen and carbondioxyde) out. The solved gases in fresh water are expelled by freezing too, but this produces heat that will delay freezing. This is however much less important than the time required to cool down hot water.

I´m sorry! I meant hot water, not tap water (terrible mistake!! ). Anyway, thanks for both answers, I ended up having a "two in one" answer.