<blockquote>Number Two: Freezing Regular water froze and the salty water didn't, and that is exactly right. You have just discovered the opposite side of the boiling point problem above. When you add an impurity to a substance, you tend to raise its boiling point, and lower its freezing point. Salt is an impurity to the water, and it prevents it from doing a phase change from liquid to solid as easily as when it is pure. Pure water freezes at 32 F or 0 C. Salt water freezes at a lower temperature though, about three or four degrees lower. The reason your salty water never froze in your freezer is your freezer never got below this temperature! It is kept at a temperature just below freezing for pure water. Not quite cold enough for your salty water.</blockquote>
via <a href="http://www.madsci.org/posts/archives/feb2001/983550047.Ch.r.html">Re: How does salt affect water?</a>.