why does the temperature continually decrease as a solution freezes instead of maintaing a constant temp. as a

Question

pure liquid would do?

Answer 1

Pure liquids and solutions both cool (lose heat) as long as they are above the ambient temperature. They stop cooling only when they get to room temperature, or get to the freezing point. This, of course, is slightly lower for solutions than for the pure liquid solvent.

Answer 2

The freezing point depression is due to the solute. As the solution freezes, pure water is removed from the solution in the form of ice crystals. The solute in the remaining liquid water is now more concentrated yielding a greater freezing point depression.