Explain why the freezing point of a pure solvent is constant, whereas the freezing point of a solution decreases with steady cooling.

Answer 1

As the solution is cooled, at some point lower than the normal freezing point of the solvent, a few crystals of pure solvent freeze. That makes the remaining solution more concd in the solute which lowers the freezing point more and the solution must be cooled more to freeze the solution (because it is more concentrated). This process continues. The result is that the freezing point is not sharp (0.5-1 degree C) but a range (2-10 C). The bottom line is that the solvent in the solution is freezing in stages over a range of temperatures.