Question

as a solution freezes, the solid that forms is essentially pure, frozen solvent without any solute molecules incorporated. With this in mind, explain why the freezing point of a solution slowly decreases with time rather than stays constant as with the freezing point of a pure solvent.

Answer 1

The freezing point of the solution is always lower than that of the pure solvent , and this is the colligative property of the solution , which is called as the freezing point depression.

The depression is given as : Kf.m

Where Kf is the freezing point constant and m is the molality that depends on the amounts of both the constituents.

Now as the time passes , and the solvent crystallises in the solution , its concentration in the solution decreases and that of solute increases.

So the value of molality increases in the formula , and the depression increases , which causes the freezing point of the solution to lower.