Question

Explain why the cooling curve for a pure solvent reaches a plateau at its freezing point whereas the cooling curve for the solution does not reach a plateau but continues to decrease slowly as the solvent freezes out of the solution.

Answer 1

Cooling of a pure solvent is approximately an isothermal process which means that the temperature doesn't change by much until the whole solvent has frozen.

But for a solution, it is not the case. Addition of solute to the solvent causes a depression of the freezing point. Higher the concentration, lower is the freezing point. When the solvent in a solution starts to freeze, the solvent also starts getting separated out of the solution. This will mean that in the remaining solution, the concentration of solute starts increasing. As we know that higher the concentration of solute, lower is the freezing point. Thus, this will cause a further depression in freezing point and you would need to lower the temperature further to cause additional freezing. Thus, the cooling curve continues to decrease slowly as the solvent freezes out of the solution.