Question

a) Why does the temperature of pure cyclohexane remain constant during freezing?

b) Why does the temperature of the solution during freezing not remain constant after the addition of solute to cyclohexane?

Answer 1

A pure substance has physical and chemical properties which remain constant throughout the sample. At the Freezing point, the vapour pressure of the solid and liquid forms are equal. Freezing point is the temperature at which the solid phase and the liquid phase coexist in equilibrium with each other. When a substance starts freezing, the entropy of molecules start decreasing and the molecules rearrange to form a solid. If solute(impurity) is added it affects the reordering process and thus changes the temperature of freezing. Since pure cyclohexane has no impurity, it will have a constant freezing point , temperatures remain constant during freezing.

B) When a non volatile solute is added to a pure solvent, freezing point of such a mixture decreases. This is a colligative property and called freezing point depression. The freezing point in this case is lower than that of pure solvent and is proportional to the molality of the solute added. Adding solute to pure solvent, according to Raoult's law, causes a decrease in vapour pressure. Since the vapour pressure of solid and liquid phase must be same freezing point, to maintain the equilibrium, the lowering of vapour pressure causes the lowering of the Temperature at which freezing occurs. The depression of freezing point when solute is added to cyclohexane is not constant and is proportional to the molality of the solute added.