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The freezing point of water is depressed when a non-volatile solute is added because:
AIce cannot form in the presence of any solute
BThe density of the solution is higher than that of pure solvent
CThe vapour pressure of the solvent in the solution is lower than that of pure solvent, so it equals the vapour pressure of the solid only at a lower temperature
DThe solute lowers the temperature of the solution by absorbing heat
Answer & Solution
Correct answer: C. The vapour pressure of the solvent in the solution is lower than that of pure solvent, so it equals the vapour pressure of the solid only at a lower temperature
Freezing point is the temperature at which solid and liquid solvent are in equilibrium, i.e. have equal vapour pressures. A non-volatile solute lowers the vapour pressure of the liquid phase only, so the equilibrium with solid occurs at a lower temperature than for the pure solvent. This is the molecular basis of $\Delta T_f = K_f m$.
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