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      Freezing Point Depression

Addition of solute molecules also depresses the freezing point of a liquid.The freezing point is the temperature at which freezing and thawing are in equilibrium. If solute ions or molecules are added until only 90% of the particles in the liquid are the original solvent molecules, then only 90% of the collisions of solvent particles with a crystal have a chance of adhering to the solid.

Hence the temperature must be lowered, to decrease the tendency for molecules to break loose from the solid and escape into the solution, before freezing and thawing again are in balance.

For dilute solutions, the lowering of freezing point is proportional to the molality of the solute:

D Tf = -kfmA

Example. The molal freezing point depression constant for water is kf = 1.86. What is the freezing point of a solution of 0.10 mole of glucose in 1000 g of water?


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