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
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?