Why does a solute depress the freezing point of a solution?

A solute depresses the freezing point of a solution because the solute can not fit perfectly into the crystal lattice of the solid solvent. The normal crystal lattice is the lowest energy arrangement of the molecules or ions of the solid solvent. Therefore, the disordered lattice of a solvent freezing in the presence of a solute has at least slightly higher energy than the lattice of a solid pure solvent and requires at least a slightly lower temperature to solidify.