A solute makes the freezing point of a solution lower than the freezing point of the pure solvent because?

Answer 1

The phenomenon you're describing is generally referred to as freezing-point depression, the lowering of the freeze point of a liquid (or solvent) by adding another compound.

Freezing point depression is a phenomenon driven by entropic changes in the system containing solvent and solute. As the system is frozen, the solvent forms crystals of high purity regardless of solute molecules being present while solvent crystallizes. Replacement of any solute in the crystal with a solvent molecule takes place spontaneously, since the inability of solute molecules to fit well into the ordered crystal makes the solute-solvent substitution thermodynamically favorable.

As the freezing proceeds, solvent molecules continue to leave the liquid state and incorporate into the solid crystal, with each such occurrence leaving behind a smaller volume of liquid in which solute molecules can occupy. The shrinking of liquid volume occupied by a fixed number of solute molecules reduces the dispersion of solute molecules in the liquid, resulting in a reduction of entropy of the solute molecules. Thus, additional energy is required to match the reduced entropy of the solute molecules with that of the solid solvent crystal.

The energy required (versus pure solvent) to gap the entropic difference (thus difference in chemical potential) to establish equilibrium but at a freezing temperature lower than that of the pure substance. Note that at low solute concentrations, freezing point depression is a property that depends solely of the number of solute particles and physical properties of the solute. Such properties are called colligative properties.