What happens to the freezing point of a solution when a non-volatile solute is added?

When a non-volatile solute is added to a solvent, the freezing point of the resulting solution decreases. This phenomenon is a result of colligative properties, which are properties that depend on the number of solute particles in a solution rather than the nature of the solute itself.

In pure solvents, molecules are able to form a solid structure at the freezing point. However, when a non-volatile solute is introduced, it disrupts the ability of solvent molecules to arrange themselves into this solid structure. This disruption occurs because the solute particles occupy space and interfere with the intermolecular forces that hold the solvent molecules together in a solid state.

The decrease in freezing point is known as freezing point depression. The extent of this decrease can be calculated using the formula ΔTf = i * Kf * m, where ΔTf is the change in freezing point, i is the van't Hoff factor (which indicates how many particles the solute breaks into), Kf is the cryoscopic constant of the solvent, and m is the molality of the solution.

Thus, the addition of a non-volatile solute lowers the freezing point, making the correct answer a decrease in freezing point.