What type of solute primarily contributes to freezing point depression and boiling point elevation?

The correct answer highlights the role of ionic solutes in the phenomena of freezing point depression and boiling point elevation. When an ionic solute, such as sodium chloride (NaCl), is dissolved in a solvent like water, it dissociates into its constituent ions. This dissociation increases the number of solute particles in the solution, which significantly influences colligative properties.

Colligative properties are determined by the number of solute particles in solution rather than their identity. The more particles present, the greater the effect on the freezing and boiling points of the solvent. In the case of ionic solutes, each dissolved unit contributes more than one particle. For instance, one formula unit of NaCl yields two ions (Na^+ and Cl^-), effectively doubling the number of solute particles compared to a molecular solute of the same molar concentration, which does not dissociate.

As a result of this greater number of particles, ionic solutes cause a more substantial depression in the freezing point and an elevation in the boiling point of the solvent, compared to other types of solutes. Thus, ionic solutes are particularly impactful in altering these physical properties of solutions, making them the principal contributors to freezing point depression and boiling point elevation.