Question

What causes boiling point elevation and freezing point depression in solutions?

Answer 1

Simply, by interactions between solvent and solute.....i.e. so-called `"colligative properties..."`

Explanation:

The dissolution of a solute in a solvent relies on the interaction between solute particles, and solvent molecules....and usually this is an attractive force. It is reasonable that such interactions should ELEVATE the boiling point of a solution by an amount proportional to the concentration of the dissolved solute... And so too for melting point... the solvent-solvent forces of interaction are SOMEWHAT diminished.

Anyway, you are going to have to read your text, or at least come again here for clarification...