How does freezing point depression occur?
The particles in the solution get in between the water molecules preventing them from forming the hexagon hydrogen bonds needed to form ice.
Pure water will freeze at 0 degrees C.
As the temperature cools the average movement of the water molecules slows down. This allows water molecules to form hydrogen bonds with neighboring water molecules. When the temperature reaches 0 degrees C. enough hydrogen bonds form to create the hexagonal structure of ice.
When there are other atoms, molecules, or ions in next to the water molecules the temperature needed to form the hexagonal crystal drops. ( needs to get colder, less movement of the molecules). The other particles get in between the water molecules. Remember these solute particles are moving as well as the water molecules. When the temperature drops these molecules also slow down allowing the water molecules to hydrogen bond to each other.
The freezing point depression is - 1.86 degrees C for every mole of solute particles in a Kg of water.