Boiling - Freezing temperatures --> Ionic or non-ionic?

Question

Why do ionic substances have a greater effect on freezing and boiling temperatures of solvents than do non-ionic substances for a given number of moles?

Answer 1

Elevation of the boiling point and depression of the freezing point are colligative properties. Colligative properties depend on the number of particles in solution.

Non-electrolytes do not dissociate when they dissolve in a solvent. So the number of particles that corresponds to the molecular formula is 1. For example ethanol is C2H5OH. So when 1 mole of C2H5OH dissolves e.g. in water you have 1 mole of particles in solution.

Electrolytes dissociate into ions when they dissolve in polar solvents like water. Thus the number of particles in solution is greater than what you would expect according to the molecular formula. E.g. NaCl dissociates 100% into Na+ and Cl-, so 1mole NaCl gives 1 mole Na+ and 1 mole Cl-, thus in total 2 moles of particles. Thus the effect will be double than what you would expect for 1 mole of a non-electrolyte.
CaCl2 dissociates 100% into 1 Ca+2 and 2 Cl-, so in total 3 particles are in the molecular formula and the effect will be 3 times greater,etc.

For weak electrolytes that don't dissociate 100% you have to consider the degree of dissociation; the total number of particles, which will determine the effect on the boiling/freezing point is equal to the particles that didn't dissociate plus the particles that are produced by the dissociation.