Colligative Property
Definition
Solution properties that depend only on HOW MANY solute particles are present, not what they are: like freezing point dropping when you salt icy roads.
Technical definition
Colligative properties (vapor-pressure lowering, boiling-point elevation, freezing-point depression, osmotic pressure) depend only on the number of dissolved particles per amount of solvent, not their identity. Ionic solutes contribute multiple particles per formula unit, counted by the van't Hoff factor.
Example
Spreading NaCl on icy roads lowers water's freezing point. Each NaCl gives 2 particles, doubling the effect per mole versus sugar.
Common confusion
Colligative means collective: the identity does not matter, but the particle COUNT does. Forgetting that NaCl splits into 2 ions is the classic exam error.