Why do colligative properties depend on the number of particles?
Colligative properties are properties of solutions that depend upon the ratio of the number of solute particles to the number of solvent molecules in a solution, and not on the type of chemical species present.
Colligative properties include:
1. Relative lowering of vapor pressure.
2. Elevation of boiling point.
3. Depression of freezing point.
For example, the freezing point of salt water is lower than that of pure water (0 °C) due to the presence of the salt dissolved in the water. It does not matter whether the salt dissolved in water is sodium chloride or potassium nitrate.
If the molar amounts of solute are the same and the number of ions are the same, the freezing points will be the same!
Colligative properties are mostly studied for dilute solutions, whose behavior may often be approximated as that of an ideal solution.