Read Colligative Properties as a Particle-Counting Exercise
Every colligative property — boiling point elevation, freezing point depression, vapour pressure lowering, osmotic pressure — depends only on the total number of dissolved particles, never on what those particles actually are.
Chemistry questions first require the material, particles or reaction condition to be identified before a formula or rule is selected.
Comparing boiling points across different salts
When equal masses of different salts are dissolved separately in the same amount of water, the highest boiling point comes from whichever salt produces the most moles of dissolved particles — this means dividing each salt's mass by its molar mass, then multiplying by its van't Hoff factor (ion count on dissociation), not just comparing formula weights.
Isotonic solutions need equal total particle concentration
Two solutions are isotonic when their total molar concentration of dissolved particles matches, accounting for dissociation — a mix of an electrolyte and a non-electrolyte can be isotonic with another such mix only if the sums of (concentration × van't Hoff factor) are equal on both sides.
Vapour pressure transfer between two open solutions
When two solutions of different concentration share a closed vapour space, solvent transfers from the more dilute (higher vapour pressure) solution to the more concentrated one until their vapour pressures — and hence concentrations — equalise; predicting direction just needs comparing initial vapour pressures via Raoult's law.
Reverse osmosis reverses the natural solvent flow
Normal osmosis moves solvent from a dilute to a concentrated solution across a semipermeable membrane; reverse osmosis applies external pressure exceeding the osmotic pressure to force solvent flow the opposite way, from concentrated to dilute — purifying the dilute side.
OlympIQ exam lens
- ✓Colligative properties depend on total particle count (moles × van't Hoff factor), never on particle identity.
- ✓Comparing boiling points across different salts of equal mass means comparing moles of particles produced, not formula weight.
- ✓Two solutions are isotonic when their total particle concentrations (accounting for dissociation) match.
- ✓Reverse osmosis needs applied pressure exceeding the solution's own osmotic pressure, reversing the natural solvent flow direction.
Don't rank colligative effects by molar mass or by 'stronger' salt — always convert to moles of actual dissolved particles (including the van't Hoff factor) before comparing.
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