What are Colligative Properties?
Colligative properties are physical properties that depend only on the number of dissolved solute particles, not on the identity or mass of those particles. They arise because solute particles disrupt the structure and motion of solvent molecules. The four main colligative properties are freezing point depression, boiling point elevation, vapor pressure lowering, and osmotic pressure.
Colligative properties depend only on the concentration of solute particles (typically expressed in molality), not on what the particles are. Examples include freezing point depression (ΔTf = Kf × m) and boiling point elevation (ΔTb = Kb × m).
- •ΔTf = Kf × m
- •Pure solvent freezes at lower temp when solute added
- •Example: saltwater freezes below 0°C
- •ΔTb = Kb × m
- •Pure solvent boils at higher temp when solute added
- •Example: saltwater boils above 100°C
Step-by-step worked examples
A 0.1 m NaCl solution has Kf(water) = 1.86 K·kg/mol. Calculate freezing point depression.
NaCl → 2 ions, so i = 2 (van 't Hoff factor) ΔTf = i × Kf × m = 2 × 1.86 × 0.1 = 0.372 K Freezing point = 0°C − 0.372°C ≈ −0.37°C
Calculate boiling point elevation for 0.5 m glucose (non-electrolyte). Kb(water) = 0.51 K·kg/mol.
Glucose doesn't ionize, so i = 1 ΔTb = i × Kb × m = 1 × 0.51 × 0.5 = 0.255 K Boiling point = 100°C + 0.255°C ≈ 100.26°C
Why does salt melt ice even though it lowers the freezing point?
Salt dissolves on ice surface → freezing point lowers Ice (solid) is now above its new freezing point → melts This effect is used for de-icing roads in winter.
Flashcards
Quick quiz
Q1.Colligative properties depend on…
Q2.Why does salt lower the freezing point of water?
Q3.Osmotic pressure formula?
Q4.0.1 m NaCl vs 0.1 m glucose: which has greater freezing point depression?
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Common mistakes
Ignoring the van 't Hoff factor for ionic compounds. — Correct: Electrolytes ionize: NaCl (i ≈ 2), CaCl₂ (i ≈ 3). Account for this in colligative property calculations.
Confusing boiling point elevation with a higher temperature. — Correct: The solution boils at a HIGHER temperature than pure solvent—a shift of the boiling point.
Thinking all solutes have the same effect at the same molality. — Correct: Ionic compounds affect colligative properties more than non-electrolytes at the same molality.
Assuming salt only lowers temperature. — Correct: Salt lowers the freezing point AND raises the boiling point.
FAQ
What are colligative properties?
Properties that depend only on the number of solute particles (molality), not the type of particles. Examples: freezing point depression, boiling point elevation, osmotic pressure, vapor pressure lowering.
Why is it called 'colligative'?
From Latin 'colligatus' (bound together). These properties result from the collective effect of all solute particles.
Do all solutions show colligative effects?
Yes, but ideal solutions show them best. Real solutions deviate slightly due to solute–solvent interactions.
What is the van 't Hoff factor?
A multiplier (i) that accounts for how many particles a solute produces when dissolving: i = 1 for glucose, i ≈ 2 for NaCl, i ≈ 3 for CaCl₂.