Colligative properties
Colligative properties depend on the number of dissolved particles, not on what they are. Adding a solute lowers the vapour pressure and freezing point and raises the boiling point. ΔTf = i·Kf·m.
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Analogy
Like adding more people to a crowded dance floor: the more bodies, the harder it is for the water molecules to line up as ice.
Example
i = 2 (Na⁺ and Cl⁻), Kf = 1.86 °C/mΔTf = 2 × 1.86 × 1.00ΔTf = 3.72 °CFreezing point ≈ −3.72 °C
In practice
Road salt melts ice because salty water freezes at a lower temperature than pure water.
Practice
Flashcards
- Write the freezing point depression formula.ΔTf = i · Kf · m.
Interview questions
- Why does road salt help melt ice, and why is CaCl₂ more effective than NaCl per mole?
- Solute lowers the freezing point
- Freezing point depends on particle count
- CaCl₂ gives three ions, i = 3
- NaCl gives two ions, i = 2
Questions people ask
What is the van 't Hoff factor, i?
The van 't Hoff factor is the number of particles one formula unit gives in solution. Glucose stays whole, so i = 1. NaCl splits into two ions, so i is about 2. Real values are slightly lower because oppositely charged ions pair up briefly.
What is osmosis?
Osmosis is the movement of solvent through a membrane from a dilute solution to a more concentrated one. The membrane lets water through but blocks the solute. The pressure needed to stop this flow is the osmotic pressure, π = iMRT, and it explains why drinking seawater dehydrates you.