# 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.

Chemistry (university) · Part 03: Intermolecular forces, gases and solutions · Card 9 of 9 · Last updated 2026-10-19

## 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

**1.00 m NaCl in water**

i = 2 (Na⁺ and Cl⁻), Kf = 1.86 °C/m
ΔTf = 2 × 1.86 × 1.00
ΔTf = 3.72 °C
Freezing 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.

## Related concepts

- [Solubility and 'like dissolves like'](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-3/chemistry-university-part-3-concept-8)
- [Molarity and dilution](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-4/chemistry-university-part-4-concept-6)

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