# Colligative Properties

> Colligative properties for JEE Chemistry: Raoult's law, boiling point elevation, freezing point depression, osmotic pressure and van't Hoff factor.

- Canonical URL: https://prepelephant.com/topics/jee/chemistry/colligative-properties
- Exam / course: JEE · Subject: Chemistry
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Colligative Properties", PrepElephant, https://prepelephant.com/topics/jee/chemistry/colligative-properties

## Direct answer

Colligative properties — relative lowering of vapour pressure, boiling point elevation, freezing point depression and osmotic pressure — depend on the number of solute particles, not their identity. Each follows a counting law: delta p/p° = x(solute), delta T_b = K_b m, delta T_f = K_f m and pi = C R T. Where solutes dissociate or associate, the van't Hoff factor i corrects the particle count: i = observed colligative effect / calculated effect.

## What you must remember

- Raoult's law: p = p° x for a volatile component; for an ideal pair, total vapour pressure p = pA° xA + pB° xB across all proportions.
- Positive deviation (weaker interactions) gives minimum-boiling azeotropes like ethanol-water; negative deviation gives maximum-boiling azeotropes like nitric acid-water (about 68 per cent HNO3).
- delta T_b = K_b × m and delta T_f = K_f × m on molality; for water K_b = 0.52 and K_f = 1.86 K kg mol^-1.
- Osmotic pressure pi = C R T; osmosis is solvent flow across a semipermeable membrane towards the concentrated side; equal pi means isotonic.
- van't Hoff factor: i = 1 + alpha(n - 1) for dissociation into n particles, i below 1 on association; observed molar mass M(observed) = M(normal)/i. Counts: NaCl 2, CaCl2 3, K4[Fe(CN)6] and Al2(SO4)3 5 each; benzoic acid dimerising in benzene gives about 0.5.
- Henry's law p = K_H x: K_H rises with temperature, so gas solubility in water falls on heating — the physics behind the bends in scuba diving and fizz escaping warm drinks.
- Osmotic pressure is the preferred route to macromolecular molar masses: measurable at room temperature with molarity.

## Common confusion

The classic error is sign logic for association versus dissociation: dissociation creates more particles, so effects grow, i exceeds 1 and apparent molar mass falls below true; association does the reverse. Students plug molarity into delta T_b or delta T_f (they need molality) or molality into pi (it needs molarity), and read azeotropes as pure compounds — an azeotrope is a mixture whose vapour matches its liquid composition, resisting distillation.

## Exam-focused takeaway

JEE Main tests the four laws as direct numericals — compute delta T_f or pi, find molar mass from a depression, pick i for a named electrolyte. JEE Advanced layers concepts: Raoult with Dalton's law over an ideal pair, vapour-composition diagrams read for deviation type, molar mass anomalies from i (dimerising benzoic acid, ionising KCl), and isotonic-condition problems. Write the concentration unit each formula wants before substituting.

## Frequently asked questions

### Why are colligative properties called colligative?

They depend on the collective number of solute particles, not their nature — equal moles of any non-volatile solute lower the freezing point equally.

### What is the van't Hoff factor?

The ratio of actual to nominal particle count: 2 for NaCl, about 0.5 for a dimerising acid in benzene; it multiplies every colligative effect.

### What is an azeotrope?

A mixture boiling at constant composition — minimum-boiling ethanol-water, maximum-boiling nitric acid-water — that simple distillation cannot separate.

### Why is osmotic pressure preferred for polymer masses?

It is measurable at room temperature on dilute solutions and avoids the heat and tiny effects that damage macromolecules.

### How does Henry's law explain the bends?

Rapid ascent lowers pressure, dissolved nitrogen's solubility drops and it bubbles out of blood — hence staged decompression.

### Does delta T_f use molality or molarity?

Molality: delta T_f = K_f × m, because solvent mass, unlike volume, does not change with temperature.