Solution Properties

20min Part 2 / Ch3 / Lesson 3

Prerequisites: 1-2-2

Objectives

Explain vapor pressure lowering, boiling point elevation, and freezing point depression
Understand osmotic pressure
Describe properties of colloidal solutions

Colligative Properties

Properties of solutions that depend only on the number of solute particles (concentration), not on their identity, are called colligative properties.

Colligative properties (dilute solutions):

Vapor pressure lowering: adding solute lowers vapor pressure
Boiling point elevation: solution boils higher than pure solvent ( $\Delta T_b = K_b \cdot m$ )
Freezing point depression: solution freezes lower than pure solvent ( $\Delta T_f = K_f \cdot m$ )
Osmotic pressure: $\Pi = cRT$ (van’t Hoff equation)

$m$ : molality (mol/kg), $K_b$ , $K_f$ : molal boiling point elevation / freezing point depression constants

Boiling point elevation and freezing point depression

Colloidal Solutions

Colloids: dispersed systems with particle sizes of about $1 \sim 100\ \mathrm{nm}$ . Between true solutions and suspensions.

Characteristic phenomena:

Tyndall effect: light scattering makes the beam path visible
Brownian motion: particles move irregularly
Electrophoresis: colloidal particles migrate in an electric field
Dialysis: separating colloids from ions using a semipermeable membrane

Check Your Understanding

Q1 Why is salt spread on roads in winter?

Q2 Colligative properties depend on:

Q3 The Tyndall effect is observed in:

Exercises

Q1. Given the freezing point depression constant for water $K_f = 1.86\ \mathrm{K \cdot kg/mol}$ , find the freezing point of a solution made by dissolving $9.0\ \mathrm{g}$ of glucose $\mathrm{C_6H_{12}O_6}$ ( $M = 180$ ) in $500\ \mathrm{g}$ of water.

Solution

Molality: $m = \dfrac{9.0/180}{500/1000} = \dfrac{0.050}{0.50} = 0.10\ \mathrm{mol/kg}$

Freezing point depression: $\Delta T_f = K_f \cdot m = 1.86 \times 0.10 = 0.186\ \mathrm{K}$

Freezing point: $0 - 0.186 = -0.19\ \mathrm{°C}$