Chapter 14 – Solutions: Properties, Interactions, and Colligative Effects

Chapter 14 – Solutions

Introduction to Solutions

Solutions are homogeneous mixtures composed of two or more substances. The major component is called the solvent, and the minor component is the solute. Solutions can be described and quantified using various concentration units.

• Solvent: The substance present in the greatest amount in a solution.

• Solute: The substance dissolved in the solvent.

• Types of solutions: Aqueous solutions (water as solvent), gaseous solutions, solid solutions.

Example: Salt dissolved in water forms an aqueous solution.

Concentration Units

Concentration expresses the amount of solute in a given quantity of solvent or solution. Common units include:

Concentration Type	Abbreviation	Unit
Molarity	M	mol/L
Molality	m	mol/kg
Mass Percent	%	g/100g
Mole Fraction	X	no unit

Solubility: The maximum amount of a solute that can dissolve in a given amount of solvent at a specific temperature.

Solution Formation and Equilibrium

Solution formation is governed by dynamic equilibrium, where the rates of dissolution and recrystallization become equal. The solubility of a solute is constant at equilibrium.

• Unsaturated solution: Contains less than the maximum amount of solute that can dissolve.

• Saturated solution: Contains the maximum amount of solute that can dissolve.

• Supersaturated solution: Contains more solute than is stable at equilibrium; excess solute may precipitate.

Example: Dissolving NaCl in water until no more dissolves forms a saturated solution.

Solubility of Liquids: Miscibility and Immiscibility

Liquids that dissolve in each other in all proportions are miscible. Liquids that do not dissolve in each other are immiscible.

• Miscible: Ethanol and water

• Immiscible: Oil and water

Driving Forces for Solution Formation

Two fundamental concepts drive mixing and solubility:

• Enthalpy (ΔH): Energy changes during solution formation.

• Entropy (ΔS): The measure of energy dispersal or disorder.

Mixing is favored when entropy increases and enthalpy change is exothermic.

Polarity and Solubility

"Like dissolves like": Polar solvents dissolve polar solutes, and nonpolar solvents dissolve nonpolar solutes.

Solvent	Solute	Solubility
Water (polar)	NaCl (ionic)	Soluble
Hexane (nonpolar)	Oil (nonpolar)	Soluble
Water	Oil	Insoluble

Example: Ethanol (polar) is miscible with water (polar).

Energetics of Solution Formation

Solution formation involves breaking and forming intermolecular forces:

• Solute-solute interactions (must be broken)

• Solvent-solvent interactions (must be broken)

• Solute-solvent interactions (must be formed)

The overall enthalpy change is the enthalpy of solution ($\Delta H_{solution}$):

• $\Delta H_{solution} = \Delta H_{solute} + \Delta H_{solvent} + \Delta H_{mix}$

If $\Delta H_{solution}$ is negative (exothermic), solution formation is favored.

Factors Affecting Solubility

• Temperature: Solubility of solids generally increases with temperature; solubility of gases decreases.

• Pressure: Solubility of gases increases with pressure (Henry's Law).

Henry's Law: $S_g = k_H P_g$

Where $S_g$ is the solubility of the gas, $k_H$ is Henry's Law constant, and $P_g$ is the partial pressure of the gas.

Colligative Properties

Colligative properties depend on the number of solute particles, not their identity. These include vapor pressure lowering, boiling point elevation, freezing point depression, and osmotic pressure.

• Vapor Pressure Lowering: Addition of solute lowers the vapor pressure of the solvent.

• Boiling Point Elevation: $\Delta T_b = i K_b m$

• Freezing Point Depression: $\Delta T_f = i K_f m$

• Osmotic Pressure: $\Pi = M R T$

Where $i$ is the van't Hoff factor (number of particles per formula unit), $K_b$ and $K_f$ are constants, $m$ is molality, $M$ is molarity, $R$ is the gas constant, and $T$ is temperature in Kelvin.

Osmosis and Osmotic Pressure

Osmosis: Movement of solvent molecules through a semipermeable membrane from low to high solute concentration.

Osmotic Pressure ($\Pi$): The pressure required to stop osmosis.

• $\Pi = M R T$

Example: Calculating osmotic pressure for a solution of known molarity and temperature.

Colloids and the Tyndall Effect

Colloid: A heterogeneous mixture where dispersed particles are intermediate in size between those in solutions and suspensions.

Dispersed Phase	Dispersion Medium	Example
Liquid	Gas	Clouds, fog
Gas	Liquid	Whipped cream
Solid	Liquid	Muddy water

Tyndall Effect: Scattering of light by colloidal particles, making the path of light visible.

Example: Light passing through fog is scattered, making the beam visible.

Additional info: These notes include expanded definitions, equations, and examples for clarity and completeness, suitable for exam preparation in General Chemistry.