Chapter 14: Solutions – Structure, Properties, and Calculations

Solutions: Composition and Types

Definition and Components

Solutions are homogeneous mixtures composed of two or more substances. The majority component is called the solvent, while the minority component is the solute. Solutions can form spontaneously unless energetically unfavorable.

Common Types of Solutions

Solutions can exist in various phases depending on the physical states of the solute and solvent.

Solution Phase	Solute Phase	Solvent Phase	Example
Gaseous solution	Gas	Gas	Air (mainly oxygen and nitrogen)
Liquid solution	Gas	Liquid	Club soda (CO2 and water)
Liquid solution	Liquid	Liquid	Vodka (ethanol and water)
Solid solution	Solid	Liquid	Seawater (salt and water)
Solid solution	Solid	Solid	Brass (copper and zinc) and other alloys

Spontaneous Mixing and Entropy

Nature’s Tendency to Mix

Mixing of substances is driven by entropy, which is a measure of energy dispersal in a system. Spontaneous mixing occurs when the barrier between two substances is removed, resulting in a uniform concentration.

Role of Entropy

- Entropy increases as energy is dispersed over a larger volume. - Formation of a solution does not always lower the system's potential energy, but the increase in entropy can drive the process.

Intermolecular Forces and Solution Formation

Types of Intermolecular Forces

Intermolecular forces determine the ability of substances to mix and form solutions. The main types are:

• Dispersion forces (London forces): Present in all molecules, especially nonpolar ones.

• Dipole-dipole interactions: Occur between polar molecules.

• Hydrogen bonding: Strong dipole-dipole interaction involving H bonded to N, O, or F.

• Ion-dipole interactions: Occur between ions and polar molecules.

Solution Interactions

Solution formation depends on the relative strengths of three interactions:

• Solvent-solvent interactions

• Solute-solute interactions

• Solvent-solute interactions

Relative Interaction	Outcome
Solvent-solute > solvent-solvent and solute-solute	Solution forms
Solvent-solute = solvent-solvent and solute-solute	Solution forms
Solvent-solute < solvent-solvent and solute-solute	Solution may or may not form, depending on disparity

Solubility: "Like Dissolves Like"

Solubility is the maximum amount of solute that can dissolve in a given amount of solvent. The general rule is "like dissolves like":

• Polar molecules and ionic compounds dissolve in polar solvents.

• Nonpolar molecules are more soluble in nonpolar solvents.

Common Polar Solvents	Common Nonpolar Solvents
Water (H2O)	Hexane (C6H14)
Acetone (CH3COCH3)	Diethyl ether (CH3CH2OCH2CH3)
Methanol (CH3OH)	Toluene (C7H8)
Ethanol (CH3CH2OH)	Carbon tetrachloride (CCl4)

Solubility of Vitamins

Vitamins can be classified as water-soluble or fat-soluble based on their molecular structure. Water-soluble vitamins dissolve in body fluids and are easily eliminated, while fat-soluble vitamins can accumulate in fatty tissues.

Energetics of Solution Formation

Three-Step Process

Formation of a solution can be visualized in three steps:

1. Separating solute particles (requires energy, endothermic)

2. Separating solvent particles (requires energy, endothermic)

3. Mixing solute and solvent particles (releases energy, exothermic)

Enthalpy of Solution Formation

The overall enthalpy change for solution formation is: $\Delta H_{\text{soln}} = \Delta H_{\text{solute}} + \Delta H_{\text{solvent}} + \Delta H_{\text{mix}}$ For aqueous solutions, $\Delta H_{\text{solvent}}$ and $\Delta H_{\text{mix}}$ are combined into $\Delta H_{\text{hydration}}$: $\Delta H_{\text{soln}} = \Delta H_{\text{solute}} + \Delta H_{\text{hydration}}$

Examples of Enthalpy Changes

• Exothermic solution formation: $\Delta H_{\text{soln}} < 0$

• Endothermic solution formation: $\Delta H_{\text{soln}} > 0$

Solution Equilibrium and Solubility Limits

Dynamic Equilibrium

When a solution reaches its solubility limit, the rate of dissolution equals the rate of recrystallization, resulting in dynamic equilibrium.

Types of Solutions Based on Solubility

• Saturated: Contains maximum solute at equilibrium; additional solute will not dissolve.

• Unsaturated: Contains less solute than saturation; more solute can dissolve.

• Supersaturated: Contains more solute than saturation; unstable and excess solute may precipitate.

Solubility and Temperature

Solubility is usually reported as grams of solute per 100 g water. For most solids, solubility increases with temperature (endothermic dissolution).

Solubility of Gases

Gases generally have lower solubility in water, and their solubility decreases with increasing temperature (exothermic dissolution).

Henry’s Law

The solubility of a gas in a liquid is proportional to the partial pressure of the gas above the liquid: $S_{\text{gas}} = k_H \times P_{\text{gas}}$ Where $S_{\text{gas}}$ is the solubility, $k_H$ is Henry’s law constant, and $P_{\text{gas}}$ is the partial pressure.

Concentration Units and Calculations

Describing Solution Concentration

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

• Molarity (M): mol solute / L solution

• Molality (m): mol solute / kg solvent

• Mass percent: (mass solute / mass solution) × 100%

• Parts per million (ppm): (mass solute / mass solution) × 106

• Mole fraction (XA): mol component / total mols

Key Formulas

• Molarity: $\text{Molarity (M)} = \frac{\text{amount solute (in mol)}}{\text{volume solution (in L)}}$

• Molality: $\text{Molality (m)} = \frac{\text{amount solute (in mol)}}{\text{mass solvent (in kg)}}$

• Percent by mass: $\text{Percent by mass} = \frac{\text{mass solute}}{\text{mass solution}} \times 100\%$

• Parts per million: $\text{ppm} = \frac{\text{mass solute}}{\text{mass solution}} \times 10^6$

Colligative Properties

Definition and Types

Colligative properties depend on the number of solute particles, not their identity. These include:

• Vapor pressure lowering

• Freezing point depression

• Boiling point elevation

• Osmotic pressure

Vapor Pressure Lowering

The presence of a nonvolatile solute lowers the vapor pressure of the solvent.

Freezing Point Depression and Boiling Point Elevation

Adding solute to a solvent lowers its freezing point and raises its boiling point.

Osmosis and Osmotic Pressure

Osmosis is the movement of solvent through a semipermeable membrane from a region of lower solute concentration to higher concentration. Osmotic pressure is the pressure required to stop this flow.

van’t Hoff Factor (i)

The van’t Hoff factor accounts for the number of particles a solute produces in solution, affecting colligative properties.

Mixtures: Solutions, Suspensions, and Colloids

Classification of Mixtures

• Solutions: Homogeneous mixtures; particles do not separate on standing.

• Suspensions: Heterogeneous; particles separate on standing.

• Colloids: Heterogeneous; particles do not separate on standing, but can coagulate and exhibit Brownian motion.

Soap and Emulsification

Soaps have hydrophilic heads and hydrophobic tails, allowing them to form micelles and emulsify nonpolar substances in water.

Tyndall Effect

Colloids scatter light, a phenomenon known as the Tyndall effect.

Summary Table: Key Solution Concepts

Concept	Definition	Example/Application
Solution	Homogeneous mixture of solute and solvent	Salt water, air
Solubility	Maximum amount of solute that can dissolve	NaCl in water
Colligative properties	Depend on number of solute particles	Boiling point elevation
Concentration units	Molarity, molality, mass percent, ppm	Lab calculations

Additional info: Academic context and explanations have been expanded for clarity and completeness.