Chapter 13: Solutions – Structure, Properties, and Calculations

Solutions: Homogeneous Mixtures

Definition and Examples

A solution is a homogeneous mixture of two or more substances. The component present in the largest amount is called the solvent, while the other component(s) are called solute(s). Solutions are found in all states of matter and are common in everyday life and laboratory practice.

• Solvent: The dissolving medium (majority component).

• Solute: The substance dissolved (minority component).

• Examples: Salt water (NaCl in H2O), air (O2 and N2), soda (CO2 in H2O).

Types of Solutions

Solutions can be classified based on the physical states of their components:

Solution Phase	Solute Phase	Solvent Phase	Example
Gaseous	Gas	Gas	Air (O2 in N2)
Liquid	Gas	Liquid	Soda (CO2 in H2O)
Liquid	Solid	Liquid	Seawater (salts in H2O)
Solid	Solid	Solid	Brass (Zn in Cu)

Properties of Solutions, Colloids, and Suspensions

Classification and Comparison

Mixtures can be classified as solutions, colloids, or suspensions based on particle size and behavior:

	Solutions	Colloids	Suspensions
Homogeneity	Homogeneous	Heterogeneous	Heterogeneous
Particle Size	0.01–1 nm	1–1000 nm	>1000 nm
Separation	Do not separate	Do not separate	Settle out
Filtration	Cannot be filtered	Cannot be filtered	Can be filtered
Light Scattering	No	Tyndall effect	May scatter

Solubility: Soluble vs. Insoluble, Miscible vs. Immiscible

Definitions

• Soluble: A solute that dissolves in a solvent.

• Insoluble: A solute that does not dissolve in a solvent.

• Miscible: Two liquids that are infinitely soluble in each other (e.g., water and vinegar).

• Immiscible: Two liquids that do not mix (e.g., oil and vinegar).

Polarity and Solubility

Polarity and "Like Dissolves Like"

The solubility of substances depends on their polarity. Polar solvents dissolve polar and ionic solutes, while nonpolar solvents dissolve nonpolar solutes. This is summarized by the rule: like dissolves like.

• Polar molecules: Unequal sharing of electrons, resulting in dipoles (e.g., water, vinegar).

• Nonpolar molecules: Equal sharing of electrons, no dipoles (e.g., oil).

Common Laboratory Solvents

Common Polar Solvents	Common Nonpolar Solvents
Water (H2O)	Hexane (C6H14)
Acetone (CH3COCH3)	Diethyl ether (CH3CH2OCH2CH3)
Methyl alcohol (CH3OH)	Toluene (C7H8)

Electrolytes vs. Nonelectrolytes

Definitions and Examples

• Electrolyte: A substance that dissolves in water to give a solution that conducts electricity (e.g., NaCl, KCl, HCl).

• Nonelectrolyte: A substance that dissolves in water but does not conduct electricity (e.g., sugar, air).

Water: The Universal Solvent

Unique Properties of Water

• Polar covalent bonds and hydrogen bonding.

• Ice is less dense than liquid water.

• High boiling point and surface tension.

• Low vapor pressure.

Formation of Solutions: Dissolving Solids in Water

Solvation Process

When an ionic solid dissolves in water, the solvent–solute attractions must overcome the solute–solute and solvent–solvent attractions. Water molecules surround and separate the ions, dispersing them throughout the solution.

Factors Affecting the Rate of Dissolving

How Quickly Does a Solute Dissolve?

• Agitation: Stirring brings fresh solvent into contact with solute, increasing the rate of dissolving.

• Temperature: Higher temperatures increase kinetic energy, leading to faster dissolving.

• Particle Size: Smaller particles have greater surface area, dissolving faster.

Solubility: How Much Solute Can Dissolve?

Definitions and Categories

• Solubility: The amount of solute that dissolves in a given quantity of solvent at a specified temperature and pressure (g/100 g H2O).

• Saturated Solution: Contains the maximum amount of solute at given conditions.

• Unsaturated Solution: Contains less than the maximum amount of solute.

• Supersaturated Solution: Contains more than the normal maximum amount of solute; unstable and excess solute may precipitate.

Solubility and Temperature

Solids and Gases

• Solubility of most solids increases with temperature.

• Solubility of gases decreases with increasing temperature.

Solubility and Pressure: Henry's Law

Gas Solubility and Pressure

The solubility of a gas in a liquid increases as the partial pressure of the gas above the solution increases. This relationship is described by Henry's Law:

• Henry's Law: $S = kP$ where S is the solubility of the gas, k is a constant, and P is the partial pressure of the gas.

Concentration Units

Molarity (M)

Molarity is the most common unit of concentration in chemistry, defined as the number of moles of solute per liter of solution:

$\text{Molarity (M)} = \frac{\text{moles of solute}}{\text{liters of solution}}$

Calculating Molarity: Example

To calculate the molarity of a solution, convert the mass of solute to moles and divide by the volume of solution in liters.

Using Molarity in Calculations

To find the mass of solute in a given volume and molarity, use dimensional analysis:

Solution Dilution

Dilution Equation

To dilute a solution, use the equation:

$M_1 V_1 = M_2 V_2$

• M1 and V1: Initial molarity and volume

• M2 and V2: Final molarity and volume

Other Concentration Measures

Percent by Volume and Mass

• Percent by volume (% v/v): $\frac{\text{volume of solute}}{\text{volume of solution}} \times 100$

• Percent by mass (% m/m): $\frac{\text{mass of solute}}{\text{mass of solution}} \times 100$

Colligative Properties of Solutions

Definition and Types

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

• Vapor-pressure lowering

• Freezing-point depression

• Boiling-point elevation

• Osmotic pressure

Osmosis and Osmotic Pressure

Osmosis

Osmosis is the flow of solvent from a less concentrated solution to a more concentrated solution through a semipermeable membrane. Osmotic pressure is the pressure required to stop this flow and is a colligative property.

Red Blood Cells in Solutions

• Isoosmotic: No net flow of water; cell retains shape.

• Hypoosmotic: Water enters cell; cell may burst.

• Hyperosmotic: Water leaves cell; cell shrinks.

Solution Stoichiometry

Using Molarity in Chemical Reactions

In reactions involving solutions, use the volume and molarity to calculate moles of reactants or products, then apply stoichiometric coefficients from the balanced equation.

Summary of Key Formulas

• Molarity: $M = \frac{\text{moles solute}}{\text{liters solution}}$

• Mass percent: $\text{mass percent} = \frac{\text{mass solute}}{\text{mass solution}} \times 100$

• Dilution: $M_1 V_1 = M_2 V_2$

• Henry's Law: $S = kP$