Chapter 9: Solutions

9.1 Mixtures and Solutions

Mixtures are combinations of two or more substances that are not chemically bonded. They can be classified based on the uniformity of their composition.

• Heterogeneous mixtures: Non-uniform composition; different regions have different properties. Example: Rocky Road ice cream.

• Homogeneous mixtures: Uniform composition throughout. Example: Seawater.

Homogeneous mixtures are further classified as:

• Solutions: Homogeneous mixtures with particle sizes similar to ions or small molecules (typically <2 nm).

• Colloids: Homogeneous mixtures with particle sizes between 2 and 500 nm.

Parts of a solution:

• Solute: The substance dissolved in a solvent (minor component).

• Solvent: The substance in which the solute is dissolved (major component).

9.2 The Solution Process

The formation of a solution depends on the relative strengths of attractions between solute and solvent particles compared to those within the pure substances.

• "Like dissolves like": Polar solvents dissolve polar and ionic solutes; nonpolar solvents dissolve nonpolar solutes.

• Example: Oil (nonpolar) does not dissolve in water (polar) due to strong water-water hydrogen bonding.

• Solvation (hydration): Water molecules surround and stabilize ions or polar molecules in solution.

• Dissolution is a physical change—the chemical identities of solute and solvent are retained.

• Enthalpy change (ΔH): Dissolution can be exothermic (releases heat) or endothermic (absorbs heat).

Example: Hexane (nonpolar) and carbon tetrachloride (nonpolar) form a solution; octane (nonpolar) and methyl alcohol (polar) do not.

9.3 Solubility

Solubility describes how much solute can dissolve in a given amount of solvent at a specific temperature.

• Miscible: Two liquids that are soluble in all proportions (e.g., ethanol and water).

• Saturated solution: Contains the maximum amount of dissolved solute at equilibrium; excess solute remains undissolved.

• Solubility: Maximum amount of solute that dissolves in a given solvent at a specified temperature (e.g., 35.8 g NaCl/100 mL H2O at 20°C).

9.4 The Effect of Temperature on Solubility

Temperature affects the solubility of solids and gases differently.

• Solids: Most become more soluble as temperature increases.

• Gases: Become less soluble as temperature increases.

• Supersaturated solution: Contains more solute than a saturated solution at a given temperature; unstable and precipitates when disturbed.

9.5 The Effect of Pressure on Solubility: Henry’s Law

Pressure affects the solubility of gases in liquids.

• Henry’s Law: The solubility of a gas is directly proportional to its partial pressure above the solution (at constant temperature).

Equation:

Where C = concentration (solubility) of the gas, k = Henry’s law constant, P = partial pressure of the gas.

• Increasing pressure increases gas solubility; decreasing pressure decreases it (explained by Le Châtelier’s principle).

Example: The solubility of O2 in blood decreases at high altitudes due to lower atmospheric pressure.

9.6 Units of Concentration

Concentration expresses the amount of solute in a given amount of solution.

Key equations:

• Percent by mass:

• Percent by volume:

• Percent by mass/volume:

• ppm:

• ppb:

• Molarity:

9.7 Dilution

Solutions are often prepared by diluting a more concentrated stock solution.

• The amount of solute remains constant; only the volume changes.

Key equation:

Where C = concentration, V = volume, and subscripts 1 and 2 refer to initial and final states.

• Dilution factor:

Example: To prepare 750 mL of 0.32 M NaOH from 1.0 M NaOH, use .

9.8 Ions in Solution: Electrolytes

Electrolytes are substances that produce ions in solution and can conduct electricity.

• Strong electrolytes: Completely ionize in water (e.g., NaCl).

• Weak electrolytes: Partially ionize in water (e.g., acetic acid).

• Nonelectrolytes: Do not produce ions in water (e.g., sugar).

Equivalents (Eq): Used to express the amount of ions in solution. 1 Eq = 1 mole of charge; 1 mEq = 0.001 Eq.

Example: The normal concentration of K+ in blood is 5.0 mEq/L.

9.9 Properties of Solutions

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

• Vapor pressure lowering: Solutions have lower vapor pressure than pure solvents.

• Boiling point elevation: Solutions boil at higher temperatures than pure solvents.

• Freezing point depression: Solutions freeze at lower temperatures than pure solvents.

• Osmosis: Solvent passes through a semipermeable membrane from lower to higher solute concentration.

Key equations:

• Boiling point elevation:

• Freezing point depression:

For water: ,

Example: 0.75 mol KBr in 1.0 kg water raises boiling point by 0.77°C (since KBr dissociates into 2 ions).

9.10 Osmosis and Osmotic Pressure

Osmosis is the movement of solvent through a semipermeable membrane from a dilute to a concentrated solution.

• Osmotic pressure (π): The pressure required to stop osmosis.

• Osmolarity (osmol/L): The total molarity of all solute particles in solution.

Equation:

Where π = osmotic pressure, M = molarity of solute particles, R = gas constant, T = temperature (K).

• Isotonic: Same osmolarity as blood plasma (~0.30 osmol/L).

• Hypotonic: Lower osmolarity than plasma; cells may swell.

• Hypertonic: Higher osmolarity than plasma; cells may shrink.

9.11 Dialysis

Dialysis is the separation of small solute particles from larger particles (like proteins) through a semipermeable membrane.

• Hemodialysis: Used to remove waste from the blood in patients with kidney failure.

• Small molecules (e.g., urea) pass through the membrane; large molecules (e.g., proteins) do not.

• Osmotic pressure of proteins (colloids) helps regulate water movement in and out of blood plasma.

Example: The balance of filtration and reabsorption in capillaries is regulated by osmotic pressure and protein content.

Additional info: Some equations, tables, and definitions were expanded for clarity and completeness. All key concepts from the provided material are included and organized for GOB Chemistry students.