Rucete ✏ AP Chemistry In a Nutshell
8. Solutions
This chapter explores the properties of solutions, how they form, how we classify them, and how their concentration and solubility are measured. It emphasizes molecular-level interactions and the role of temperature and pressure in solubility.
Introduction and Definitions
• A solution is a uniform mixture of solute(s) dissolved in a solvent.
• Common types include gases dissolved in gases or liquids, liquids in liquids, solids in liquids, and solid alloys.
• Solutions are often formed in the liquid phase due to high collision frequency between molecules.
Solution Terminology
• Solute: the substance being dissolved.
• Solvent: the substance doing the dissolving (usually the greater amount).
• Dilute solution: small amount of solute.
• Concentrated solution: large amount of solute.
• Saturated: contains maximum amount of solute at equilibrium.
• Unsaturated: can dissolve more solute.
• Supersaturated: contains more solute than normally possible under given conditions; unstable and may crystallize.
Solubility and the Solution Process
• Solubility: maximum amount of solute that dissolves in a solvent at a given temperature (g/L or mol/L).
• Dissolution involves three steps: 1. Solute particles separate (requires energy), 2. Solvent particles separate (requires energy), 3. Solute and solvent mix (releases energy).
• Net energy change and increase in entropy (disorder) determine whether the solution process is favorable.
Dissolving Ionic Compounds
• Ionic compounds dissolve in water when hydration energy offsets lattice energy.
• Water’s polarity allows it to attract and stabilize ions in solution.
• Some ionic compounds remain insoluble because hydration does not compensate for lattice energy.
Dissolving Gases
• Gases mix freely due to entropy increase (randomness).
• Little to no intermolecular attraction exists between gas molecules.
Rates of Dissolution
• Increased by heating, grinding the solute, or stirring the solution.
• These actions increase the surface area and kinetic energy, enhancing molecular collisions.
• Final concentration depends only on solute identity and temperature.
Aqueous Solutions and Classification
• Aqueous solution: water is the solvent.
• Electrolytes: substances that form ions in solution and conduct electricity.
• Strong electrolytes: completely dissociate (e.g., NaCl, HCl).
• Weak electrolytes: partially ionize (e.g., CH₃COOH, NH₃).
• Nonelectrolytes: dissolve but do not ionize (e.g., sugar, alcohols).
Solubility Rules
• Help predict whether an ionic compound will dissolve in water.
• Most alkali metal and ammonium salts are soluble.
• Most nitrates, chlorates, and acetates are soluble.
• Most halides (Cl⁻, Br⁻, I⁻) are soluble except with Ag⁺, Pb²⁺, and Hg₂²⁺.
• Most sulfates are soluble except with Ba²⁺, Pb²⁺, and Ca²⁺.
• Most carbonates, phosphates, and sulfides are insoluble unless with alkali metals or ammonium.
Concentration Units
• Molarity (M) = moles of solute / liters of solution.
• Molality (m) = moles of solute / kilograms of solvent (used when temperature changes are involved).
• Percent by mass = (mass of solute / mass of solution) × 100
• Mole fraction = moles of component / total moles in solution.
Preparing Dilutions
• Use M₁V₁ = M₂V₂ to calculate volumes/concentrations before and after dilution.
• Only volume of solvent is changed during dilution; amount of solute stays the same.
Colligative Properties
• Depend on the number of solute particles, not their identity.
• Include vapor pressure lowering, boiling point elevation, freezing point depression, and osmotic pressure.
Vapor Pressure Lowering
• Solute particles block solvent molecules from escaping, reducing vapor pressure.
• Raoult’s Law: P(solution) = X(solvent) × P⁰(solvent)
Boiling Point Elevation and Freezing Point Depression
• Boiling point increases, freezing point decreases when solute is added.
• ΔT = i × K × m - i = van’t Hoff factor (number of particles formed), - K = constant depending on solvent, - m = molality.
Osmotic Pressure
• Pressure required to prevent osmosis (movement of solvent through a semipermeable membrane).
• π = iMRT (similar to PV = nRT)
Colloids and Suspensions
• Colloids: mixtures where particles (1–1000 nm) remain suspended (e.g., milk, fog).
• Exhibit Tyndall effect (scattering of light).
• Suspensions: larger particles that settle over time unless stirred (e.g., muddy water).
In a Nutshell
Solutions are homogeneous mixtures whose properties depend on molecular interactions and concentration. Solubility is influenced by temperature, pressure, and particle nature. Understanding electrolyte behavior, colligative properties, and concentration units is crucial for analyzing chemical mixtures in both lab and real-life contexts.