Lecture on Solutions and Solubility

Definition of a Solution

• A solution is a homogeneous mixture of two or more substances.
• Can involve different combinations:
  • Solid and liquid (e.g., seawater)
  • Gas and liquid (e.g., carbonated water)
  • Two different liquids or other combinations.

Aqueous Solutions

• Water acts as the solvent.
• Solutes can be solid, liquid, or gas.
• Examples:
  • Sugar water
  • Salt water
  • Ethyl alcohol mixing with water
  • Carbon dioxide in water forming club soda.

Solubility

• Not all solutes dissolve in all solvents (e.g., grease in water vs. paint thinner).
• Definition: The amount of a substance that dissolves in a given amount of solvent.
• Factors Influencing Solubility:
  • Nature’s tendency towards mixing
  • Types of intermolecular forces

Influence of Intermolecular Forces on Solubility

• Solvent-solute interactions influence solution formation.
• Types of forces:
  • Dispersion forces
  • Dipole-dipole forces
  • Hydrogen bonding
  • Ion-dipole forces
• Rules:
  • If solvent-solute interactions are stronger, a solution is likely to form.
  • If solvent-solvent and solute-solute interactions are stronger, a solution may not form.
  • "Like dissolves like":
    • Polar solvents dissolve polar/ionic solutes.
    • Nonpolar solvents dissolve nonpolar solutes.

Role of Entropy

• Entropy: Measure of energy dispersal/randomization.
• Tendency towards energy dispersal leads to mixing.
• Example: Mixing of ideal gases (e.g., neon and argon) without change in potential energy due to entropy.
• Thermal energy dispersal: Heat flowing from hot to cold.

Practical Example

• Question: Is methanol (CH3OH) more soluble in water or a nonpolar solvent?
• Answer: Water (methanol is polar and forms hydrogen bonds).