Chemistry: The Central Science - Chapter 13: Properties of Solutions

13.1 The Solution Process

• Solutions are homogeneous mixtures of two or more substances.
• Solute is uniformly dispersed throughout the solvent.
• Solution formation depends on:
  • Natural tendency toward mixing.
  • Intermolecular forces.

Natural Tendency Toward Mixing

• Mixing of gases is spontaneous and increases randomness (entropy).
• Solutions are favored by increased entropy.

Intermolecular Forces

• Intermolecular forces between solute and solvent molecules affect solution formation.
• Three interactions:
  1. Overcoming solute-solute interactions.
  2. Overcoming solvent-solvent interactions.
  3. Solvent-solute interactions facilitate mixing.
• Vapor pressure is related to the equilibrium between liquid and gas phases.

13.2 Saturated Solutions and Solubility

• Solution-making and crystallization are opposing processes.
• Saturated solution: rate of precipitation equals rate of dissolving; no more solute dissolves.
• Solubility: maximum amount that dissolves in a solvent.
• Unsaturated solutions can dissolve more solute.

Supersaturated Solutions

• Solute concentration higher than in saturated solutions.
• Crystallization is slow; a seed crystal can speed up the process.

13.3 Factors That Affect Solubility

• Solubility depends on the nature of solute and solvent.
• "Like dissolves like": polar dissolves polar; nonpolar dissolves nonpolar.
• Solute-solvent interaction strength affects solubility.
• Larger gases are more soluble in water.

Organic Molecules in Water

• Polar organic molecules dissolve better in water.
• As hydrocarbons increase, solubility in water increases.

Solubility and Biological Importance

• Fat-soluble vitamins are stored in fat; water-soluble vitamins need daily intake.

Liquid/Liquid Solubility

• Miscible: liquids mix in all proportions.
• Immiscible: do not mix (e.g., hexane and water).

Pressure and Temperature Effects

• Pressure primarily affects gas solubility.
• Temperature generally increases solubility of solids but decreases gas solubility.

13.4 Expressing Solution Concentration

• Concentrations may be dilute or concentrated.
• Expressed using:
  • Mass percent
  • Parts per million (ppm) and billion (ppb)
  • Mole fraction
  • Molarity (moles/liter)
  • Molality (moles/kg solvent)

Molarity vs Molality

• Molarity varies with temperature; molality does not.

13.5 Colligative Properties

• Depend on the number of solute particles, not the identity.
• Include vapor pressure lowering, boiling point elevation, freezing point depression, osmotic pressure.

Raoult's Law

• Vapor pressure of a solution is proportional to the mole fraction of the solvent.

Boiling and Freezing Point Changes

• Solutions have higher boiling points and lower freezing points than pure solvents.

Osmosis and Osmotic Pressure

• Movement of solvent through a semipermeable membrane from low to high solute concentration.
• Osmotic pressure calculated using van 't Hoff factor, molarity, temperature.

Types of Solutions

• Isotonic: equal osmotic pressure.
• Hypotonic: lower osmotic pressure.
• Hypertonic: higher osmotic pressure.

13.6 Colloids

• Particles larger than ions but not settling out by gravity.
• Exhibit Tyndall effect: scattering of light.
• Stabilized by adsorption.

Biological Importance

• Emulsifiers aid digestion by stabilizing colloids.
• Colloids in nanoscience involve metal and semiconductor nanocrystals.