I3i: Describe the Colligative Properties


  • A solution behaves differently from a pure solvent
  • Because particles of solute interfere with physical processes
  • COLLIGATIVE PROPERTIES = properties of a solution that depend only on the concentration of solute & not on the type of solute particles
  • Adding solute to a solvent with ∆:
    3. FREEZING PT ↓
    4. OSMOTIC P ↑

Vapour Pressure

  • All liquids have a Vapour Pressure
  • The molecules on liquid surface are liquid ⮂ gas spontaneously
  • No. of gas particles resent determines the Vapour Pressure
  • Adding solute → some particles wil occupy the liquid surface SA
  • ∴↓ability of some molecules to enter gas phase
  • ∴Adding solute = ↓VAPOUR PRESSURE

Boiling Point

  • Every solution has a Boiling Pt
  • When the liquid irreversibly enters gas phase
  • Adding solute particles to solvent
  • Blocks molecules going into gas phase
  • ∴more heat required

= ↑Boiling Pt

Freezing Point

  • The freezing pt/ is the interface liquid ⮂ solid
  • For liquid to freeze it must form a lattice
  • Adding solute → interferes with solvent particles forming a lattice
  • ∴to compensate, the system needs to be at an even lower temp. to freeze
  • ∴Freezing pt ↓

Osmotic Pressure ↑

  • Osmotic P ∝ of solute particles
  • ∴Adding solute to solvent ↑ of solute particles
  • ∴↑Osmotic pressure