Colligative Properties of a Solution

• Colligative properties

  • Properties of a solution that depend only on the number and not on the identity of the solute particles
  • Depend on the collective effect of the concentration of solute particles present in an ideal solution
  • Useful for characterizing the nature of a solute after it is dissolved in a solvent and for determining the molar masses of substances
  • Etymology – From the Latin word colligatus, meaning “tied or bound together”
• Vapor Pressure Lowering

  1. A substance that has no measurable vapor pressure is nonvolatile, while one that exhibits a vapor pressure is volatile

  2. As a non-volatile solute is added to the solution and dissolved into a volatile solvent, it results in diluting the water and lowering the vapor pressure of the solvent

  3. According to Raoult’s Law, a solvent’s partial vapor pressure in a solution is equal to the vapor pressure of the pure solvent multiplied by its mole fraction in the solution
• Three Colligative Properties

  • Vapor Pressure Lowering
  • Boiling Point Elevation
  • Freezing Point Depression
• How is vapor pressure affected by the electrolytes?

  Strong electrolytes dissociate into ions when dissolved into a solution, resulting in a large number of dissolved particles, which lowers the vapor pressure of the solvent more than nonelectrolyte solutions
• Electrolyte vs Nonelectrolyte Solutions


  • Electrolyte solutions conduct electricity and contain ions, atoms, or molecules that have either lost or gained electrons
  • Nonelectrolyte solutions do not conduct electricity
• Volatile vs Nonvolatile Solutions

  • Volatile substances evaporate readily at normal temperatures and have a measurable vapor pressure
  • Nonvolatile substances do not have a measurable vapor pressure
• Boiling Point Elevation

  The addition of a nonvolatile solute lowers the vapor pressure of the solution, requiring the temperature to be raised to restore the vapor pressure to the value conforming to the pure solvent
• The magnitude of the freezing point depression is directly proportional to the molality of the solution
• Salt, being an electrolyte that dissociates into ions once dissolved, has the tendency to increase a solution’s boiling point more than sugar, which is not an electrolyte
• Electrolytes produce more particles than non-electrolytes, providing a higher boiling point elevation
• As the concentration of sucrose increases in the solution, the higher the pure water’s boiling point elevation would be
• Lightning strikes oceans because they conduct way more electricity than any other bodies of water due to the presence of NaCl (salt) in the seas. In freshwaters, however, non-electrolytes are present
• Lowering of the vapor pressure in a solution

  Causes the temperature to be raised to restore the vapor pressure of the solution to the value conforming to the pure solvent
• Freezing Point Depression

  A solution must be cooled to a lower temperature than the pure solvent for freezing to occur
• Human bodies are good conductors of electricity due to the presence of various ions like sodium ion, potassium ion, chloride ion, etc., which have the tendency to conduct electricity
• Boiling Point Elevation

  Change in boiling point of the solvent above a solution from that of the pure solvent is directly proportional to the molal concentration of the solute
• Salt is added to ice by the ice cream vendor because it lowers the freezing temperature of the ice and makes the ice cream colder faster
• The lowering of the vapor pressure in a solution causes the boiling point of the solution to be higher than the pure solvent. As a result, the freezing point of a solvent decreases when any solute is dissolved into it
• As more solute particles are added, the lower the freezing point will be. Electrolytes, producing more particles when dissolved in water, result in a much lower freezing point than when a non-electrolyte is dissolved
• When a solute is added to a solvent forming a solution, the boiling point increases and the freezing point decreases