Solids in Solution and Colligative Properties: Summary

The answers to the ConcepTests are given below and will open in a separate window. 
Key points from this module:
  • A solid solute dissolved in solution is saturated when the solution is in equilibrium with solid solute (i.e., solute that is not dissolved).
  • Solubility is temperature dependent and for most solids, solubility increases with temperature.
  • A solution is supersaturated when the solute concentration is higher than the saturated concentration.
  • This is not an equilibrium condition and is usually obtained by cooling a saturated solution.
  • When a solid is filtered from a solution, some of that solution, still containing dissolved solute, is retained.
  • Adding a solute to a liquid lowers its vapor pressure, raises its boiling point temperature, and lowers its freezing point temperature.
  • Colligative properties of solutions (changes in saturation pressure, boiling point, freezing point) depend only on the solute mole fraction, not on which solute is used (for non-dissociating solids).
From studying this module, you should now be able to:
  • Conduct mass balances on an evaporative crystallization system by taking into account temperature-dependent solubility.
  • Calculate boiling point elevation for a given mole fraction of solute.
  • Calculate freezing point depression for a given mole fraction of solute.
  • Explain the difference between saturation and supersaturation for a solid dissolved in a liquid.

Prepared by John L. Falconer, Department of Chemical and Biological Engineering, University of Colorado Boulder