Enduring Understanding 2.A.3: Solutions, Distillation and Chromatography

  • A Solution is a homogenous mixture in which a solute is dissolved in a solvent.
  • Solutions can be solids, liquids, or gases. In solutions where the solvent is liquid, the solute can be a solid, liquid, or gas.
  • Properties of liquid solutions include:
  • The solute cannot be removed by filtration.
  • The solution is clear - the solute particles are not large enough to scatter visible light.
  • Components of a solution can be separated by processes that exploit the different properties of the solutes.
  • Solubility of a solute depends on interactions between molecules of the solute and solvent. This is often summed up as 'like dissolves like', so polar or ionic solutes dissolve well in polar solvents like water, while nonpolar solutes do not.
  • Example: The solubility of very polar ammonia (NH3) in water at 20 °C at 1 atm pressure is about 500 g/kg, while nonpolar methane is about 0.025 g/kg.

  • Distillation is one technique to separate volatile liquids. Different substances have different boiling points, and will boil at different temperatures and can be separated.

  • Chromatography is a technique that exploits different polarities of molecules in a mixture to separate them.
  • It involves running a mobile 'phase' containing the molecules to be separated over a solid, stationary 'phase'. The molecules stick to the solid phase to different degrees, based on their polarity, and therefore emerge from the stationary phase at different times.
  • Two types of chromatography are:
  • Thin layer chromatography (TLC): a sheet coated with a thin layer of silica. A small amount of a mixture is 'spotted' on the bottom of the sheet. The sheet is placed in a solvent (mobile phase) and the solvent runs up the sheet of silica (stationary phase) by capillary action. Different substances travel to different heights on the sheet.
  • Column chromatography: A stationary phase of small particles of silica or some other solid, packed into a column. A mixture is placed on top of the column and a solvent is forced through. Different substances come out ('elute') at different times.



Related Links:
Chemistry
Chemistry Quizzes
AP Chemistry Notes
Conservation of Matter and Gravimetric Analysis



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