Separation Techniques

Separation Techniques

1. Chromatography

   • can be used: separate dyes in ink/pigments; identify poisons, drugs and banned substances.
   • common solvent used: water or ethanol
     • the more soluble the dye is in the solvent, the further it will travel up the chromotagram
     • to choose the right solvent: the dyes must be able to dissolve in the solvent
   • Why must a pencil be used instead of a pen?
     • A pencil is used since pencil lead will not dissolve in the solvent, it will not interfere with the chromatogram results.
   • Why must the pencil line be above the solvent?
     • Since the dyes are placed on the pencil line, if the pencil line is below the solvent, the dyes can dissolve in it causing inaccurate results.
   • A pure substance gives only one spot on a chromatogram

2. Filtration

   • To separate solid from liquid
   • Residue : insoluble solid Filtrate : liquid

3. Evaporation

   • Used to separate a dissolved solid from a solid-liquid mixture
     • Only suitable for solids that will not decompose

4. Crystallization

   • To obtain pure solids from a solution
   • Why must the solution be heated?
     • To obtain a hot saturated solution.
   • Why must the crystals be rinsed?
     • To rinse off any impurities on the crystals
   • Steps
     • 1.Solution is heated to evaporate most of the solvent.
       • 2. Hot saturated solution is allowed to cool
         • 3.Pure crystals appear as solids. Cold solution is filtered to obtain crystals
           • 4. Crystals are rinsed with cold distilled water Dry crystals by pressing them between sheets of filter paper

5. Simple Distillation

   • To obtain a liquid from a solution
   • What is the purpose of using boiling stones?
     • To ensure smooth boiling
   • Why is the thermometer placed at the mouth of the condenser?
     • Ensures that the thermometer measures the boiling point of the substance that is being distilled.
   • Why is the ‘water in’ at the bottom of the condenser?
     • To ensure all the vapor is condensed into liquid.

6. Fractional Distillation

   • To separate more than 2 liquids
     • Eg: Liquid air; mixture of alcohol and water
   • Relies on the boiling point of all the liquids The liquid with the lower boiling point distillates first.
   • What is the purpose of the fractionating column?
     • To condense the liquid with the higher boiling point back into the flask.

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