Separation Techniques

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Separation Techniques

• Chemistry 11

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• ALL of the following techniques (after this
  slide) only separate MIXTURES (not compounds)
• The mixtures are separated based on differences
  in physical properties, not chemical reactions
• Electrolysis could be used to separate compounds
  by breaking the bonds in the molecules

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Hand Separation / Mechanical Separation

• mechanical mixture can often be separated by hand
  or by the use of a sieve or magnet.

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Settling

• Uses gravity to pull denser particles to the
  bottom of a mixture

• Special Case Centrifugation
• Uses a machine (centrifuge) to spin the sample at
  high speeds to pull the particles down faster

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Filtration

• Allows the separation of solids from liquids or
  gases
• Ex. Air filters
• The solid which remains behind on filter paper is
  called the residue
• The liquid which passes through filter paper is
  called the filtrate

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Filtration Continued

• NOTE
• Filtration cannot be used to separate dissolved
  solids from a liquid.
• Filtration only works when the solid particles
  present are big enough to be seen smaller
  particles (such as dissolved salt) simply pass
  right through the filter paper.

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Extraction

• Two Types
• Extraction of a solid from a mechanical mixture
  of solids
• Extraction of a dissolved liquid or solid from a
  liquid solution

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1. Extraction of a solid from a mechanical
mixture of solids

• Uses a liquid to dissolve one or more of the
  solids present but leaves others undissolved
• Ideally only two solids will be present so that
  the desired solid is either
• i) Left behind, or
• ii) Dissolved and subsequently separated
• by simple evaporation of the solvent.

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A couple of definitions

• Two liquids are MISCIBLE if they are mutually
  soluble in each other in all proportions
• ie. the liquids mix together
• Two liquids are IMMISCIBLE if they are insoluble
  in each other (form two phases)
• ie. The liquids dont mix together
• Note liquids can be partially miscible

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2. Extraction of a dissolved liquid or solid from
a liquid solution

• A solvent is added that
• is immiscible with the solvent already present
• dissolves one or more desired substances from the
  solution and leaves unwanted substances behind
  (or vice versa)
• Two solvent layers form and are separated using a
  separatory funnel
• Process may need to be repeated to remove more of
  the desired component

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Distillation

• Uses differences in boiling point to separate two
  or more liquids
• The liquids are heated
• Liquid with lowest boiling point evaporates first
  
• This vapour is cooled and collected and called
  the distillate
• This process gives good results as long as the
  boiling points of the liquids are not too close
  together

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Evaporation

• separates solids or dissolved solids from liquid
  by removing the liquid
• Can wait for the liquid to evaporate or boil it
  off

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Recrystallization

• causes a dissolved solid to crystallize in a pure
  state
• commonly used to purify solids
• Dissolve solid in minimum hot solvent possible
• Cool down the solution or evaporate off the
  solvent slowly to cause the solid to reform
  without the impurities in the solid that were
  present before

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Chromatography

• Used to separate multiple dissolved solids from a
  liquid solution
• Multiple types
• We will look at paper chromatography as our
  example
• 3 components
• Mobile phase (developing solution)
• Stationary phase (paper)
• Analyte (chemicals being analyzed)

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Paper Chromatography

• analyte (dissolved solids) are put on the
  chromatography paper at a marked location
• Paper is put into a developing solution (a
  solvent that will carry the analyte up the paper)
• The different solids interact with the paper
  differently
• The solids that interact strongly are harder to
  move up the paper and are separated out first
• Weaker interacting solids travel farther up the
  paper

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• After some time passes, the paper is removed from
  the developing solution and the solvent front
  is marked
• Solvent front is the farthest point the solvent
  moved to
• By analyzing how far the dissolved solids
  traveled compared to how far the solvent front
  moved (as well as other properties) the solids
  can sometimes be identified
• Rf distance substance traveled
• distance solvent front traveled

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Paper Chromatography