separating mixtures

• Compound


  Composed of two or more elements
• Formation of compounds


  1. Formed from elements by chemical reactions

  2. Chemical reactions always involve the formation of one or more new substances

  3. Often involve a detectable energy change
• Separation of compounds


  Can only be separated into elements by chemical reactions
• Mixture


  Two or more elements or compounds not chemically combined together
• The chemical properties of each substance in a mixture are unchanged
• Compounds


  Have to undergo chemical reactions to be separated into their constituent elements
• Mixtures


  Can be separated by physical processes
• Physical processes are NOT the same as chemical reactions
• No new substances are made via physical processes
• Physical processes


  • Simple distillation
  • Fractional distillation
  • Chromatography
  • Crystallisation
  • Filtration
• Distillation


  1. Separates liquid mixtures based on the different boiling points of the components

  2. The mixture is heated and cooled

  3. Simple distillation - can purify a liquid

  4. Fractional distillation - separates numerous liquid fractions in a mixture
• Liquids have different boiling points
• The liquid with the lowest boiling point will evaporate first
• The temperature shown on the thermometer when the liquid is received is its boiling point
• Filtration


  1. Separates insoluble solid - liquid mixture

  2. Can be a means of purifying a liquid as it removes impurities
• Crystallisation


  1. Used when there is a soluble solid within the liquid mixture

  2. The liquid mixture is slowly heated

  3. The liquid/solvent evaporates

  4. Once crystals start to form, the solid is transferred to an evaporating dish and left to slowly evaporate

  5. Crystals will form in a few days

  6. If there is liquid present, filter the crystals out of the solution

  7. Pat the crystals dry with the filter paper
• Crystallisation is a slow process
• When salt and sand are combined, they both maintain their chemical properties since no chemical reaction has occurred
• When oxygen gas reacts with hydrogen gas


  Water is made
• Both oxygen and hydrogen gases are colourless, but oxygen is reactive and hydrogen is flammable
• Chromatography


  1. Used to separate mixtures

  2. Can help identify substances

  3. Many different types: paper chromatography, thin layer chromatography (TLC), gas chromatography, column chromatography
• Paper Chromatography


  1. Draw a horizontal pencil line 2 cm from the bottom short edge of the chromatography paper

  2. Use a pipette to spot one drop of the liquid

  3. Clip the top short edge of the paper to the wooden spill

  4. Pour ethanol into a beaker and place a watch glass over it

  5. Carefully rest the wooden spill on the top edge of the beaker

  6. Once the solvent has travelled 3/4 of the way up the paper, remove it and mark the solvent front
• Rf (Ratio to Front)


  • Different substances have different Rf values
  • Different solvents for the same substance results in different Rf values
  • The Rf value is a ratio and has no units
• Rf = distance travelled by the spot / distance travelled by the solvent front
• Limitations of Paper Chromatography: Spots can be hidden, it is a qualitative method good for comparisons if there are sufficient control variables, two substances might have the same Rf value in the same solvent