methods of separating and purifying substances

• a pure substance - a single element or compound not mixed with any other substance, they melt and boil at specific temperatures
• a mixture consists of two or more elements or compounds not chemically combined together
• simple distillation

  used to separate a solvent from a solution e.g. pure water from slat water

  • dissolved solute has a much higher boiling point than the solvent
  • when the solution is heated, solvent vapour evaporates from the solution, the gas moves away and is cooled and condensed
  • the remaining solution becomes more concentrated in solute as the amount of solvent decreases

  A) distillate

  B) condenser

  C) thermometer

  D) bunsen

  
• fractional distillation apparatus

  A) condenser

  B) substance

  C) fractionating column

  D) solution

  E) thermometer

  
• filtration


  used to separate an insoluble solution from a solution

  • filter the solution using filter paper and a funnel
  • the soluble solution will filter through and the insoluble solution will be left on the filter paper
• crystallisation


  used to separate a soluble salt from a solution that this salt was dissolved in

  • first warm the solution in an open container, allowing the solvent to evaporate, leaving a saturated solution
  • allow this solution to cool
  • the solid will be come out of the solution and crystals will begin to grow which can later be collected

  
• paper chromatography


  used to separate mixtures and give information to help identify substances

  • involves a stationary and mobile phase
  • the more soluble a substance the further it has moved up the paper

  Rf value = distance moved by substance / distance moved by solvent
• paper chromatography


  separation of mixtures of soluble substances by running a solvent (mobile phase) through the mixture on the paper (the paper contains the stationary phase) which causes the substance to move at different rates over the paper
• interpret a paper chromatograph

  to distinguish between pure and impure substances

  • pure should only have one spot on the diagram
  • impure substances will show up with more than one spot

  to identify substances by comparison with known values

  • carry out paper chromatography with both the known substance and one your testing, if both spots are at the same height then you know theyre the same substance

  to identify substances by calculation and use of Rf values

  • calculate the Rf values then compare them to known values for different substances
• potable water is water that has low levels of microbes and low levels of contaminating substances, its not the same as pure water but it is still safe
• making waste and ground water potable

  • sedimentation

  large insoluble particles will sink to the bottom of the water

  • filtration

  water is filtered through beds of sand which removes small insoluble particles

  • chlorination

  chlorine gas is put through water to kill microbes
• making sea water potable using distillation

  • filter the seawater
  • boil it
  • water vapour is cooled and condensed
• water used in analysis

  must be pure because any dissolved salts could react with the substances you are analysing, leaving you with a false result