Methods of Separating & Purifying Substances

2.5 Explain the difference between the use of ‘pure’ in chemistry compared with its everyday use and the differences in chemistry between a pure substance and a mixture
  • A mixture
    • Consists of 2 or more elements or compounds not chemically combined together
    • Chemical properties of each substance in the mixture are unchanged
  • A pure substance = a single element or compound, not mixed with any other substance
  • In everyday language, a pure substance = substance that has had nothing added to it, so it is unadulterated and in its natural state, e.g. pure milk 
2.6 Interpret melting point data to distinguish between pure substances, which have a sharp melting point and mixtures, which melt over a range of temperatures
  • Pure substances melt and boil at specific temperatures
    • This melting and boiling points data can be used to distinguish pure substances from mixtures (which melt over a range of temperatures due to them consisting of 2 or more elements or compounds)
2.7 Explain the experimental techniques for separation of mixtures by: simple distillation, fractional distillation, filtration, crystallisation, and paper chromatography
  • Simple distillation
    • Used to separate a pure liquid from a mixture of liquids
      • Works when the liquids have different boiling points
      • Commonly used to separate ethanol from water
      • (Taking the example of ethanol…) ethanol has a lower bp than water so it evaporates first. The ethanol vapour is then cooled and condensed inside the condenser to form a pure liquid.
      • Sequence of events in distillation is as follows: heating -> evaporating -> cooling -> condensing
    • Fractional distillation
      • The oil is heated in the fractionating column and the oil evaporates and condenses at a number of different temperatures.
      • The many hydrocarbons in crude oil can be separated into fractions each of which contains molecules with a similar number of carbon atoms
      • The fractionating column works continuously, heated crude oil is piped in at the bottom. The vaporised oil rises up the column and the various fractions are constantly tapped off at the different levels where they condense.
      • The fractions can be processed to produce fuels and feedstock for the petrochemical industry.
        • Many of the fuels on which we depend for our modern lifestyle, such as petrol, diesel oil, kerosene, heavy fuel oil and liquefied petroleum gases, are produced from crude oil.
        • Many useful materials on which modern life depends are produced by the petrochemical industry, such as solvents, lubricants, polymers, and detergents.
        • The vast array of natural and synthetic carbon compounds occur due to the ability of carbon atoms to form families of similar compounds.
  • Filtration
    • If you have produced e.g. a precipitate (which is an insoluble salt), you would want to separate the salt/precipitate from the salt solution.
      • You would do this by filtering the solution, leaving behind the precipitate
  • Crystallisation
    • If you were to have produced a soluble salt and you wanted to separate this salt from the solution that it was dissolved in
      • You would 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 come out of the solution and crystals will start to grow, these can then be collected and allowed to dry
    • Paper chromatography
      • Chromatography…
        • Used to separate mixtures and give information to help identify substances
        • Involves a stationary phase and a mobile phase
        • Separation depends on the distribution of substances between the phases
      • Rf value = distance moved by substance / distance moved by solvent ( / represents a dividing sign)
      • Different compounds have different Rf values in different solvents, which can be used to help identify the compounds
      • Compounds in a mixture may separate into different spots depending on the solvent but a pure compound will produce a single spot in all solvents
Paper Chromatography Analytical technique separating compounds by their relative speeds in a solvent as it spreads through paper.

 

The more soluble a substance is, the further up the paper it travels.

 

Separates different pigments in a coloured substance.

Pigment Solid, coloured substance
2.8 Describe an appropriate experiment technique to separate a mixture, knowing the properties of the components of the mixture

*use 2.7 in application for this

2.9 Describe paper chromatography as the 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 substances to move at different rates over the paper
2.10 Interpret a paper chromatogram: to distinguish between pure and impure substances, to identify substances by comparison with known substances and to identify substances by calculation and use of Rf values

*see 2.7

2.11 Core practical: Investigate the composition of inks using simple distillation and paper chromatography

*see 2.7

2.12 Describe how: waste and ground water can be made potable, including the need for sedimentation, filtration and chlorination, sea water can be made potable by using distillation and water used in analysis must not contain any dissolved salts

Purifying water
  • Water of the correct quality is essential for life. It must be free of poisonous salts and harmful microbes.
How correct quality water is produced:
  1. An appropriate source is found (usually rainwater which is stored in reservoirs)
  2. Water is passed through a mesh screen to remove large bits e.g twigs
  3. Chemicals are added to make solids and microbes stick together and sink
  4. Water is then passed through filter beds to remove smaller solids.
  5. The water is then sterilised with chlorine to kill any microbes left.
Filters and water softeners
  • Water filters contain silver and carbon to remove substances from their tap water.
  • Some people buy water softeners which contain ion exchange resins.
  • These can both improve the taste and quality of tap water.
Chlorine and fluoride

Advantages

  • Fluorine is added to water to improve dental health.
  • Chlorine is added to water to reduce microbes.

Disadvantages

  • Adding fluoride can cause cancer and bone problems.
  • People have no choice, is adding fluoride ‘mass medicating’?
Distillation
  • Pure water can be produced by distillation from sea water
  • Large amounts of energy would be needed to boil the water
  • So high costs are involved
  • Water used in analysis must not contain any dissolved salt as to not interfere with the analysis