Chemistry Notes: Separation and Purification Techniques

States of Matter and Mixtures: Methods of Separating and Purifying Substances

Purity in Chemistry vs. Everyday Use

  • Mixture:
    • Composed of two or more elements or compounds that are not chemically combined.
    • Each substance retains its original chemical properties.
  • Pure Substance (Chemistry):
    • A single element or compound, not mixed with any other substance.
  • Pure Substance (Everyday):
    • A substance with nothing added, unadulterated and in its natural state (e.g., pure milk).

Melting Point and Purity

  • Pure substances have sharp, specific melting and boiling points.
  • Mixtures melt and boil over a range of temperatures.
    • This difference allows for distinguishing between pure substances and mixtures.

Separation Techniques

Fractional Distillation

  • Used to separate a pure liquid from a mixture of liquids.
  • Based on different boiling points of the liquids.
  • Example: Separating ethanol from water.
    • Ethanol has a lower boiling point than water, so it evaporates first.
    • Ethanol vapor is cooled and condensed to form a pure liquid.
  • Sequence: Heating -> Evaporating -> Cooling -> Condensing
  • Also used to separate oil; oil is heated in a fractionating column where it evaporates and condenses at different temperatures.
  • Crude oil hydrocarbons are separated into fractions based on the number of carbon atoms.
  • The fractionating column continuously processes heated crude oil, with vaporized oil rising and condensing at different levels.
  • Fractions are tapped off and processed into fuels and feedstock for the petrochemical industry.

Simple Distillation

  • Used to separate a solvent from a solution, such as producing water from a salt solution.
  • Works because the solute has a much higher boiling point than the solvent.
  • Process:
    • Heating the solution causes the solvent vapor to evaporate.
    • The vapor is cooled and condensed.
    • The remaining solution becomes more concentrated in the solute.

Filtration

  • Used to separate an insoluble salt (precipitate) from a salt solution.
  • The solution is filtered, leaving the precipitate on the filter paper.

Crystallisation

  • Used to separate a soluble salt from a solution.
  • Process:
    • Warm the solution in an open container to evaporate the solvent, creating a saturated solution.
    • Allow the solution to cool.
    • The solid salt crystallizes out of the solution.
    • Collect the crystals and allow them to dry.

Paper Chromatography

  • Used to separate mixtures and identify substances.
  • Involves a stationary phase (paper) and a mobile phase (solvent).
  • Separation depends on the distribution of substances between the phases.
  • Rf value formula: Rf = \frac{\text{distance moved by substance}}{\text{distance moved by solvent}}
  • Different compounds have different R_f values in different solvents, aiding in identification.
  • Pure compounds produce a single spot in all solvents; mixtures may separate into multiple spots.

Identifying Substances Using Chromatography

  • Pure Substances: Should only have one spot on a chromatogram.
  • Impure Substances/Mixtures: Will show up with more than one spot.
  • Comparison with Known Substances:
    • Run paper chromatography with both known and unknown substances on the same paper.
    • If the spots are at the same height, the substances are the same.
  • Calculation of R_f Values:
    • Calculate R_f values and compare them to known values for identification.

Other keypoints on Chromatography

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

Core Practical: Ink Composition

  • Investigate ink composition using simple distillation and paper chromatography (refer to 2.7 for methods).

Determining appropriate technique to separate a mixture

*Identify which types of substance you have in the mixture and so which technique is most appropriate (from 2.7)

Making Water Potable

Potable Water

  • Suitable for drinking.
    • Low levels of microbes.
    • Low levels of contaminating substances.
    • Not the same as pure water but still safe.

Making Waste and Ground Water Potable

  1. Sedimentation: Large insoluble particles sink to the bottom.
  2. Filtration: Water is filtered through sand beds to remove small insoluble particles.
  3. Chlorination: Chlorine gas is used to kill microbes.

Making Sea Water Potable using Distillation

  1. Filter the seawater.
  2. Boil it.
  3. Cool and condense the water vapor.

Water Used in Analysis

  • Must be pure.
  • Dissolved salts could react with the substances being analyzed, leading to false results.