Purity and Separating Mixtures

  • Created on: 03-04-17 10:20
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  • Purity and Separating Mixtures
    • A pure substance is a sample of matter that has definite chemical and physical properties and has not been contaminated by an other substances
    • Compounds
      • Pure substance composed of two or more different elements joined by chemical bonds
      • Made of elements in a specific ratio that is always the same
      • Have a chemical formula
      • Can only be separated by chemical means, not physical
    • Elements
      • Contains just one type of atom
    • Mixtures
      • Can contain both elements and compounds
      • A combination of two or more pure substances that are physically, not chemically, combined
      • Can be separated physically
      • No chemical change takes place
      • Each item retains its properties in the mixture
    • To distinguish a pure from an impure substance, you can use melting point data
      • A pure substance possesses a sharp melting point and melts over a narrow temperature range
      • An impure substance will have a wide melting point range
    • Calculating Relative Formula Mass
      • Add all the atomic masses of elements in the compound together
        • e.g. CO C=12, O=16 12 + 16 = 28
    • Empirical Formula
      • The simplest whole number ratio of the atoms in a compound
    • Methods of Separation
      • Filtration
        • A method for separating an insoluble solid from a liquid. For example, sand and water
        • 1. One beaker contains a mixture of solid and liquid, the other contains a funnel with filter paper
          • 2. The solid and liquid mixture is poured into the filter funnel
            • 3.The liquid particles are small enough to pass through the filter paper as a filtrate.
              • The solid particles are too large to pass through the filter paper and stay behind as a residue.
      • Fractional Distillation
        • Fractional distillation can be used to separate two or more liquids
          • For example, ethanol can be separated from water because the two liquids have different boiling points.
        • Fractional distillation is used to separate crude oil into useful liquids that have different boiling points
        • 1. Water and ethanol solution is heated
          • 2. The ethanol evaporates first, cools, then condenses
            • 3. The water is left behind
      • Simple Disrtillstion
        • Distillation separates a liquid from a solution. For example, water and salty water
          • This method works because the water evaporates from the solution, but is then cooled and condensed into a separate container
            • The salt does not evaporate and so it stays behind.
        • 1. Salty water is heated
          • 2. The water vapour cools in the condenser and drips into a beaker
            • 3. The water has condensed and is now in the beaker, the salt stays behind
      • Chromatography
        • Used to separate mixtures of coloured compounds, including inks, dyes and colouring agents in food.
        • Simple chromatography is carried out on paper. A spot of the mixture is placed near the bottom of a piece of chromatography paper. The paper is then placed upright in a suitable solvent, such as water.
        • A pure substance will only produce one spot on the chromatogram during paper chromatography. Two substances will be the same if they produce the same colour of spot, and their spots travel the same distance up the paper
        • Rf Values
          • = distance travelled by the component / distance travelled by the solvent
        • Mobile Phase
          • Suitable Solvent (water, ethanol)
        • Stationary Phase
          • Chromatography Paper
    • Formulation
      • A mixture that has been designed as a useful product
        • e.g. Ketchup, Aspirin
  • This involves evaporating a solution (for example salt water) to a much smaller volume and then leaving it to cool
    • As the solution cools, crystals form, and these can be obtained by filtration


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