Elements, Compounds & Mixtures

• Element


  Substance made from only one type of atom
• Compound


  Substance made from two or more elements that have reacted chemically with each other
• 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 has a fixed melting and boiling point, but a mixture may melt or boil over a range of temperatures
• Pure substance


  A single element or compound, not mixed with any other substance
• Melting and boiling points


  Can be used to distinguish pure substances from mixtures
• Simple distillation


  1. Used to separate a solvent from a solution

  2. Solvent vapour evaporates from the solution, gas moves away and is cooled and condensed
• Fractional distillation


  1. Used to separate a pure liquid from a mixture of liquids

  2. Liquids have different boiling points

  3. Oil is heated in the fractionating column and the oil evaporates and condenses at different temperatures

  4. Fractions can be processed to produce fuels and feedstock for the petrochemical industry
• Filtration


  Used to separate a precipitate (insoluble salt) from a salt solution
• Crystallisation


  1. Used to separate a soluble salt from the solution it was dissolved in

  2. Solution is warmed to evaporate solvent, leaving a saturated solution

  3. Solution is allowed to cool, solid comes out of solution and crystals grow
• Paper chromatography


  1. Analytical technique separating compounds by their relative speeds in a solvent as it spreads through paper

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

  3. Separates different pigments in a coloured substance
• A chromatogram provides information about the composition of a mixture
• Rf value


  • Distance moved by substance / distance moved by solvent
  • Different compounds have different Rf values in different solvents, which can be used to help identify the compounds
• Investigate paper chromatography using inks/food colourings