Pure substances and mixtures

• • An element contains atoms with the same atomic number
  • A compound contains two or more elements chemically joined together
  • A mixture contains two or more different substances that are not chemically joined together
• Pure substances have a sharp melting point but mixtures melt over a range of temperatures. This difference is most easily seen when the temperature of a liquid is measured as it cools and freezes. The graph shows the cooling curve for a sample of a compound called salol.
• Filtration

  used to separate an insoluble solid from liquid. It works because the filtration paper has tiny holes in it which only lets the liquid through.

  A) solid and liquid

  B) filter paper

  C) filter funnel

  
• Crystallisation

  used to produce solid crystals from a solution, when the solution is warmed the solvent evaporates leaving crystals behind.

  A) solution

  B) evaporating basin

  C) heat

  
• Distillation

  • Simple distillation is used to separate a solvent from a solution. It is useful for producing water from salt solution.
  • Simple distillation works because the dissolved solute has a much higher boiling point than the solvent. When the solution is heated, solvent vapour evaporates from the solution. The vapour is cooled and condensed. The remaining solution becomes more concentrated as the amount of solvent in it decreases.
• Fractional distilation

  • Fractional distillation is used to separate the different liquids from a mixture of liquids. It works because the different liquids have different boiling points. When the mixture is heated:
  • vapours rise through a column which is hot at the bottom, and cold at the top
  • vapours condense when they reach a part of the column that is below the temperature of their boiling point
  • the liquid flows out of the column

  A) in

  B) out

  C) fractionating collum

  D) condenser

  
• Chromatography

  used to separate mixtures of soluble substances.Chromatography relies on two different ‘phases’:

  • the stationary phase is paper, or a thin layer of powder coated onto a glass or plastic plate
  • the mobile phase is a solvent that moves past the stationary phase, carrying different substances with it

  
• Rf values

  • Rf value=

  
• Calculating Rf value