Criteria of purity

• Pure substances have a definite, sharp melting point / boiling point
• Impure substances have a lower m.p and a higher b.p
• This assessment of substance purity is important, especially in food consumption, as its intake can be dangerous.
• Method to separate substances in a solvent with different solubilities

  1. Drop substance onto the start line on chromatography paper
  2. Paper placed in beaker with solvent; paper must touch the surface of the solvent; the line must be above the liquid
  3. Solvent travels up the paper by capillary action
  4. Different solubilities lead to different travel rates
• High solubility -> high travel rate
• Chromatograms are the visual outputs on the chromatography paper
• Interpreting Chromatograms:

  • Number of rings/dots = number of substances
  • If two dots travel the same distance up the paper they are the same substance.
• Retention Value: Used to identify a substance
• Retention value is calculated by the formula: Distance moved by solute / Distance moved by solvent
• Locating Agents: Used to make colorless chromatograms visible
• Process for locating agents

  • Dry paper in oven
  • Spray it with locating agent
  • Heat it for 10 minutes in oven