testing for ions

• what are qualiative tests

  tell you which ions are present , but not how much
• carbonate ions - co3 2-

  sulphate ions - so4 2-

  halide ions - Cl- , Br-, I-
• carbonate ions

  • add a dilute strong acid to the suspected carbonate
  • collect any gas formed and pass it through limewater
  • fizzing/colourless gas is produced
  • gas turns limewater cloudy
• Sulphate ions

  • add dilute hydrochloric acid and barium chloride to the suspected sulphate
  • a white precipitate of barium sulphate is produced
• order of tests :

  Carbonate

  If you test for sulfate first, Ba2+ ions form white ppt

  with carbonate ions.

  It’s the only test that produces bubbles

  
  

  Sulfate

  - Once carbonate is eliminated, you can now add Ba2+

  ions and test for sulfate

  
  

  Halide

  -Silver ions react with carbonate and sulfate ions to

  produce a white precipitate. So Halide test must be

  carried out last once they are eliminated.
• Halide ions

  • halides react with silver ions to form different coloured silver halide precipitates , AgX
  • silver chloride - white precipitate , soluble in dilute ammonia
  • Silver bromide - cream precipitate , soluble in concentrated ammonia
  • silver Iodide - yellow precipitate, insoluble in dilute + concentrated ammonia
• Halide ions 2

  • dissolve suspected halide in water
  • add an aqueous solution of silver nitrate
  • note the colour of any precipitation formed
  • if the colour is hard to distinguish, add aqueous ammonia (first dilute then concentrated)
  • note solubility of precipitate in aqueous ammonia
• Ammonium , Nh4 +

  • add sodium hydroxide solution to the suspected ammonium compound and warm very gently
  • test any gas evolved with red litmus paper
  • ammonia gas turns red litmus paper blue
  • ammonia gas has a distinctive smell (hazardous - should be tested with care)
• what happens when halide ions react with silver ions

  • AgNo3 (aq) supplies Ag+(aq) ions
  • the halide compound e.g KBr, supplies Cl- (aq) or I -(aq)
  • No3- and halide was bonded with are called spectator ions , and do not need to be included in the ionic equation