3.1.4 qualitative analysis

• Test for positive ions means test for ammonium ion NH4+
• NH4+ (aq) + OH- (aq) --> NH3 (g) + H2O (l)
• Ammonium ion test

  • Add few drops of sodium hydroxide and heat it
  • (Gas is given off which has pungent smell)
  • Add damp litmus paper to ammonia gas
  • Positive turns from red to blue
• Ba2+ (aq) + SO4 2- (aq) --> BaSO4 (s)
• Sulphate ions

  • Add HCl or nitric acid
  • Add BaCl2 to the solution
  • If positive a white precipitate will form
• X- (aq) + Ag+ (aq) --> AgX (s)
• Halide ions

  • Add few drops of nitric acid
  • Add silver nitrate
  • If positive a silver halide precipitate forms
• Ammonia can be added as the different silver halides as they have different solubilities in ammonia
• Cl- dissolves in dilute NH3 solution
• Br- dissolves in concentrated NH3 solution
• I - is insoluble in ammonia
• CO3 2- (aq) + 2H+ (aq) --> CO2 (g) + H2O (l)
• Carbonate ions

  • Add hydrochloric acid or nitric acid
  • If fizzes it's positive as CO2 has is made
  • CO2 will turn limewater cloudy
• Test carbonate then sulfate then halide
• bond enthalpy decreases down the group (group 7)
• heat up the mixture to speed up the reaction