RP4

• Mg2+ cation

  • Ammonia solution: white ppt of Mg(OH)2
  • NaOH solution: white ppt of Mg(OH)2
  • sulphuric acid: no observed change
• • Ca2+ cation
  • Ammonia solution: no observation
  • NaOH solution: white ppt of Ca(OH)2
  • sulphuric acid: no observed change
• Ammonium Cation

  Test: Add sodium hydroxide solution and gently warm. Positive result: A gas that has a choking smell is evolved, this gas will turn damp red litmus paper blue.
• F- Anions

  • Acidified silver nitrate: no observed change as AgF is soluble
  • dilute ammonia solution: no observed change
  • concentrated ammonia solution: no observed change
• Cl- anions

  • Acidified silver nitrate: white ppt
  • dilute ammonia solution: white ppt dissolves
  • concentrated ammonia solution: no observed change
• The silver nitrate is acidified with nitric acid. The acid reacts with and removes other ions that may be present and react with silver nitrate to form a precipitate (OH- or CO32-).
• The precipitates of AgCl, AgBr and AgI may look similar colours, ammonia is added as a confirmable test. The test must therefore be performed in the specified order.
• The solubility's of the silver halides in ammonia decreases down the group.
• Alternative test for identification of group VII anions

  Add sulfuric acid to the solid halide compound

  Does not distinguish between HF and HCl as in both cases, there are steamy fumes

  Orange fumes of Bromine vapour

  Purple fumes of iodine vapour
• Equations for alternative test

  • NaCl (s) + H2SO4 (l) NaHSO4 (s) + HCl (g)
  • 2H+ + 2Br- + H2SO4 (l) SO2 (g) + 2H2O (l) + Br2 (l)
  • 8H+ + 8I- + H2SO4 (l) H2S (g) + 4H2O (l) + 4I2 (s)
• Testing for hydroxide ions (OH-)

  Add a small amount of solid ammonium chloride

  A gas that has a choking smell is evolved, this gas will turn damp red litmus paper blue.

  NH4+(aq) + OH-(aq) --> NH3(g) + H20(l)
• Testing for carbonate ions (CO32-)

  Add dilute hydrochloric acid, and bubble the gas evolved through limewater.

  Fizzing will occur, and the evolved gas will form a white precipitate with limewater.
• Testing for sulfate ions (SO42-)

  Add dilute hydrochloric acid followed by barium chloride solution

  A white precipitate of barium sulfate forms.
• Reaction of strontium hydroxide with sulfuric acid

  Sr(OH)2 + H2SO4 SrSO4 + 2H2O
• Aqueous silver nitrate and aqueous sodium nitrate

  1. Add any soluble chloride/bromide/iodide (HCL, HBr or HI)

  2. Observation with silver nitrate: white/cream/yellow precipitate

  3. Observation with sodium nitrate: remains a colourless solution
• Aqueous magnesium chloride and aqueous barium chloride

  1. Add soluble sulfate (H2SO4) OR NaOH / KOH

  2. Observation with magnesium chloride: remains colourless OR white precipitate

  3. Observation with barium chloride: white precipitate OR remains colourless
• Solid silver bromide and solid silver iodide

  1. Add concentrate sulfuric acid OR concentrate ammonia solution

  2. Observation with silver bromide: orange vapours OR cream precipitate dissolves

  3. Observation with silver iodide: purple vapours OR does not dissolve
• Aqueous hydrochloric acid and aqueous nitric acid

  1. Add aqueous silver nitrate

  2. Observation with hydrochloric acid: white precipitate

  3. Observation with nitric acid: remains colourless
• Ba2+ cation

  • Ammonia solution: no observed change
  • NaOH solution: no observed change
  • sulphuric acid: white ppt of BaSO4
• Br- anions

  • Acidified silver nitrate: cream ppt
  • dilute ammonia solution: no observed change
  • concentrated ammonia solution: ppt dissolves
• I- ions 


  • Acidified silver nitrate: yellow ppt
  • dilute ammonia solution: no observed change
  • concentrated ammonia solution: no observed change