RP7 Identifying Ions

• How do you carry out a flame test to identify metal ions?

  1. Clean a platinum wire loop by dipping it in HCl and then holding it in the blue flame until it burns without any colour

  2. Dip the loop into the sample you want to test and put it into the blue flame of the Bunsen burner

  3. Record the colour of the flame
• List precautions to take when using a Bunsen burner

  • Don't leave unattended
  • Turn off gas or leave on orange safety flame when not in use
  • Tie back long hair
  • Keep flammable chemicals away from the flame
• Lithium ions (Li+)


  Crimson flame
• Sodium ions (Na+)


  Yellow flame
• Potassium ions (K+)


  Lilac flame
• Calcium ions (Ca2+)


  Orange-red flame
• Copper ions (Cu2+)


  Green flame
• The wire must be cleaned before carrying out a flame test to remove any unwanted ions that might obscure the colour of the flame
• A flame test cannot be used when a compound contains a mixture of metal ions, as the flame colours of some ions may be hidden by the colours of other metal ions
• How can you test for carbonate ions?

  1. Add a few drops of HCl to the sample in a test tube

  2. Connect this test tube to a test tube of limewater

  3. If carbonate ions are present, carbon dioxide will be produced. Limewater will turn cloudy when CO2 is bubbled through.
• Na2CO3 + 2HCl


  CO2 + 2NaCl + H2O
• How can you test for sulfate ions?

  1. Add HCl to remove any CO3^2- ions as these will obscure the results

  2. Add a couple of drops of barium chloride

  3. If sulfate ions are present a white precipitate of barium sulfate will form
• BaCl2 + MgSO4


  BaSO4 + MgCl2
• How do you carry out a test for halide ions?

  1. Add a couple of drops of nitric acid to react with any carbonate ions which might obscure the experiment

  2. Add a couple of drops of silver nitrate

  3. Observe the colour of the precipitate
• Silver nitrate added to chloride solution


  White precipitate of silver chloride
• Silver nitrate added to bromide solution

  Cream precipitate of silver bromide
• Silver nitrate added to iodide solution


  Yellow precipitate of silver iodide
• Sodium hydroxide reacts with calcium ions

  White precipitate
• Sodium hydroxide reacts with copper(II) ions


  Blue precipitate
• Sodium hydroxide reacts with iron(II) ions

  Green precipitate
• Sodium hydroxide reacts with iron(III) ions

  Brown precipitate
• Sodium hydroxide reacts with aluminium ions


  White precipitate at first, redissolves with excess NaOH to form a colourless solution
• Sodium hydroxide reacts with magnesium ions


  White precipitate
• Given two solutions, how can you identify which contains aluminium ions and which contains magnesium ions?

  Add excess sodium hydroxide. Both will form white precipitates but the one containing aluminium ions will redissolve to form a colourless solution.