A-Level Chemistry OCR Notes

Tests for Ions

Qualitative tests give information on the identity of ions present in a sample
To test for the presence of carbonate ions in a sample:
- Add a dilute strong acid to the sample. If carbonate ions are present CO2 (g) will evolve
- Test the gas evolved by bubbling it through limewater, if it is CO2 the limewater turns cloudy due to the formation of CaCO3
To test for the presence of halide ions in solution, add dilute nitric acid and an aqueous solution of AgNO3

Silver Halide	Colour of Precipitate	Addition of Dilute Ammonia	Addition of Concentrated Ammonia
AgCl	White	Dissolves to give a colourless solution	Dissolves to give a colourless solution
AgBr	Cream	Remains	Dissolves to give a colourless solution
AgI	Yellow	Remains	Remains

To test for the presence of sulfate ions in a sample, add barium ions.
- Ba2+ (aq) + SO42- (aq) → BaSO4 (s).
- BaSO4 is a white precipitate
To test for ammonium ions add sodium hydroxide solution and warm gently
NH4+ (aq) + OH- (aq) → NH3 (g) + H2O (l)
- Test the gas evolved with damp red litmus paper. If the sample contains ammonium ions, the red litmus paper will turn blue
To test for transition metal ions, add aqueous ammonia or sodium hydroxide dropwise
- Cu(II) → Blue precipitate
- Fe(II) or Cr (III) → Green precipitate
- Mn(II) → Brown precipitate
To distinguish Fe(II) or Cr(III) add excess ammonia
- Fe(II) → No change
- Cr(III) → Purple solution