Identification of Anions (CIE IGCSE Chemistry: Co-ordinated Sciences (Double Award))

• Negatively charged non-metal ions are known as anions
• You must be able to describe the tests for the following ions:
  • Carbonate ions, CO₃^2– 
  • Halide ions, Cl^– , Br^– , I^– 
  • Nitrate ions, NO₃^–
  • Sulfate ions, SO₄^2– 
  • Sulfite ions, SO₃^2– 

Test for carbonate ions

• Carbonate compounds contain the carbonate ion, CO₃^2-
• The test for the carbonate ion is:
  • Add dilute acid 
  • Bubble the gas released through limewater
  • Limewater turns cloudy if the carbonate ion is present
• If a carbonate compound is present then fizzing / effervescence should be seen as CO₂ gas is produced, which forms a white precipitate of calcium carbonate when bubbled through limewater:

CO₃^2- (aq) + 2H⁺ (aq) → CO₂ (g) + H₂O (l)

CO₂ (g) + Ca(OH)₂ (aq) → CaCO₃(s) + H₂O(l)

• The white precipitate turns limewater cloudy 

Testing for carbonate ions

Test for halide ions

• Halide ions are the negative ions / anions formed by the elements in Group 7 
• The test for the halide ions is:
  • Acidify the sample with nitric acid
  • Add silver nitrate solution, AgNO₃,  
  • A silver halide precipitate forms if a halide ion is present
    • The precipitate is indicated by the state symbol (s)
• The colour of the silver halide precipitate depends on the halide ion:
  • The chloride ion forms a white precipitate of silver chloride 

potassium chloride +  silver nitrate   →  potassium nitrate + silver chloride 

KCl (aq)   +     AgNO₃ (aq)   →  KNO₃ (aq)  +  AgCl (s) 

• • The bromide ion forms a cream precipitate of silver bromide 

potassium bromide +  silver nitrate   →  potassium nitrate + silver bromide

KBr (aq)   +     AgNO₃ (aq)   →  KNO₃ (aq)  +  AgBr (s) 

• • The iodide ions forms a yellow precipitate of silver iodide 

potassium iodide +  silver nitrate   →  potassium nitrate + silver iodide

KI (aq)   +     AgNO₃ (aq)   →  KNO₃ (aq)  +  AgI (s) 

Testing for halide ions

Test for nitrate ions

• Nitrate compounds contain the nitrate ion, NO₃^–
• The test for the nitrate ion is
  • Add aqueous NaOH and aluminium foil
  • Warm gently and test the gas released
  • The gas given off is ammonia, NH₃
• Ammonia is a gas with a characteristic sharp choking smell that turns damp red litmus paper blue

Test for sulfate ions

• Sulfate compounds contain the sulfate ion, SO₄^2-
• The test for the sulfate ion is:
  • Acidify the sample with dilute hydrochloric acid 
  • Add a few drops of barium chloride solution
  • A white precipitate of barium sulfate is formed, if the sulfate ion is present

Ba²⁺ (aq) + SO₄^2- (aq) → BaSO₄ (s)

• The test can also be carried out with barium nitrate solution

Testing for sulfate ions