Identification of Anions (Cambridge (CIE) IGCSE Combined Science): Revision Note

Identification of anions

• 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^– 

  • Sulfate ions, SO₄^2– 

Test for carbonate ions (CO₃^2-)

• Carbonate compounds contain the carbonate ion, CO₃^2-

Test

• Add dilute acid to the sample

• A gas is produced

• Bubble the gas through limewater

Result

• Limewater turns cloudy if carbon dioxide is present

• This happens because carbon dioxide reacts with limewater to form a white precipitate of calcium carbonate

Key observation

• Fizzing / effervescence when acid is added

• Limewater turns cloudy

Test for halide ions (Cl^-, Br^-, I^-)

• Halide ions are negative ions (anions) formed by Group 7 elements.

Test

• Acidify the sample with dilute nitric acid

• Add aqueous silver nitrate

Result

• A silver halide precipitate forms.
  The colour of the precipitate identifies the halide ion:

  • Chloride (Cl⁻): white precipitate (silver chloride)

  • Bromide (Br⁻): cream precipitate (silver bromide)

  • Iodide (I⁻): yellow precipitate (silver iodide)

• 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) 

Test for sulfate ions (SO₄^2-)

• Sulfate compounds contain the sulfate ion, SO₄²⁻.

Test

• Acidify the sample with dilute nitric acid

• Add aqueous barium nitrate

Result

• A white precipitate of barium sulfate forms if sulfate ions are present

Ba²⁺ (aq) + SO₄^2- (aq) → BaSO₄ (s)

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