How to Test for Anions (Edexcel IGCSE Chemistry (Modular))

Tests for 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

• 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

Limewater turns milky in the presence of carbon dixoide caused by the formation of insoluble calcium carbonate

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

Each silver halide produces a precipitate of a different colour

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 

A white precipitate of barium sulfate is a positive result for the presence of sulfate ions