Halide Ions: Tests and Reactions

Halide ions (F⁻, Cl⁻, Br⁻, I⁻) are the anions formed when halogens gain an electron. Testing for halide ions and understanding their reactions with concentrated sulfuric acid are key topics for Edexcel A-Level chemistry. These reactions reveal important differences in the reducing power of the halide ions.

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Testing for Halide Ions with Silver Nitrate

The standard test for halide ions involves adding dilute nitric acid (to remove any carbonate or sulfate ions that might interfere) followed by silver nitrate solution (AgNO₃).

The silver ions react with halide ions to form insoluble silver halide precipitates:

• Ag⁺(aq) + Cl⁻(aq) → AgCl(s) — white precipitate
• Ag⁺(aq) + Br⁻(aq) → AgBr(s) — cream precipitate
• Ag⁺(aq) + I⁻(aq) → AgI(s) — yellow precipitate

Confirming with Ammonia

If the precipitate colours are difficult to distinguish, ammonia (NH₃) can be added as a further test:

Precipitate	Colour	Dilute NH₃	Concentrated NH₃
AgCl	White	Dissolves	Dissolves
AgBr	Cream	Does not dissolve	Dissolves
AgI	Yellow	Does not dissolve	Does not dissolve