Testing for Halide Ions

What are halide ions?

Negatively charged ions formed from halogen atoms.

Which halide ions are tested at GCSE?

Chloride, bromide and iodide.

What reagent is used to test for halide ions?

Silver nitrate solution, after acidifying with dilute nitric acid.

Why is dilute nitric acid added before silver nitrate?

To remove interfering ions, particularly carbonate ions, that could form misleading precipitates.

What colour precipitate indicates chloride ions?

White.

What colour precipitate indicates bromide ions?

Cream.

What colour precipitate indicates iodide ions?

Yellow.

What compound forms when chloride ions react with silver ions?

Silver chloride, AgCl.

What compound forms when bromide ions react with silver ions?

Silver bromide, AgBr.

What compound forms when iodide ions react with silver ions?

Silver iodide, AgI.

What is the ionic equation for silver chloride formation?

Ag⁺ + Cl⁻ → AgCl.

What is the ionic equation for silver bromide formation?

Ag⁺ + Br⁻ → AgBr.

What is the ionic equation for silver iodide formation?

Ag⁺ + I⁻ → AgI.

Why do halide ions form precipitates with silver nitrate?

They form insoluble silver halides.

Why is hydrochloric acid not used to acidify the solution?

It introduces chloride ions, potentially producing a false positive.

Why is sulfuric acid generally avoided for acidifying the halide test?

It can introduce sulfate ions that interfere with silver-ion tests.

What is the correct order of reagents in the halide test?

Add dilute nitric acid, then silver nitrate solution.

What is a common mistake in halide tests?

Confusing white chloride, cream bromide and yellow iodide precipitates.