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Anion tests: carbonate, sulfate and halide

Chemical analysis • Identification of ions

Key concepts

What you'll likely be quizzed about

  • Carbonate ions react with dilute acids to produce carbon dioxide gas and a salt.
  • The addition of dilute acid causes effervescence due to CO2 evolution.
  • The gas turns limewater cloudy by reacting with calcium hydroxide to form insoluble calcium carbonate.
  • Limitations include potential reactions with other acids and slow reactions with certain carbonates.
  • Proper testing involves directing the gas through limewater to confirm carbon dioxide instead of misinterpreting bubbling from other reactions.

Flashcards

Test your knowledge with interactive flashcards

Appearance of a positive sulfate test

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A white, insoluble precipitate of barium sulfate forms on a positive sulfate test.

Key notes

Important points to keep in mind

Add dilute acid to test for carbonate; effervescence indicates CO2 formation.

Confirm carbon dioxide by bubbling gas through limewater to produce cloudiness.

Acidify samples before adding barium chloride to test for sulfate to avoid false positives from carbonates.

A white, acid-insoluble precipitate with barium indicates sulfate.

Acidify with nitric acid before silver nitrate to avoid introducing chloride contamination.

Silver chloride is white, silver bromide is cream, silver iodide is yellow.

Test halide precipitate solubility with dilute ammonia then concentrated ammonia to distinguish halides.

Low concentrations and sample impurities reduce test reliability; repeat tests or use fresh reagents if results are unclear.

Observe precipitate colour in good lighting and compare with known standards for accurate identification.

Order of reagents and correct acids prevents false positives from carbonate or chloride contaminants.

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