Qualitative Analysis (Very High Yield): Tests For Anions (Cl⁻, Br⁻, I⁻, SO₄²⁻, CO₃²⁻, NO₃⁻) (Copy)

Qualitative Analysis (Very High Yield)

Tests For Anions (Cl⁻, Br⁻, I⁻, SO₄²⁻, CO₃²⁻, NO₃⁻)

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Purpose of Anion Tests in ATP

• Anion tests are used to:
  • Identify negative ions in solutions or solids
• ATP questions assess:
  • Correct reagent selection
  • Correct observations
  • Correct inferences
• Examiners are extremely strict about:
  • Order of steps
  • Observation wording
  • Use of confirmatory tests

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Core Rule for Anion Testing in ATP

• Always follow this structure:
  • Reagent added
  • Observation
  • Inference (only if asked)
• Writing chemical identity as observation:
  • Loses marks
• Missing confirmatory steps:
  • Loses marks

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General Preliminary Rule for Anion Tests

• Many anion tests require:
  • Acidification first
• Purpose of acidification:
  • Remove interfering ions
• Common acid used:
  • Dilute nitric acid (HNO₃)

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Tests for Halide Ions (Cl⁻, Br⁻, I⁻)

Reagents Used

• Dilute nitric acid
• Aqueous silver nitrate (AgNO₃)

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Test Method for Halide Ions

• Add dilute nitric acid to the sample
• Then add aqueous silver nitrate

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Chloride Ion (Cl⁻)

Observation

• A white precipitate forms

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Confirmation (With Aqueous Ammonia)

• The precipitate dissolves in dilute ammonia

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Inference

• Chloride ions are present

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Examiner Notes

• Must mention:
  • White precipitate
• Must include:
  • Solubility in dilute ammonia
• Writing “silver chloride” as observation:
  • Loses marks

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Bromide Ion (Br⁻)

Observation

• A cream precipitate forms

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Confirmation (With Aqueous Ammonia)

• The precipitate dissolves in concentrated ammonia

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Inference

• Bromide ions are present

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Examiner Notes

• Cream colour must be stated
• Do not write:
  • “Light yellow” (not accepted)

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Iodide Ion (I⁻)

Observation

• A yellow precipitate forms

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Confirmation (With Aqueous Ammonia)

• The precipitate does not dissolve in ammonia

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Inference

• Iodide ions are present

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Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change O Level And IGCSE Chemistry Full Scale Course

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Summary Table: Halide Ion Tests

Ion	Observation with AgNO₃	Ammonia Solubility
Cl⁻	White precipitate	Dissolves in dilute NH₃
Br⁻	Cream precipitate	Dissolves in conc. NH₃
I⁻	Yellow precipitate	Insoluble in NH₃

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Test for Sulfate Ion (SO₄²⁻)

Reagents Used

• Dilute hydrochloric acid
• Aqueous barium chloride

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Test Method

• Add dilute hydrochloric acid to the sample
• Then add aqueous barium chloride

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Observation

• A white precipitate forms

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Inference

• Sulfate ions are present

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Examiner Notes

• Acidification is compulsory
• Writing “barium sulfate” as observation:
  • Loses marks

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Test for Carbonate Ion (CO₃²⁻)

Reagents Used

• Dilute acid
• Limewater

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Test Method

• Add dilute acid to the sample
• Pass the gas produced into limewater

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Observations

• Effervescence is observed
• Limewater turns milky

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Inference

• Carbonate ions are present

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Examiner Notes

• Both steps must be included
• Writing “carbon dioxide gas” as observation:
  • Loses marks

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Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change O Level And IGCSE Chemistry Full Scale Course

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Test for Nitrate Ion (NO₃⁻)

Reagent Used

• Aqueous sodium hydroxide
• Aluminium foil
• Gentle heating

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Test Method (Reduction Test)

• Add aqueous sodium hydroxide to the sample
• Add aluminium foil
• Warm gently

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Observations

• A colourless gas is produced
• Gas has a pungent smell
• Damp red litmus paper turns blue

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Inference

• Nitrate ions are present

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Examiner Notes

• This is a confirmatory test
• Do not write:
  • “Ammonia gas produced” as observation

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Common Examiner Traps in Anion Tests

• Forgetting to acidify before silver nitrate
• Mixing up halide precipitate colours
• Writing inference as observation
• Missing ammonia solubility test
• Using wrong acid

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Acceptable Observation Vocabulary (Anions)

• White precipitate
• Cream precipitate
• Yellow precipitate
• Effervescence observed
• Limewater turns milky
• Colourless gas produced
• Pungent smell
• Red litmus turns blue

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Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change O Level And IGCSE Chemistry Full Scale Course

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Examiner Expectations for Anion Questions

• Examiner expects:
  • Correct reagent order
  • Correct observation wording
  • Correct confirmatory step
• Examiner penalises:
  • Missing steps
  • Incorrect colours
  • Vague language

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ATP-Focused Strategy for Anion Identification

• Memorise:
  • Reagent + colour + confirmation
• Always:
  • Acidify when required
  • Write observation first
• Never:
  • Write chemical names unless inference is asked

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Core Scientific Principle

• Anion identification relies on:
  • Formation of characteristic precipitates
  • Gas evolution tests
  • Confirmatory solubility behaviour
• Precise observation and sequence:
  • Guarantees full ATP marks