Qualitative Analysis (Very High Yield): Tests For Cations (NH₄⁺, Cu²⁺, Fe²⁺, Fe³⁺, Zn²⁺, Al³⁺, Ca²⁺) (Copy)

Qualitative Analysis (Very High Yield)

Tests For Cations (NH₄⁺, Cu²⁺, Fe²⁺, Fe³⁺, Zn²⁺, Al³⁺, Ca²⁺)

---

Purpose of Cation Tests in ATP

• Cation tests are used to:
  • Identify positive ions present in a solution or solid
• ATP questions test:
  • Correct reagent choice
  • Correct observations
  • Correct inferences
• Examiners strictly assess:
  • Observation wording
  • Logical sequence of tests
  • Correct linkage between test and ion

---

Core Rule for Cation Testing in ATP

• Always write answers in this order:
  • Reagent added
  • Observation
  • Inference (only if asked)
• Writing inference instead of observation:
  • Loses marks
• Using chemical names in observations:
  • Loses marks

---

Group Reagents Used for Cation Testing

• Aqueous sodium hydroxide (NaOH)
• Aqueous ammonia (NH₃)
• Warmth (for ammonium ion test)
• Flame tests (for calcium)

---

Test for Ammonium Ion (NH₄⁺)

Reagent Used

• Aqueous sodium hydroxide
• Gentle heating

---

Correct Test Method

• Add aqueous sodium hydroxide to the sample
• Warm the mixture gently

---

Observation

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

---

Inference

• Ammonium ions (NH₄⁺) are present

---

Examiner Notes

• Smell alone:
  • Not enough
• Litmus test:
  • Must be mentioned
• Do not write:
  • “Ammonia gas is produced” as observation

---

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

---

Test for Copper(II) Ion (Cu²⁺)

Reagent: Aqueous Sodium Hydroxide

Observation

• A blue precipitate forms

Inference

• Copper(II) ions are present

---

Reagent: Aqueous Ammonia (Few Drops)

Observation

• A blue precipitate forms

---

Reagent: Aqueous Ammonia (Excess)

Observation

• The precipitate dissolves
• A deep blue solution forms

---

Examiner Notes

• Deep blue solution:
  • Is diagnostic for Cu²⁺
• Must mention:
  • Excess ammonia
• Do not confuse:
  • Pale blue precipitate with deep blue solution

---

Test for Iron(II) Ion (Fe²⁺)

Reagent: Aqueous Sodium Hydroxide

Observation

• A green precipitate forms
• The precipitate slowly turns brown on standing

---

Reagent: Aqueous Ammonia

Observation

• A green precipitate forms
• The precipitate slowly turns brown

---

Inference

• Iron(II) ions are present

---

Examiner Notes

• Green → brown change:
  • Very high yield
• Must mention:
  • Initial green colour
• Writing only “brown precipitate”:
  • Loses marks

---

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

---

Test for Iron(III) Ion (Fe³⁺)

Reagent: Aqueous Sodium Hydroxide

Observation

• A brown precipitate forms

---

Reagent: Aqueous Ammonia

Observation

• A brown precipitate forms

---

Inference

• Iron(III) ions are present

---

Examiner Notes

• Colour must be:
  • Brown (not red, not orange)
• No dissolving in excess:
  • Mentioning dissolving is incorrect

---

Test for Zinc Ion (Zn²⁺)

Reagent: Aqueous Sodium Hydroxide (Few Drops)

Observation

• A white precipitate forms

---

Reagent: Aqueous Sodium Hydroxide (Excess)

Observation

• The precipitate dissolves
• A colourless solution forms

---

Reagent: Aqueous Ammonia (Few Drops)

Observation

• A white precipitate forms

---

Reagent: Aqueous Ammonia (Excess)

Observation

• The precipitate dissolves
• A colourless solution forms

---

Inference

• Zinc ions are present

---

Examiner Notes

• Zn²⁺ dissolves in:
  • Excess NaOH
  • Excess NH₃
• Colourless solution must be stated

---

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

---

Test for Aluminium Ion (Al³⁺)

Reagent: Aqueous Sodium Hydroxide (Few Drops)

Observation

• A white precipitate forms

---

Reagent: Aqueous Sodium Hydroxide (Excess)

Observation

• The precipitate dissolves
• A colourless solution forms

---

Reagent: Aqueous Ammonia (Few Drops)

Observation

• A white precipitate forms

---

Reagent: Aqueous Ammonia (Excess)

Observation

• The precipitate does not dissolve

---

Inference

• Aluminium ions are present

---

Examiner Notes

• Key difference from Zn²⁺:
  • Does not dissolve in excess ammonia
• Very common comparison question

---

Test for Calcium Ion (Ca²⁺)

Reagent: Aqueous Sodium Hydroxide

Observation

• A white precipitate forms

---

Reagent: Aqueous Ammonia

Observation

• No precipitate forms

---

Flame Test

Observation

• Brick-red / orange-red flame

---

Inference

• Calcium ions are present

---

Examiner Notes

• Flame test:
  • Must be included for Ca²⁺ confirmation
• Do not write:
  • “Red flame” only (too vague)

---

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

---

Summary Table: Cation Tests (Very High Yield)

Ion	NaOH (few drops)	NaOH (excess)	NH₃ (few drops)	NH₃ (excess)	Special Test
NH₄⁺	Gas on heating	–	–	–	Turns red litmus blue
Cu²⁺	Blue ppt	Insoluble	Blue ppt	Deep blue solution	–
Fe²⁺	Green ppt → brown	Insoluble	Green ppt → brown	Insoluble	–
Fe³⁺	Brown ppt	Insoluble	Brown ppt	Insoluble	–
Zn²⁺	White ppt	Dissolves	White ppt	Dissolves	–
Al³⁺	White ppt	Dissolves	White ppt	Insoluble	–
Ca²⁺	White ppt	Insoluble	No ppt	No ppt	Brick-red flame

---

Common Examiner Traps in Cation Tests

• Writing inference as observation
• Forgetting excess reagent behaviour
• Mixing Zn²⁺ and Al³⁺ results
• Using wrong flame colour
• Writing “ammonia gas” instead of observation

---

ATP-Focused Strategy for Cation Questions

• Memorise:
  • Colour of precipitates
  • Solubility in excess reagents
• Always:
  • Write observation first
  • Use correct examiner words
• When stuck:
  • Describe what you see
  • Avoid chemical names unless inference is asked

---

Core Scientific Principle

• Cation tests rely on:
  • Formation and solubility of hydroxide precipitates
• Correct identification depends on:
  • Careful observation
  • Correct reagent use
  • Precise wording
• Mastery of these tests:
  • Guarantees high ATP marks