Testing for Ions

OCR

GCSE

Qualitative analysis employs specific chemical protocols to identify aqueous cations and anions based on unique observable changes. Cations are distinguished via flame tests, relying on electron excitation, or precipitation reactions with sodium hydroxide to form insoluble metal hydroxides. Anion identification utilizes silver nitrate for halides, barium chloride for sulfates, and dilute acids for carbonates, producing diagnostic precipitates or effervescence. Candidates must link these macroscopic observations to the underlying formation of insoluble ionic compounds through balanced symbol and ionic equations.

Objectives

Exam Tips

Pitfalls

Key Terms

Mark Points

What You Need to Demonstrate

Key skills and knowledge for this topic

Award 1 mark for stating the flame test colour for Calcium is orange-red (reject 'red' alone which implies Lithium)
Credit the observation of a white precipitate that dissolves in excess sodium hydroxide to identify Zinc ions
Award 1 mark for specifying the addition of dilute nitric acid *before* silver nitrate when testing for halides to remove carbonate interference
Credit the ionic equation: Ag⁺(aq) + Cl⁻(aq) → AgCl(s) (state symbols are essential for the mark at Higher Tier)

Example Examiner Feedback

Real feedback patterns examiners use when marking

"You correctly identified the ion, but missed the mark for the observation—be specific (e.g., 'white precipitate' not just 'white')."
"Good recall of the reagents. Now explain *why* we add acid first—what would happen if carbonate ions were present?"
"Check your equation: you included spectator ions. Focus only on the ions that form the solid precipitate."
"You confused the test for the element (Chlorine) with the ion (Chloride). Review the difference between litmus and silver nitrate tests."

Marking Points

Key points examiners look for in your answers

Award 1 mark for stating the flame test colour for Calcium is orange-red (reject 'red' alone which implies Lithium)
Credit the observation of a white precipitate that dissolves in excess sodium hydroxide to identify Zinc ions
Award 1 mark for specifying the addition of dilute nitric acid *before* silver nitrate when testing for halides to remove carbonate interference
Credit the ionic equation: Ag⁺(aq) + Cl⁻(aq) → AgCl(s) (state symbols are essential for the mark at Higher Tier)

Examiner Tips

Expert advice for maximising your marks

💡Create a mental flowchart for anion tests: Carbonate (Acid) → Sulfate (BaCl2) → Halide (AgNO3). The order is crucial to avoid incorrect identification.
💡When asked to distinguish between Calcium and Zinc ions, focus on the addition of *excess* sodium hydroxide; only Zinc precipitate redissolves.
💡For 6-mark identification questions, structure your response clearly: Test → Reagent → Observation → Conclusion.

Common Mistakes

Pitfalls to avoid in your exam answers

Stating that 'sulfuric acid' is used to acidify the silver nitrate test (incorrect as it introduces sulfate ions which may precipitate)
Confusing the test for Chlorine gas (bleaches damp litmus) with the test for Chloride ions (white precipitate with AgNO3)
Describing precipitates vaguely as 'cloudy' or 'misty' rather than using the required term 'precipitate' with a specific colour

Key Terminology

Essential terms to know

Flame tests and electron excitation

Cation identification via precipitation (NaOH)

Anion identification (Halides, Sulfates, Carbonates)

Ionic equations and state symbols

Instrumental analysis (Flame Emission Spectroscopy)

Likely Command Words

How questions on this topic are typically asked

Describe

Identify

Explain

Write

Plan

Practical Links

Related required practicals

•{"code":"PAG 7","title":"Tests for ions","relevance":"Core practical involving flame tests and wet chemical analysis of unknown salts"}

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