When identifying an unknown compound, follow this logical order:
- Observe: Note colour, state, and any obvious properties
- Flame test first: Quick visual check for Li⁺, Na⁺, K⁺, Ca²⁺, Cu²⁺
- NaOH test: For transition metal cations and Al³⁺, Ca²⁺
- Add dilute HCl: Does it fizz? If yes, CO₃²⁻ is present
- BaCl₂ + HCl: White precipitate = SO₄²⁻
- AgNO₃ + HNO₃: White/cream/yellow precipitate identifies Cl⁻/Br⁻/I⁻
- Confirm identity: Check that cation + anion combination is chemically sensible
Example: Identifying an Unknown White Solid
- Flame test → red flame (suggests Ca²⁺ or Li⁺)
- Add acid → fizzing; gas turns limewater milky → CO₃²⁻ present
- NaOH → white precipitate that does NOT redissolve (confirms Ca²⁺, not Al³⁺)
- Conclusion: Calcium carbonate (CaCO₃)
This key facts covers Sequential Testing Strategy within Tests for Ions for GCSE Chemistry. Revise Tests for Ions in Chemical Analysis for GCSE Chemistry with 20 exam-style questions and 14 flashcards. This is a high-frequency topic, so it is worth revising until the explanation feels precise and repeatable. It is section 8 of 15 in this topic. Use this key facts to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for Tests for Ions
Which reagents are used to test for carbonate ions in a solution?
Describe how sodium hydroxide solution can be used to distinguish between iron(II) ions and iron(III) ions in solution, including the expected observations.
Quick recall flashcards
How do you test for sulfate ions (SO₄²⁻)?
Add barium chloride solution + dilute HCl. White precipitate of BaSO₄ forms. Equation: Ba²⁺ + SO₄²⁻ → BaSO₄(s)
What is a precipitation reaction?
A reaction where two soluble ionic compounds react to form an insoluble precipitate. General form: A⁺(aq) + B⁻(aq) → AB(s)