HSC Chemistry — Module 8
Qualitative Analysis — Flashcards & Quiz
Qualitative analysis identifies WHICH ions are present in an unknown solution (not how much), and HSC Chemistry Module 8 asks you to apply systematic tests such as flame tests for metal cations, precipitate reactions with specific reagents (Cl⁻, Ba²⁺, Ag⁺), and acid-base indicators. You should be able to follow a flowchart through multiple tests and justify each step based on expected reactions and observations.
Key Points
- Flame tests identify alkali and alkaline earth metal cations by their characteristic colour: Na⁺ yellow, K⁺ lilac, Ca²⁺ orange-red, Cu²⁺ blue-green.
- Chloride test: add AgNO₃ → white precipitate of AgCl that dissolves in NH₃(aq). Bromide gives cream AgBr, iodide gives yellow AgI.
- Sulfate test: add BaCl₂(aq) → white precipitate of BaSO₄, insoluble in dilute HCl (distinguishes from carbonate which dissolves).
- Carbonate test: add dilute acid → effervescence of CO₂ that turns limewater milky.
- Systematic approach: follow a flowchart, eliminating possibilities step by step; always justify each test based on the expected reaction.
- HSC skill: write ionic equations for each observed reaction, identifying spectator ions that are not involved in the precipitate formation.
Common Mistakes to Avoid
- Confusing flame tests (cation ID) with precipitate tests (anion ID).
- Forgetting that some precipitate colours are similar — e.g. AgCl and AgBr are both white-ish.
- Adding acid before a carbonate test — CO₂ effervescence is the positive indicator.
- Missing the order in systematic analysis — flame test first for spectator cations, then precipitate tests for anions.
- Not writing balanced ionic equations for each observed reaction.
Exam Strategy
HSC Module 8 qualitative analysis questions ask you to follow a systematic test sequence and justify each step. Method: (1) flame test for metal cation, (2) precipitate tests for specific anions (Cl⁻, Br⁻, I⁻, SO₄²⁻, CO₃²⁻), (3) write ionic equations, (4) link observations to the identified ions.
Sample Flashcards
Q1: Which of the syllabus cations can be identified by a flame test, and what are its limitations?
Heating excites electrons in the metal ions; as they return to lower energy levels they emit light of characteristic wavelengths. Useful colours: Ba²⁺ — pale (apple) green; Ca²⁺ — brick red (orange-red); Cu²⁺ — blue-green. Mg²⁺, Ag⁺, Pb²⁺, Fe²⁺ and Fe³⁺ give no distinctive colour. Limitations: colours are subjective, a trace of sodium gives an intense yellow that masks other colours, and mixtures are hard to interpret.
Q2: How can precipitation reactions be used to identify the syllabus cations?
Add reagents in a planned order so that each step removes a group of ions. 1) Dilute HCl: white precipitate = Ag⁺ (AgCl) or Pb²⁺ (PbCl₂, which dissolves when heated). 2) Dilute H₂SO₄: white precipitate = Ba²⁺ (BaSO₄), Pb²⁺ or Ca²⁺ (CaSO₄, slightly soluble). 3) NaOH: Mg²⁺ gives a white Mg(OH)₂ precipitate, Cu²⁺ a blue Cu(OH)₂ precipitate, Fe²⁺ a green Fe(OH)₂ precipitate and Fe³⁺ a red-brown Fe(OH)₃ precipitate.
Q3: What are complexation reactions and how are they used in ion tests?
In a complexation reaction, a central metal ion bonds to surrounding molecules or ions (ligands) that donate lone pairs, forming a complex ion — often with a distinctive colour. Fe³⁺ + SCN⁻ → [FeSCN]²⁺ gives a blood-red solution; Fe²⁺ does not give this colour, so the test distinguishes Fe³⁺ from Fe²⁺. Adding excess ammonia to Cu²⁺ first forms a pale blue Cu(OH)₂ precipitate, which then dissolves to form the deep blue [Cu(NH₃)₄]²⁺ complex.
Q4: How are halide and sulfate ions identified?
Halides: acidify with dilute nitric acid (to remove carbonate and hydroxide, which would also precipitate), then add silver nitrate. AgCl is white, AgBr is cream and AgI is yellow. Sulfate: acidify with dilute HCl or HNO₃, then add barium chloride or barium nitrate; a white BaSO₄ precipitate confirms SO₄²⁻.
Q5: How are carbonate, hydroxide, phosphate and acetate ions identified?
Carbonate: add dilute acid — effervescence of CO₂, which turns limewater milky. Hydroxide: the solution is basic (turns red litmus blue / gives a high pH) and forms coloured precipitates with Cu²⁺ (blue) or Fe³⁺ (red-brown). Phosphate: add ammonium molybdate in nitric acid and warm — a yellow precipitate forms. Acetate: add neutral iron(III) chloride — a deep red colour forms (a complex); alternatively, warming with ethanol and concentrated H₂SO₄ gives the fruity smell of ethyl ethanoate.
Sample Quiz Questions
Q1: Barium ions give a pale (apple) green flame colour.
Answer: TRUE
Ba²⁺ gives a pale green flame, which distinguishes it from Ca²⁺ (brick red) after a sulfate precipitate has formed.
Q2: Magnesium ions give a distinctive flame colour, so a flame test is enough to identify them.
Answer: FALSE
Mg²⁺ gives no distinctive flame colour. It is identified by a white Mg(OH)₂ precipitate with NaOH after other ions have been ruled out.
Q3: Adding dilute HCl to a solution containing Ag⁺ or Pb²⁺ produces a white precipitate.
Answer: TRUE
AgCl and PbCl₂ are both white and insoluble in cold water. PbCl₂ dissolves when the mixture is heated, which distinguishes it from AgCl.
Revision Tip
Precipitate tests are colour-based recall — drill Revizi flashcards with reagent + observed colour + ion identified for the common tests.
Related Concepts
Last updated: 9 October 2026 · 6 sample flashcards · 9 sample quiz questions