Identification of Anions
Section: 12. Experimental Techniques & Chemical Analysis | Syllabus: Cambridge AS Level Physics 9702
What are Anions? Anion A negatively charged ion formed when an atom or group of atoms gains one or more electrons. Common Anions to Identify: In IGCSE Chemistry, you need to be able to identify these anions using chemical tests: Carbonate (CO₃²⁻) Chloride (Cl⁻) Bromide (Br⁻) Iodide (I⁻) Sulfate (SO₄²⁻) Nitrate (NO₃⁻) Summary of Anion Tests Anion Test Method Positive Result Carbonate CO₃²⁻ Add dilute acid; test gas produced with limewater Effervescence; limewater turns milky/cloudy Chloride Cl⁻ Add dilute nitric acid, then silver nitrate solution White precipitate (soluble in dilute ammonia) Bromide Br⁻ Add dilute nitric acid, then silver nitrate solution Cream precipitate (slightly soluble in concentrated ammonia) Iodide I⁻ Add dilute nitric acid, then silver nitrate solution Yellow precipitate (insoluble in ammonia) Sulfate SO₄²⁻ Add dilute hydrochloric acid, then barium chloride solution White precipitate (insoluble in dilute acid) Nitrate NO₃⁻ Add aluminum powder and sodium hydroxide, then warm; test gas with damp red litmus Ammonia gas produced; turns damp red litmus blue 1.
Test for Carbonate Ions (CO₃²⁻) Test Method: Add a few drops of dilute acid (e.g., dilute hydrochloric acid or dilute nitric acid) to the substance If carbonate is present, effervescence (fizzing) will occur as carbon dioxide gas is produced To confirm, bubble the gas through limewater (calcium hydroxide solution) Chemical Equation: Carbonate + Acid → Salt + Water + Carbon dioxide Example with hydrochloric acid: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂ Observation What It Indicates Effervescence (fizzing, bubbles) Carbon dioxide gas is being produced Limewater turns milky/cloudy Confirms the gas is carbon dioxide; carbonate ion is present No effervescence Carbonate ion is NOT present Why limewater turns cloudy: Carbon dioxide reacts with calcium hydroxide (limewater) to form calcium carbonate, which is insoluble and appears as a white cloudy precipitate.
CO₂ + Ca(OH)₂ → CaCO₃ + H₂O 2. Tests for Halide Ions (Cl⁻, Br⁻, I⁻) General Test Method for Halides: Dissolve the substance in water if necessary Add a few drops of dilute nitric acid (to remove any carbonate impurities that might interfere) Add a few drops of silver nitrate (AgNO₃) solution Observe the color of any precipitate formed Optional: Test solubility in ammonia solution to distinguish between halides Chemical Equations: Silver nitrate reacts with halide ions to form silver halide precipitates: Ag⁺(aq) + Cl⁻(aq) → AgCl(s) (white precipitate) Ag⁺(aq) + Br⁻(aq) → AgBr(s) (cream precipitate) Ag⁺(aq) + I⁻(aq) → AgI(s) (yellow precipitate) Halide Ion Precipitate Color Solubility in Dilute Ammonia Solubility in Concentrated Ammonia Chloride Cl⁻ White Soluble (dissolves) Soluble Bromide Br⁻ Cream (pale yellow) Insoluble Slightly soluble (partially dissolves) Iodide I⁻ Yellow (deeper than cream) Insoluble Insoluble Why use dilute nitric acid first?
Dilute nitric acid is added to: Remove carbonate ions (which would also form a precipitate with silver nitrate) Provide an acidic environment for the test Ensure no other interfering ions are present Important: Don't use dilute hydrochloric acid or dilute sulfuric acid - they would introduce chloride or sulfate ions that would interfere with the test!
Interactive Diagram: Visual comparison showing three test tubes after adding silver nitrate to solutions containing chloride, bromide, and iodide ions. Should clearly show white precipitate (AgCl), cream precipitate (AgBr), and yellow precipitate (AgI) with labeled colors.
3. Test for Sulfate Ions (SO₄²⁻) Test Method: Dissolve the substance in water if necessary Add a few drops of dilute hydrochloric acid (to remove carbonate impurities) Add a few drops of barium chloride solution (BaCl₂) Observe for the formation of a white precipitate Chemical Equation: Barium chloride reacts with sulfate ions to form barium sulfate precipitate: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) (white precipitate) Full equation example: BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq) Observation What It Indicates White precipitate forms Sulfate ion (SO₄²⁻) is present Precipitate is insoluble in dilute acid Confirms it is barium sulfate (distinguishes from other white precipitates) No precipitate Sulfate ion is NOT present Why use dilute hydrochloric acid first?
To remove carbonate ions (which would also form a white precipitate with barium chloride) To ensure the white precipitate is definitely barium sulfate Important: Don't use dilute sulfuric acid - it contains sulfate ions that would give a false positive result!
Key Feature of BaSO₄: Barium sulfate is insoluble in dilute acids, which distinguishes it from other white precipitates like barium carbonate (which would dissolve in acid). 4. Test for Nitrate Ions (NO₃⁻) Test Method: Add a small amount of aluminum powder (or Devarda's alloy) to the solution Add sodium hydroxide solution (NaOH) Warm the mixture gently (DO NOT boil) Test the gas evolved with damp red litmus paper Wh…
Interactive revision notes, videos and practice questions load below.