Identification of Cations
Section: 12. Experimental Techniques & Chemical Analysis | Syllabus: Cambridge AS Level Physics 9702
What are Cations? Cation A positively charged ion formed when an atom or group of atoms loses one or more electrons. Common Cations to Identify: In IGCSE Chemistry, you need to be able to identify these metal cations and the ammonium ion using chemical tests: Aluminum (Al³⁺) Calcium (Ca²⁺) Copper(II) (Cu²⁺) Iron(II) (Fe²⁺) Iron(III) (Fe³⁺) Zinc (Zn²⁺) Ammonium (NH₄⁺) Summary of Cation Tests Cation Add NaOH (few drops) Add NaOH (excess) Flame Test Color Aluminum Al³⁺ White precipitate Precipitate dissolves (forms colorless solution) No distinctive color Calcium Ca²⁺ White precipitate Insoluble (precipitate remains) Brick red/orange-red Copper(II) Cu²⁺ Blue precipitate Insoluble (precipitate remains) Blue-green Iron(II) Fe²⁺ Green precipitate Insoluble (precipitate remains) No distinctive color Iron(III) Fe³⁺ Brown/rusty precipitate Insoluble (precipitate remains) No distinctive color Zinc Zn²⁺ White precipitate Precipitate dissolves (forms colorless solution) No distinctive color Ammonium NH₄⁺ No precipitate; ammonia gas released when warmed Ammonia gas (pungent smell; turns damp red litmus blue) Not applicable Test Using Sodium Hydroxide Solution General Method: Dissolve the compound in water to make a solution containing the cation Add a few drops of dilute sodium hydroxide (NaOH) solution Observe the color of any precipitate formed Add excess sodium hydroxide solution and observe if the precipitate dissolves What Happens: Metal cations react with hydroxide ions (OH⁻) to form metal hydroxide precipitates: Metal ion + Hydroxide ions → Metal hydroxide precipitate Example: Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s) 1.
Aluminum Ions (Al³⁺) Test Observation Add few drops of NaOH White precipitate forms Add excess NaOH White precipitate dissolves to form a colorless solution Equations: Formation of precipitate: Al³⁺(aq) + 3OH⁻(aq) → Al(OH)₃(s) Dissolving in excess NaOH: Al(OH)₃(s) + OH⁻(aq) → Al(OH)₄⁻(aq) Aluminum hydroxide is amphoteric - it reacts with both acids and bases Key Point: The white precipitate dissolves in excess NaOH.
This distinguishes aluminum from calcium (which also forms a white precipitate but doesn't dissolve in excess NaOH). 2. Calcium Ions (Ca²⁺) Test Observation Add few drops of NaOH White precipitate forms Add excess NaOH White precipitate remains (insoluble) Flame test Brick red or orange-red flame Equation: Ca²⁺(aq) + 2OH⁻(aq) → Ca(OH)₂(s) (white precipitate) Key Points: White precipitate that does NOT dissolve in excess NaOH Distinctive brick red flame color helps confirm calcium Ca(OH)₂ is slightly soluble in water (forms limewater) 3.
Copper(II) Ions (Cu²⁺) Test Observation Add few drops of NaOH Blue precipitate forms Add excess NaOH Blue precipitate remains (insoluble) Solution color Solutions containing Cu²⁺ are blue or blue-green Flame test Blue-green flame Equation: Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s) (blue precipitate) Key Points: Very distinctive blue color makes copper(II) easy to identify Blue precipitate is insoluble in excess NaOH Copper(II) solutions are typically blue or blue-green in color 4.
Iron(II) Ions (Fe²⁺) Test Observation Add few drops of NaOH Green precipitate forms Add excess NaOH Green precipitate remains (insoluble) On standing in air Green precipitate turns brown as it oxidizes to iron(III) Solution color Pale green solution Equations: Fe²⁺(aq) + 2OH⁻(aq) → Fe(OH)₂(s) (green precipitate) Oxidation in air: 4Fe(OH)₂(s) + O₂(g) + 2H₂O(l) → 4Fe(OH)₃(s) (brown) Key Points: Green precipitate distinguishes Fe²⁺ from Fe³⁺ (which is brown) The green color may turn brown on standing as iron(II) oxidizes to iron(III) in air Iron(II) solutions are typically pale green 5.
Iron(III) Ions (Fe³⁺) Test Observation Add few drops of NaOH Brown/rusty precipitate forms Add excess NaOH Brown precipitate remains (insoluble) Solution color Yellow/orange or brown solution Equation: Fe³⁺(aq) + 3OH⁻(aq) → Fe(OH)₃(s) (brown precipitate) Key Points: Brown/rusty precipitate distinguishes Fe³⁺ from Fe²⁺ (which is green) Brown precipitate is insoluble in excess NaOH Iron(III) solutions are typically yellow, orange, or brown in color 6.
Zinc Ions (Zn²⁺) Test Observation Add few drops of NaOH White precipitate forms Add excess NaOH White precipitate dissolves to form a colorless solution Equations: Formation of precipitate: Zn²⁺(aq) + 2OH⁻(aq) → Zn(OH)₂(s) Dissolving in excess NaOH: Zn(OH)₂(s) + 2OH⁻(aq) → Zn(OH)₄²⁻(aq) Zinc hydroxide is amphoteric - it reacts with both acids and bases Key Point: Zinc behaves very similarly to aluminum: Both form white precipitates with NaOH Both precipitates dissolve in excess NaOH Both hydroxides are amphoteric To distinguish them, you may need additional tests or information from the question.
7. Ammonium Ions (NH₄⁺) Test Observation Add NaOH solution No precipitate forms Warm gently with NaOH Ammonia gas (NH₃) is released - pungent smell Test gas with damp red litmus Damp red litmus turns blue (ammonia is alkaline) Equation: NH₄⁺(aq) + OH⁻(aq) → NH₃(g) + H₂O(l) …
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