Tests for Group 2 and Group 7 Ions, Hydroxides, and Gases

Qualitative Tests for Group 2 and Group 7 Compounds  

Group 2 Salts with Sulfuric Acid  

- Barium chloride: Forms a white precipitate in both 1 mol/dm³ H₂SO₄ and excess H₂SO₄.  

- Calcium bromide: Forms a slight white precipitate in both 1 mol/dm³ H₂SO₄ and excess H₂SO₄.  

- Magnesium chloride: Forms a slight white precipitate in 1 mol/dm³ H₂SO₄ but dissolves to give a colourless solution in excess H₂SO₄.  

- Strontium chloride: Forms a slight white precipitate in both 1 mol/dm³ H₂SO₄ and excess H₂SO₄.  

Safety Notes: 

- Barium salts are toxic; always wear gloves and a lab coat.  

- Concentrated ammonia is corrosive; use splash‑proof eye protection and nitrile gloves.  

- Dilute nitric acid is an irritant; concentrated sulfuric acid is corrosive.  

- Gases produced may be toxic and corrosive; always work in a fume cupboard with small quantities.  

Group 2 Ions with Hydroxides  

- Ammonium solution + Mg²⁺ → White precipitate of Mg(OH)₂.  

- Excess NaOH + Mg²⁺ → White precipitate of Mg(OH)₂.  

- Excess NaOH + Ca²⁺ → Slight white precipitate of Ca(OH)₂.  

- Excess NaOH + Sr²⁺ → White precipitate.  

- Excess H₂SO₄ + Mg²⁺ → Colourless solution.  

- Excess H₂SO₄ + Ca²⁺ → Slight white precipitate.  

- Excess H₂SO₄ + Sr²⁺ → White precipitate.  

- Excess H₂SO₄ + Ba²⁺ → White precipitate.  

Test for Hydroxide Ions  

- Using aqueous solution: Place about 1 cm depth of solution in a test tube. Test with red litmus paper or universal indicator paper. Sodium hydroxide will turn damp red litmus paper blue.  

- Using ammonia vapour: Place 5 drops of 1.0 mol/dm³ ammonia solution on filter paper inside a petri dish. Dampen red litmus paper with distilled water and place it on the opposite side. Seal with lid. Ammonia vapours will turn damp red litmus paper blue.  

Group 7 Halide Tests  

With Silver Nitrate:  

- Potassium chloride → White precipitate.  

- Potassium bromide → Cream precipitate.  

- Potassium iodide → Yellow precipitate.  

With Dilute Ammonia:  

- Potassium chloride → Colourless solution.  

- Potassium bromide → Cream precipitate.  

- Potassium iodide → Yellow precipitate.  

With Concentrated Ammonia:  

- Potassium chloride → Colourless solution.  

- Potassium bromide → Colourless solution.  

- Potassium iodide → Yellow precipitate.  

With Concentrated Sulfuric Acid:  

- Potassium chloride → White, steamy fumes.  

- Potassium bromide → Orange fumes.  

- Potassium iodide → Purple fumes and purple/black solid.  

With Blue Litmus Paper:  

- Potassium chloride → Turns red.  

- Potassium bromide → Turns red.  

- Potassium iodide → Turns red.  

Identification of Group 7 Ions  

- Place a small spatula of solid potassium chloride in a dry test tube.  

- In a fume cupboard, add a few drops of concentrated sulfuric acid.  

- Record observations and test any gas evolved with moist blue litmus or universal indicator paper.  

- Repeat with potassium bromide and potassium iodide, noting colour changes and fumes.