Required Practical 4 - Identifying Anions & Cations (AQA A-Level Chemistry): Revision Notes

4.2.2 Required Practical 4 - Identifying Anions & Cations

Aim

To carry out simple test-tube reactions to identify the presence of different cations (positively charged ions) and anions (negatively charged ions) in a solution or solid sample using chemical tests.

Equipment

Chemicals

• 0.1 mol dm⁻³ solutions of Group 2 salts (e.g., magnesium chloride, calcium bromide)
• 1 mol dm⁻³ sodium hydroxide ($NaOH$)
• 1 mol dm⁻³ sulfuric acid ($H₂SO₄$)
• Ammonium chloride ($NH₄Cl$)
• Sodium carbonate ($Na₂CO₃$)
• Barium chloride ($BaCl₂$)
• Nitric acid (dilute and concentrated)
• Silver nitrate ($AgNO₃$)
• Ammonia solution (dilute and concentrated)
• Sodium hydroxide for testing ammonium ions
• Calcium hydroxide solution (limewater)
• Philtre paper and deionised water

Apparatus

• Test tubes (clean and dry)
• Test tube rack
• Measuring cylinders (for solution volumes)
• Dropper pipettes
• Thermometer
• Spatula (for solid samples)
• Fume cupboard (for toxic fumes)
• Litmus paper (red and blue)
• Universal indicator paper
• Bunsen burner (for gentle heating if required)
• Water bath (optional)

Risk Assessment

• Barium salts can be toxic:
  • Wear gloves when handling barium compounds.
• Hydrochloric acid and sulfuric acid are irritants:
  • Use safety goggles, gloves, and a lab coat.
• Ammonia vapours are irritating:
  • Use in a well-ventilated area or fume cupboard.
  • Avoid inhalation of vapours.
• Concentrated sulfuric acid is corrosive and produces toxic fumes:
  • Always handle in a fume cupboard with protective gear.
  • Keep away from skin and eyes.

Method

Cation Tests

Group 2 Cations

1. Sodium Hydroxide Test:

• Add 10 drops of 0.1 mol dm⁻³ Group 2 salt solution (e.g., calcium bromide) to a clean test tube.
• Add 10 drops of 1 mol dm⁻³ sodium hydroxide. Observe any precipitate.
• Continue adding $NaOH$ dropwise until in excess.
• Repeat for magnesium chloride, strontium chloride, and barium chloride.

2. Dilute Sulfuric Acid Test:

• Add 10 drops of 0.1 mol dm⁻³ Group 2 salt solution to a clean test tube.
• Add 10 drops of 1 mol dm⁻³ sulfuric acid dropwise, noting any changes.
• Continue until in excess.

Ammonium Ions

1. Add 10 drops of ammonium chloride solution to a test tube.
2. Add 10 drops of sodium hydroxide, mix, and gently warm in a water bath.
3. Use damp red litmus paper at the mouth of the tube to detect ammonia gas (turns blue if ammonium ions are present).

Anion Tests

Hydroxide Ions

1. Test a small volume (1 cm depth) with red litmus paper or a universal indicator.

• Observe any colour change (blue for hydroxide ions).

Carbonate Ions

1. Add a few drops of dilute hydrochloric acid to sodium carbonate solution in a test tube.
2. Use a delivery tube to pass any gas produced into a second test tube with limewater.

• If carbonate ions are present, limewater turns cloudy.

Sulphate Ions

1. Add dilute hydrochloric acid followed by barium chloride to the solution being tested.

• The formation of a white precipitate indicates the presence of sulphate ions.

2. Confirm by adding dilute $HCl$. The precipitate remains if sulphate ions are present.

Halide Ions

1. Add dilute nitric acid to remove interfering ions, followed by silver nitrate.

• Observe precipitate colour:
• White (chloride ions)
• Cream (bromide ions)
• Yellow (iodide ions)

2. To distinguish halides:

• Add dilute ammonia: Chloride ions dissolve.
• Add concentrated ammonia: Bromide ions dissolve.
• Iodide ions remain insoluble.

Results

Cation/Anion	Test Reagent	Observation	Conclusion
$Mg²⁺$(Magnesium)	Sodium hydroxide ($NaOH$)	White precipitate forms	Presence of $Mg²⁺$confirmed
$Ca²⁺$(Calcium)	Excess sodium hydroxide ($NaOH$)	White precipitate remains	Presence of $Ca²⁺$confirmed
$Ba²⁺$(Barium)	Dilute sulfuric acid ($H₂SO₄$)	White precipitate forms	Presence of $Ba²⁺$confirmed
$NH₄⁺$(Ammonium)	Sodium hydroxide ($NaOH$), heat, red litmus paper	Ammonia gas released, litmus turns blue	Presence of $NH₄⁺$confirmed
$OH⁻$(Hydroxide)	Red litmus paper or universal indicator	Paper turns blue	Presence of $OH⁻$confirmed
$CO₃²⁻$(Carbonate)	Dilute hydrochloric acid ($HCl$) and limewater	Effervescence; limewater turns cloudy	Presence of $CO₃²⁻$confirmed
$SO₄²⁻$(Sulphate)	Barium chloride ($BaCl₂$) after $HCl$ addition	White precipitate remains after adding HCl	Presence of $SO₄²⁻$confirmed
$Cl⁻$(Chloride)	Silver nitrate ($AgNO₃$) and dilute ammonia	White precipitate dissolves in dilute ammonia	Presence of $Cl⁻$confirmed
$Br⁻$(Bromide)	Silver nitrate ($AgNO₃$) and concentrated ammonia	Cream precipitate dissolves in concentrated ammonia	Presence of $Br⁻$confirmed
$I⁻$(Iodide)	Silver nitrate ($AgNO₃$) and concentrated ammonia	Yellow precipitate does not dissolve	Presence of $I⁻$confirmed

Errors and Improvements

• Errors:
  • Cross-contamination of reagents can lead to false positives.
  • Subjective interpretation of colour changes.
• Improvements:
  • Use freshly cleaned test tubes for each reaction to avoid contamination.
  • Conduct halide tests in bright lighting for better observation of subtle colour differences.