4. Qualitative Analysis
Identification of ions and gases
Tests for Cations
Differentiating Pb2+ and Al3+
Add aqueous potassium iodide. If Pb2+ is present, a bright yellow precipitate (lead(II) iodide) is formed. If Al3+ is present, no precipitate is formed and solution remains colourless.
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Tests for Anions
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Test for Gases
- An ionic compound contains a cation (positive ion) and an anion (negative ion)
- To identify a compound, you will have to describe the tests carried out and the expected observations for both cation and anion.
Tests for Cations
- Add aqueous sodium hydroxide dropwise first, then in excess.
- Add aqueous ammonia dropwise first, then in excess.
Differentiating Pb2+ and Al3+
Add aqueous potassium iodide. If Pb2+ is present, a bright yellow precipitate (lead(II) iodide) is formed. If Al3+ is present, no precipitate is formed and solution remains colourless.
Click here to download a FREE summary chart for Tests for Cations.
Tests for Anions
Click here to download a FREE summary chart for Tests for Anions.
Test for Gases
- Hydrogen: Gas extinguishes a lighted splint with a ‘pop’ sound
- Oxygen: Gas relights a glowing splint
- Ammonia: Gas turned moist red litmus blue
- Carbon dioxide: Gas forms white ppt when bubbled into limewater
- Chlorine: Gas turns moist blue litmus red, then bleaches it
- Sulfur dioxide: Gas turns acidified potassium manganate (VII) from purple to colourless OR acidified potassium dichromate(VI) from orange to green
