Module 3: Periodic Table and Energy · Year 12
3.1.4 Qualitative analysis
Use a short sequence of ion tests, observations and ionic equations to identify common anions and ammonium ions.
What you need to know
Open a line for a quick recap. If it feels obvious, move straight to the linked practice.
3.1.4 a(i) Describe tests for carbonate, sulfate and halide ions in the correct sequence, with observations and equations. Quick revision
For OCR, the order matters: carbonate → sulfate → halide. Start by adding dilute nitric acid. If carbonate is present you get effervescence; the CO₂ turns limewater milky. Let the effervescence finish, because you need the carbonate out of the way before the sulfate test.
Next add Ba(NO₃)₂: a white BaSO₄ precipitate shows sulfate. Filter, then add AgNO₃ to the filtrate for the halide test. Using barium nitrate here is deliberate — BaCl₂ would add chloride before you test for halides and could manufacture a false result.
- 1. carbonate: dilute HNO₃; CO₂ effervescence
- 2. sulfate: Ba(NO₃)₂; white BaSO₄
- 3. filter, then halide: AgNO₃; AgCl/AgBr/AgI precipitate
3.1.4 a(ii) Describe and interpret the test for ammonium ions using warm sodium hydroxide. Quick revision
To test for NH₄⁺, you add aqueous sodium hydroxide and warm gently. If ammonium ions are present, ammonia gas is released.
Test the gas with damp red litmus paper: ammonia turns it blue. The ionic equation is worth knowing because it shows exactly what the hydroxide ions are doing.