Chemical tests: aqueous ions (including magnesium)
I can perform and interpret sodium hydroxide precipitate tests for metal ions and other tests for identifying ammonium, carbonate, sulfate, and halide ions.
Chemical tests: aqueous ions (including magnesium)
I can perform and interpret sodium hydroxide precipitate tests for metal ions and other tests for identifying ammonium, carbonate, sulfate, and halide ions.
Lesson details
Key learning points
- Some metal ions form coloured precipitates in a reaction with sodium hydroxide solution.
- Ammonium ions can be identified by heating with NaOH and using damp red litmus paper, which turns blue if positive.
- A dilute acid is usually added before testing for carbonates, sulfates or halides to avoid false positives.
- Halide ions produce different coloured precipitates when silver nitrate is added to a solution containing them.
- A white precipitate is formed when barium chloride is added to an acidified solution containing sulfate ions.
Common misconception
All white precipitates in chemical tests indicate the presence of the same ion without the need for further testing.
Explain that white precipitates can indicate different ions depending on the test. For instance, barium sulfate indicates sulfate ions, silver chloride indicates chloride ions, and sodium hydroxide tests distinguish between cations.
Keywords
Precipitate - An insoluble solid formed when two solutions react together.
Ion - An ion is an atom or group of atoms with a positive or negative charge due to the loss or gain of electrons.
Cation - A positively charged ion.
Anion - A negatively charged ion.
Equipment
NaCl, CaSO₄, Na₂SO₄, BaCl₂, HCl, AgNO₃, HNO₃, pipettes, test tubes, test tube racks, limewater, delivery tubes, bungs, boiling tubes
Content guidance
- Risk assessment required - chemicals
- Risk assessment required - equipment
Supervision
Adult supervision required
Licence
This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).
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