Qualitative analysis
Use G3 Science Chemistry cation, anion and gas tests to identify unknowns, separating test observations from the chemical inferences they support.
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Before you begin
Use G3 Science Chemistry cation, anion and gas tests to identify unknowns, separating test observations from the chemical inferences they support.
Learning goals
- Use aqueous sodium hydroxide and aqueous ammonia to identify aluminium, ammonium, calcium, copper(II), iron(II), iron(III) and zinc ions
- Describe the tests for carbonate, chloride, nitrate and sulfate ions
- Describe the tests for ammonia, carbon dioxide, chlorine, hydrogen, oxygen and sulfur dioxide
Syllabus statements covered
- describe the use of aqueous sodium hydroxide and/or aqueous ammonia to identify the following aqueous cations through the formation of precipitates (if any) and their subsequent solubility: aluminium, ammonium (together with evolution of ammonia gas upon warming), calcium, copper(II), iron(II), iron(III) and zinc (formulae of complex ions are not required)
- describe tests to identify the following anions: carbonate (by the addition of dilute acid and subsequent use of limewater); chloride (by reaction of an aqueous solution with nitric acid and aqueous silver nitrate); nitrate (by reduction with aluminium in aqueous sodium hydroxide to ammonia and subsequent use of damp red litmus paper) and sulfate (by reaction of an aqueous solution with nitric acid and aqueous barium nitrate)
- describe tests to identify the following gases: ammonia (using damp red litmus paper); carbon dioxide (using limewater); chlorine (using damp litmus paper); hydrogen (using a burning splint); oxygen (using a glowing splint) and sulfur dioxide (using acidified potassium manganate(VII)).
Lessons
Work through them in order.
- Cation tests: precipitates and excess reagentIdentify the prescribed cations from the precipitate each forms with aqueous sodium hydroxide or aqueous ammonia and what happens in excess.
- Anion tests and reagent orderTest for carbonate, chloride, nitrate and sulfate ions, keeping reagent contamination out of the conclusion.
- Identifying gasesUse each prescribed gas test and distinguish positive evidence from a gas that has not been detected.
- Identifying an unknown from observationsCombine test results to identify an unknown, or state which identities the evidence still allows.
Practise and check
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