Full Salt Analysis Scheme
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flagWhat you'll discover
- arrow_forwardUse preliminary observations (colour, dry heating, flame) to shortlist cations
- arrow_forwardApply the group reagents in the correct order and explain what each separates
- arrow_forwardInterpret the colour and form of each group precipitate
- arrow_forwardConfirm a cation with its specific confirmatory test
- arrow_forwardExplain why skipping a group can give a completely wrong conclusion
The logic of the group scheme
Dozens of cations cannot each be tested individually — so qualitative analysis sorts them into groups by solubility, using one group reagent at a time. Group I: dilute HCl precipitates the insoluble chlorides (Pb²⁺ as white PbCl₂). Group II: H₂S in acidic medium precipitates the least soluble sulphides (Cu²⁺ as black CuS). Group III: NH₄OH in presence of NH₄Cl precipitates the hydroxides (Fe³⁺ as reddish-brown Fe(OH)₃). Group IV: H₂S in ammoniacal medium gets the more soluble sulphides (Zn²⁺ as dirty-white ZnS). Group V: (NH₄)₂CO₃ precipitates the alkaline-earth carbonates (Ca²⁺ as white CaCO₃). What remains — Na⁺, K⁺, NH₄⁺, Mg²⁺ — is Group VI.
The order is everything. Each reagent is chosen so that it precipitates its own group but leaves later groups in solution. Test out of order and the logic collapses: add ammoniacal H₂S to a solution still containing Cu²⁺ and you get a Group II precipitate while believing you are testing Group IV.
Preliminary tests: free information
Before any wet chemistry, the salt itself talks. Colour: blue or blue-green suggests Cu²⁺, yellow-brown suggests Fe³⁺, pale green Fe²⁺, and a white salt rules them all out. Dry heating in a clean tube is rich in clues: CuSO₄·5H₂O turns from blue to white; zinc salts leave a residue that is yellow hot and white cold (ZnO); lead nitrate decrepitates (crackles) and gives brown NO₂ fumes; ammonium salts sublime entirely or release ammonia; calcium carbonate releases CO₂ on strong heating.
The flame test adds another column: brick-red points at calcium, blue-green at copper. None of this is proof — preliminary tests only shortlist. Their real value is telling you where in the group scheme to expect the hit, so a contradiction (flame says calcium, but a precipitate appears in Group II) warns you immediately that something is wrong.
Confirmation: the second, independent witness
A group precipitate identifies the group, not the ion — Group III contains Fe³⁺, Al³⁺ and Cr³⁺ alike. So every analysis ends with a confirmatory test specific to one cation. Pb²⁺: potassium iodide gives a brilliant yellow PbI₂ precipitate (the "golden shower" when recrystallised). Cu²⁺: excess ammonia gives the deep royal-blue [Cu(NH₃)₄]²⁺. Fe³⁺: potassium ferrocyanide gives Prussian blue, or KSCN gives blood-red. Zn²⁺: NaOH gives a white Zn(OH)₂ precipitate that redissolves in excess — the amphoteric signature. Ca²⁺: ammonium oxalate gives a white precipitate insoluble in acetic acid. NH₄⁺: warming with NaOH releases ammonia that turns moist red litmus blue, confirmed by Nessler's reagent turning brown.
The pattern to internalise: group test to locate, confirmatory test to convict. Reporting a cation on the group precipitate alone is the classic way to lose marks — and to be wrong.