Salt Analysis — NEET Notes

Qualitative analysis of inorganic salts

The word "analysis" usually means breaking something apart — but in the chemistry lab, it has a quieter meaning. Qualitative analysis is the identification of what is present, not how much. For an inorganic salt, this reduces to a pair of questions: which cation, and which anion, combined to form it? NCERT puts the principle plainly — in CuSO₄, Cu²⁺ is the cation (contributed by the base) and SO₄²⁻ is the anion (contributed by the acid); in NaCl, Na⁺ and Cl⁻ play the same roles. Find these two ions and you have named the salt.

Two ideas from physical chemistry do almost all the work. The first is the solubility product (K_sp): when the ionic product of a sparingly soluble salt exceeds K_sp, precipitation begins. The second is the common-ion effect: adding a salt that shares an ion with the precipitating species shifts equilibrium and either forces or prevents precipitation, depending on which side of K_sp you are working. Every group reagent in this chapter — dilute HCl, H₂S in dilute acid, NH₄OH with NH₄Cl, (NH₄)₂CO₃ in NH₄OH — works by exploiting one or both of these principles. The analysis is not a list of recipes; it is two equilibrium ideas applied with care.

NCERT's systematic scheme proceeds in three layers. First, preliminary examination — colour, smell, solubility, dry heating, flame test — narrows the candidates within ten or fifteen minutes. Second, anion analysis by wet tests with dilute and concentrated H₂SO₄, followed by specific confirmatory reactions for sulphate, phosphate, and borate. Third, cation analysis by sequential precipitation through six numbered groups, each defined by a single group reagent. The order matters: if you skip ahead, ions precipitate in the wrong group and the diagnosis fails.

Qualitative analysis is not a recipe — it is the solubility product and the common-ion effect applied with care.

The systematic-analysis principle, NCERT Lab Manual Unit 7

Preliminary tests — colour, smell, solubility

Before a single drop of reagent is added, the salt itself gives evidence. Physical examination notes three things: the colour of the solid, its smell, and its solubility in water. Colour alone narrows the cation considerably. A blue salt almost always contains Cu²⁺. A bright green salt suggests Ni²⁺. Pink, red, violet, or blue tones point to Co²⁺. Light pink is the signature of Mn²⁺. Light green, yellow, or brown shades indicate Fe²⁺ or Fe³⁺. White salts give no colour clue and require the full analytical sequence.

Smell is a second early clue. Ammonium salts smell of ammonia, especially when warmed. Acetates have a faint vinegar smell. Sulphides smell of rotten eggs even before any acid is added. NCERT warns explicitly: fan vapours gently towards your nose — never lean over the test tube and inhale directly.

Solubility in water and the pH of the resulting solution give the third clue. If the salt dissolves and the solution is basic, the anion is likely carbonate or sulphide. If acidic, the salt is either an acid salt or the salt of a strong acid with a weak base — and the solution should be neutralised with Na₂CO₃ before anion testing. If the salt does not dissolve in water at all, NCERT prescribes a fixed sequence of solvents: dilute HCl, then concentrated HCl, then dilute HNO₃, then aqua regia (3 parts conc. HCl : 1 part conc. HNO₃). A salt insoluble even in aqua regia is treated as insoluble.

Flame test colours

The flame test exploits a quirk of the chlorides of certain metals: heated in a non-luminous Bunsen flame, they vaporise and excite valence electrons, which then emit characteristic colours as they fall back. The procedure is fixed. Clean a platinum-wire loop in concentrated HCl, hold it in the flame until no colour remains, dip it into a paste of the salt with conc. HCl, and introduce it into the oxidising flame. Observe first with the naked eye, then through cobalt blue glass — which filters out sodium's yellow contamination and allows finer discrimination between cations.

Platinum is used because it has no colour of its own, is chemically inert, and tolerates high temperatures. NCERT lists five cations confirmable by flame colour, plus two more (Na, K) that are universally recognised:

Dry heating test

A second preliminary test heats about 0.1 g of the dry salt in a clean, dry test tube for roughly a minute. Two pieces of information emerge: the colour of the residue while hot and after cooling, and any gas evolved. NCERT's Table 7.7 catalogues the colour transitions that point to specific cations. Blue when cold, white when hot indicates Cu²⁺ (loss of water of crystallisation from CuSO₄·5H₂O). Green to dirty white or yellow indicates Fe²⁺. White to yellow indicates Zn²⁺ — yellow hot, white cold, a reversible colour change due to anion vacancies in the lattice. Pink to blue indicates Co²⁺ (loss of water from pink CoCl₂·6H₂O to blue anhydrous CoCl₂).

The gases that escape are equally diagnostic. A colourless, odourless gas turning lime water milky suggests a carbonate. Rotten-egg smell signals sulphide. Brown fumes indicate a nitrate or nitrite. Pungent ammoniacal smell points to an ammonium salt. These observations are flagged for verification by the wet anion tests that follow.

Cation analysis — the six-group scheme

Group analysis is precipitation chemistry organised in order of decreasing K_sp. Each group has a single group reagent chosen so that, at that step's conditions, the ions of that group precipitate while ions of later groups stay in solution. NCERT's master flow chart proceeds: dilute HCl precipitates Group I; H₂S in dilute HCl precipitates Group II; NH₄OH in the presence of NH₄Cl precipitates Group III; H₂S in NH₄OH precipitates Group IV; (NH₄)₂CO₃ in NH₄OH precipitates Group V; and Mg²⁺ alone defines Group VI. Ammonium ion (NH₄⁺) does not precipitate at any step — it is the zero group, tested separately by NaOH on the original solid.

Group I — lead (Pb²⁺)

Add a few drops of dilute HCl to the original solution. A white precipitate signals Group I, which contains only Pb²⁺. The reaction is straightforward — Pb²⁺ + 2Cl⁻ → PbCl₂ (white). The key feature distinguishing Group I PbCl₂ from other white precipitates is its solubility in hot water: dissolve the precipitate by warming, and the hot solution is used for confirmatory tests in three parts.

The first part is treated with potassium iodide solution — a yellow precipitate of PbI₂ forms, which dissolves in boiling water and reappears on cooling as shining golden crystals (the "golden rain" demonstration). The second part is treated with potassium chromate solution — a yellow precipitate of PbCrO₄ forms, soluble in NaOH (forming sodium tetrahydroxoplumbate(II)) but insoluble in ammonium acetate. The third part receives a few drops of alcohol and dilute H₂SO₄ — a white precipitate of PbSO₄ appears, which dissolves in ammonium acetate due to formation of the tetraacetoplumbate(II) complex.

Lead has the rare property of appearing in both Group I (as PbCl₂) and Group II (as PbS) because PbCl₂ is only sparingly insoluble — some Pb²⁺ remains in solution after Group I precipitation and is caught again by H₂S in Group II. NCERT acknowledges this overlap explicitly.

Group II — copper, lead, arsenic

If Group I gave no precipitate (or after the Group I precipitate has been removed), pass H₂S gas through the warm acidic solution. Hydrogen sulphide is a weak acid; in the presence of dilute HCl, its dissociation is suppressed by the common-ion effect, keeping sulphide concentration low. This low [S²⁻] is enough to exceed K_sp for only the most insoluble sulphides — Cu²⁺ and Pb²⁺ (black, Group II-A) and As³⁺ (yellow As₂S₃, Group II-B). Sulphides of Group IV cations (Zn, Mn, Co, Ni) have higher K_sp values and remain in solution at this acidity.

The precipitate is treated with yellow ammonium sulphide to separate the two sub-groups. Group II-A sulphides (CuS, PbS) are insoluble and remain. Group II-B sulphide (As₂S₃) dissolves, forming ammonium thioarsenate, which decomposes on acidification to give a yellow precipitate — confirming As³⁺. A canary-yellow precipitate with ammonium molybdate after treatment with concentrated HNO₃ is the second confirmatory test for arsenic.

Within Group II-A, the precipitate is boiled with dilute HNO₃. PbS dissolves as Pb(NO₃)₂, and addition of dilute H₂SO₄ with alcohol gives the white PbSO₄ precipitate — confirming Pb²⁺. CuS also dissolves in HNO₃ to give Cu(NO₃)₂; a blue solution forms on adding excess NH₄OH due to the deep-blue tetraamminecopper(II) complex [Cu(NH₃)₄]²⁺. Acidifying this blue solution with acetic acid and adding potassium ferrocyanide K₄[Fe(CN)₆] gives a chocolate-brown precipitate of Cu₂[Fe(CN)₆] — the classic confirmatory test for Cu²⁺.

Group III — iron and aluminium

After Group II is absent (or removed), boil the solution with a few drops of concentrated HNO₃ to oxidise any Fe²⁺ to Fe³⁺. Cool, add solid NH₄Cl, then excess NH₄OH until the solution smells of ammonia. A precipitate at this step belongs to Group III. NCERT lists two cations here: Fe³⁺ as reddish-brown Fe(OH)₃, and Al³⁺ as white gelatinous Al(OH)₃. (Cr³⁺ also precipitates as green Cr(OH)₃ at this step in extended schemes, but is not in the NCERT Lab Manual cation list.)

The colour of the precipitate distinguishes the cations at a glance. Reddish-brown demands the iron tests; white-gelatinous demands the aluminium tests. Dissolve the precipitate in dilute HCl, divide into two parts.

For Fe³⁺, the first part is treated with potassium ferrocyanide K₄[Fe(CN)₆] — a deep blue precipitate or coloration of Prussian blue Fe₄[Fe(CN)₆]₃ appears. The second part is treated with potassium thiocyanate KSCN — a blood-red colouration of [Fe(SCN)]²⁺ confirms ferric ion. Both reactions are NEET favourites and appear in the Coordination Compounds chapter as well.

For Al³⁺, the first part is treated with NaOH — a white gelatinous precipitate of Al(OH)₃ forms, which dissolves in excess NaOH because Al(OH)₃ is amphoteric, forming sodium meta-aluminate NaAlO₂. The second part is the famous lake test: add blue litmus solution, then NH₄OH dropwise. Al(OH)₃ re-precipitates and adsorbs the blue dye, producing a blue floating mass in a colourless solution — a distinctive image once seen, never forgotten.

Group IV — cobalt, nickel, manganese, zinc

If Group III is absent, pass H₂S gas through the ammoniacal solution. The high pH from NH₄OH increases sulphide-ion concentration enough to precipitate the next tier of sulphides — Group IV. The colour of the precipitate is the first clue: black indicates Ni²⁺ or Co²⁺, flesh (pale buff) indicates Mn²⁺, and white indicates Zn²⁺.

For Zn²⁺, dissolve the precipitate in dilute HCl by boiling. To one part, add NaOH — a white precipitate of Zn(OH)₂ forms, soluble in excess NaOH as sodium zincate Na₂ZnO₂ (zinc hydroxide is amphoteric, like aluminium hydroxide). To another part, neutralise with NH₄OH and add potassium ferrocyanide — a bluish-white precipitate of zinc ferrocyanide appears.

For Mn²⁺, dissolve in dilute HCl and add excess NaOH — a white precipitate of Mn(OH)₂ forms, which turns brown on standing due to atmospheric oxidation to hydrated MnO(OH)₂. This colour change is the simplest Mn²⁺ confirmation.

For Ni²⁺ and Co²⁺, dissolve the black precipitate in aqua regia. Ni²⁺ is confirmed by adding dimethyl glyoxime (DMG) in ammoniacal medium — a brilliant rose-red / bright-red precipitate of nickel dimethylglyoximate forms. Co²⁺ is confirmed by adding excess solid potassium nitrite KNO₂ to a neutralised acetic-acid solution — a yellow precipitate of potassium hexanitritocobaltate(III) K₃[Co(NO₂)₆] appears.

Group V — barium, strontium, calcium

If Group IV is absent, add solid NH₄Cl followed by NH₄OH, then solid (NH₄)₂CO₃. A white precipitate at this step is the alkaline earth carbonate group — BaCO₃, SrCO₃, or CaCO₃. Dissolve the precipitate by boiling with dilute acetic acid (carbonates evolve CO₂ and form acetates). Divide into three parts and preserve a fourth for the flame test.

For Ba²⁺, the first part receives potassium chromate K₂CrO₄ — a yellow precipitate of BaCrO₄ forms. The flame test on the preserved precipitate gives a grassy / apple-green flame — confirming barium.

For Sr²⁺ (when Ba is absent), the second part receives ammonium sulphate (NH₄)₂SO₄. Heat and scratch the test-tube wall with a glass rod — a white precipitate of SrSO₄ forms. The flame test gives a crimson-red flame.

For Ca²⁺ (when both Ba and Sr are absent), the third part receives ammonium oxalate (NH₄)₂C₂O₄ — a white precipitate of CaC₂O₄ forms. The flame test gives a brick-red flame, viewed greenish-yellow through cobalt blue glass.

Group V is one of the rare places in qualitative analysis where the wet test and the flame test corroborate each other — the precipitate names the cation, the flame confirms it. NEET examiners exploit this redundancy by giving one colour observation and asking which test you would perform next.

Group VI — magnesium

Magnesium is the sole Group VI cation in the NCERT scheme. If Group V is absent, MgCO₃ does not precipitate at the previous step because in the presence of NH₄⁺ ions (from the NH₄Cl added earlier), the equilibrium NH₄⁺ + CO₃²⁻ ⇌ NH₃ + HCO₃⁻ shifts to the right, lowering [CO₃²⁻] below what is needed for MgCO₃ precipitation. This is the common-ion effect again, but now used to prevent precipitation rather than cause it.

To confirm Mg²⁺, take the filtrate from Group V (or fresh original solution if all groups have been negative so far) and add disodium hydrogen phosphate Na₂HPO₄ in the presence of NH₄OH and NH₄Cl. Scratch the test-tube wall with a glass rod. A white crystalline precipitate of magnesium ammonium phosphate Mg(NH₄)PO₄ forms — sometimes only after a few minutes of warming and scratching. The crystalline (not gelatinous) texture distinguishes Mg(NH₄)PO₄ from Al(OH)₃ at sight.

Zero group (NH₄⁺) is tested separately on a pinch of the original solid, since the group reagents above use ammonium-containing solutions and would mask the test. Heat the salt with NaOH solution — ammonia gas escapes (white fumes with a glass rod dipped in conc. HCl, brown precipitate with Nessler's reagent K₂HgI₄), confirming the ammonium ion.

Anion analysis — dilute H₂SO₄ test

The anion sequence parallels the cation sequence but uses gas evolution rather than precipitation as the primary signal. Take 0.1 g of the salt in a test tube, add 1–2 mL of dilute H₂SO₄, and observe gas evolution first at room temperature, then on warming. Dilute H₂SO₄ is preferred over dilute HCl because it is non-volatile and does not introduce chloride ions that would interfere with later halide tests. Five anions reveal themselves at this step.

Anion analysis — concentrated H₂SO₄ test

If dilute H₂SO₄ gives no positive result, the second step uses concentrated H₂SO₄. Take 0.1 g of fresh salt, add 3–4 drops of conc. H₂SO₄, and observe the change cold, then warm. Concentrated sulphuric acid is both a stronger acid and an oxidising agent — it reveals halides and nitrate, which are unaffected by dilute acid.

For chloride (Cl⁻), conc. H₂SO₄ produces a colourless, pungent gas of HCl which gives dense white fumes when a glass rod dipped in NH₄OH is brought near. Confirmatory: addition of a pinch of MnO₂ produces greenish-yellow Cl₂ gas, and silver nitrate gives a curdy white AgCl precipitate soluble in NH₄OH (forming [Ag(NH₃)₂]Cl). The chromyl chloride test — salt + K₂Cr₂O₇ + conc. H₂SO₄ → red CrO₂Cl₂ vapours, absorbed in NaOH → yellow Na₂CrO₄ → acetic acid + lead acetate → yellow PbCrO₄ precipitate — is the most distinctive confirmation, but is performed sparingly because of chromium pollution.

For bromide (Br⁻), conc. H₂SO₄ gives reddish-brown Br₂ vapours, intensified by addition of MnO₂. Layer test: shake the aqueous solution with CCl₄/CHCl₃ and excess chlorine water — bromine dissolves in the organic layer giving an orange-brown colour. Silver nitrate test gives a pale yellow AgBr precipitate, dissolving in NH₄OH only with difficulty.

For iodide (I⁻), conc. H₂SO₄ gives deep violet I₂ vapours that turn starch paper blue and deposit a violet sublimate on the test-tube wall. Layer test: with CCl₄ and chlorine water, the organic layer turns violet. Silver nitrate gives a yellow AgI precipitate insoluble in NH₄OH — solubility in ammonia distinguishes AgCl (soluble), AgBr (partially soluble), and AgI (insoluble).

For nitrate (NO₃⁻), conc. H₂SO₄ alone gives light brown fumes. Add copper turnings or chips and heat — dense brown NO₂ fumes evolve and the solution turns blue from Cu²⁺ formation. The classical confirmation is the brown ring test: take 1 mL of nitrate solution, add 2 mL conc. H₂SO₄, cool under tap water, then layer freshly prepared FeSO₄ along the side of the tube. A brown ring of [Fe(NO)]SO₄ (nitroso ferrous sulphate) forms at the junction.

Sulphate, phosphate, borate

If neither dilute nor concentrated H₂SO₄ has revealed the anion, tests for sulphate, phosphate, and borate are done separately on the aqueous extract or sodium carbonate extract.

Sulphate (SO₄²⁻): acidify the water extract with dilute HCl and add BaCl₂ solution. A white precipitate of BaSO₄ forms — and crucially, this precipitate is insoluble in concentrated HCl or HNO₃. The acid-insolubility is what distinguishes BaSO₄ from BaCO₃ and BaSO₃ (both of which dissolve in dilute acid with effervescence). Lead acetate also gives a white PbSO₄ precipitate. Sulphate is the most commonly examined anion in NEET-adjacent contexts.

Phosphate (PO₄³⁻): acidify the salt solution with concentrated HNO₃ and add ammonium molybdate solution, then boil. A canary-yellow precipitate of ammonium phosphomolybdate (NH₄)₃[P(Mo₃O₁₀)₄] appears. Each oxygen of the phosphate is replaced by an Mo₃O₁₀ group, and the colour is so distinctive that the test name "canary yellow" has become diagnostic. (The same canary yellow appears for arsenate in Group II-B, so the cation context matters.)

Borate (BO₃³⁻): the classical borate test uses ethanol and concentrated H₂SO₄. Place a small amount of salt in a porcelain dish, add ethanol and a few drops of conc. H₂SO₄, mix and ignite. Volatile triethyl borate (C₂H₅O)₃B burns with a characteristic green-edged flame. Oily droplets of the ester floating on the surface combined with the green flame edge confirm borate. (Borate is not in NCERT Lab Manual Unit 7 syllabus but appears in extended schemes and NEET-style conceptual questions.)

NEET PYQ Snapshot

Salt analysis itself is a laboratory chapter and has very few standalone NEET MCQs, but the underlying chemistry — coordination complexes, halide tests, group reagents, and confirmatory colours — appears every year through related chapters. The questions below mix the few salt-analysis-flavoured questions with closely related conceptual items that test the same observations.