Principles of Qualitative Analysis — Complete Exam-Ready Notes (NEET, JEE, IIT-JAM, BITSAT, GATE, CSIR-NET, TGT/PGT)
I've spent enough years around inorganic qualitative analysis to know exactly where students lose marks — not in the big concepts, but in the small confirmatory details. Which precipitate dissolves in excess ammonia. Which sulphide survives hot dilute nitric acid. Which flame colour gets confused with another. This chapter, Principles of Qualitative Analysis, rewards precision more than memorisation, and I've built these notes to train that precision. I'll walk you through preliminary physical tests, the full acidic radical scheme, the basic radical group classification, dry tests, and then close with a large bank of practice questions.
Let me be upfront about scope: I'm covering acidic radicals — nitrate, halides other than fluoride, sulphate, sulphide, and the related anions the chapter tests — along with basic radicals across Groups I to VI, restricted to the cations this chapter actually deals with: Ag⁺, Hg₂²⁺/Hg²⁺, Cu²⁺, Pb²⁺, Bi³⁺, Fe³⁺, Cr³⁺, Al³⁺, Ca²⁺, Ba²⁺, Zn²⁺, Mn²⁺, Ni²⁺, Co²⁺ and Mg²⁺.
Table of Contents
- What Qualitative Analysis Actually Tests
- Preliminary Physical Examination
- Effect of Heating the Mixture (Dry Heating Test)
- Acidic Radical Analysis with Dilute H₂SO₄
- Acidic Radical Analysis with Concentrated H₂SO₄
- Confirmatory Tests for Halides — Chloride, Bromide, Iodide
- Chromyl Chloride Test in Detail
- Nitrate — Brown Ring Test
- Sulphate, Sulphite, Sulphide, Carbonate, Acetate, Nitrite
- Borate and Phosphate Tests
- Basic Radical Group Classification (Zero to VI)
- Test of Ammonium Ion
- Group I Cations — Ag⁺, Pb²⁺, Hg₂²⁺
- Group II Cations — Cu²⁺, Bi³⁺
- Group III Cations — Al³⁺, Cr³⁺, Fe³⁺
- Group IV Cations — Zn²⁺, Mn²⁺, Ni²⁺, Co²⁺
- Group V Cation — Ba²⁺
- Group VI Cation — Mg²⁺
- Fusion Mixture Test for Mn and Cr
- Dry Tests — Flame Test, Borax Bead, Microcosmic Salt Bead, Charcoal Cavity, Cobalt Nitrate Test
- Interference of Acid Radicals — The Guidelines You Must Not Skip
- Solved Examples from the Chapter
- Practice MCQs (From the Chapter)
- Additional Practice MCQs (40 Questions With Answers)
1. What Qualitative Analysis Actually Tests
Qualitative analysis identifies what's present in a compound rather than how much. A given salt or mixture is broken down into two categories: acidic radicals (anions) and basic radicals (cations), and each gets its own systematic scheme of separation. I always tell students not to memorise this as a list of unconnected facts — every test in this chapter exploits either a colour change, a precipitate's solubility behaviour, or a gas's specific reaction with another reagent. Once you see that pattern, half the "memorisation" disappears.
2. Preliminary Physical Examination
Before you touch a single reagent, look at the sample. Colour, smell, and density already narrow things down considerably, and examiners love testing this because it costs nothing and separates careful students from careless ones.
Colourless gases commonly encountered: H₂S, O₂, CO, CO₂, SO₂, NH₃, CH₃COOH vapour, HCl vapour.
Coloured gases: NO₂ is brown, Cl₂ is yellowish-green, Br₂ is brown, and I₂ vapour is violet. This four-item list appears constantly in mixed-gas identification questions, so know it cold.
Cation colours are worth a dedicated look too, since they let you make an educated guess before any test tube touches a reagent.
- Light green — Fe²⁺
- Deep green — Cr³⁺
- Greenish — Ni²⁺
- Blue or bluish-green — Cu²⁺ or Ni²⁺
- Pink — Co²⁺
- Light pink, flesh, or earthy colour — Mn²⁺
Smell tests come next. Rub a pinch of the mixture between your fingers with a drop of water. Ammoniacal smell points to NH₄⁺. Rotten-egg smell is the signature of S²⁻. A vinegar-like smell suggests CH₃COO⁻. Burning sulphur smell indicates sulphates (more precisely, decomposition products of sulphites), and a pungent smell can flag Br⁻.
Density gives one more clue. Salts of Hg²⁺, Pb²⁺, or Ba²⁺ tend to feel heavy, while carbonates are typically light and fluffy powders. A separate but related observation: some substances — CaCl₂, MgCl₂, ZnCl₂, and nitrites — absorb atmospheric moisture and get visibly wet. That property is called deliquescence, and I've seen it tested directly as a definition question more than once.
3. Effect of Heating the Mixture (Dry Heating Test)
Heating a solid mixture directly, without dissolving it first, tells you a surprising amount. Sublimation — a solid turning to vapour and re-solidifying without melting — happens with NH₄X salts, HgCl₂, Hg₂Cl₂, AlCl₃, As₂O₃, and Sb₂O₃, all giving white sublimates. As₂S₃ and HgI₂ give yellow sublimates instead.
Several metal oxides change colour depending on temperature, and this hot/cold colour pair is a favourite exam trick because a student who only memorises one colour gets caught out.
- ZnO — yellow when hot, white when cold
- PbO — reddish-brown when hot, yellow when cold
- HgO and PbO — black when hot, red when cold
Gas evolution on heating is equally diagnostic. Alkali nitrates release O₂. Carbonates and oxalates release CO₂, which turns lime water milky. NH₄NO₂ decomposes to give N₂ gas — worth remembering precisely because it's an exception to the usual ammonium-salt behaviour. Ordinary ammonium salts evolve NH₃, which turns red litmus blue and blackens mercurous nitrate paper. Sulphites and thiosulphates evolve SO₂, recognisable by its burning-sulphur smell; this gas turns acidified K₂Cr₂O₇ paper green and also turns lime water milky, which is why SO₂ and CO₂ get compared so often in "distinguish between" questions. Nitrites and heavy-metal nitrates evolve brown NO₂, which turns starch-iodide paper blue.
4. Acidic Radical Analysis with Dilute H₂SO₄
This is where the systematic scheme really begins. Treating the salt with dilute H₂SO₄ separates radicals into those that react (evolving a gas or showing visible change) and those that don't.
Carbonate (CO₃²⁻): brisk effervescence, colourless and odourless gas, turns lime water milky, and the milkiness clears again if you keep passing excess gas through (formation of soluble bicarbonate). This last detail — the disappearing milkiness — is exactly the kind of thing that separates a student who's memorised the reaction from one who's actually followed the logic.
Acetate (CH₃COO⁻): the solution develops a vinegar smell. On confirmatory testing, neutral aqueous solution plus FeCl₃ gives a blood-red colouration that eventually forms a brownish precipitate.
Nitrite (NO₂⁻): brown fumes appear, and on adding KI plus starch solution, the mixture turns blue — iodide is oxidised to iodine by the nitrite, and starch detects the liberated iodine.
Sulphide (S²⁻): rotten-egg smell, and the gas blackens lead acetate paper due to PbS formation.
Sulphite (SO₃²⁻): colourless gas with the pungent burning-sulphur smell, turning acidified K₂Cr₂O₇ green.
5. Acidic Radical Analysis with Concentrated H₂SO₄
Some radicals don't react with dilute acid but do react with concentrated H₂SO₄, which is a stronger dehydrating and oxidising agent.
| Radical | Observation | Confirmatory Test |
|---|---|---|
| Cl⁻ | Colourless gas, white fumes with NH₄OH | Pale green Cl₂ with MnO₂; also chromyl chloride test |
| Br⁻ | Reddish-brown fumes | Yellowish-brown Br₂ evolved with MnO₂ |
| I⁻ | Violet pungent vapours, turns starch paper blue | Sodium extract + HNO₃ + AgNO₃ gives yellow AgI, insoluble in NH₄OH |
| NO₃⁻ | Brown pungent fumes | Brown ring test |
Notice how the pattern repeats: a physical observation with the acid alone, then a separate, independent confirmatory reaction. Examiners will often give you only the first half and ask you to name the second, so learn both halves as a pair, not as isolated facts.
6. Confirmatory Tests for Halides — Chloride, Bromide, Iodide
For bromide, treating the mixture with conc. H₂SO₄ gives reddish-brown fumes, and adding a little MnO₂ with heating liberates yellow-brown Br₂ gas:
KBr + H₂SO₄ → KHSO₄ + HBr
4HBr + MnO₂ → Br₂ + 2H₂O + MnBr₂
In aqueous or sodium carbonate extract, acidifying with dilute HNO₃ and adding AgNO₃ gives a pale yellow precipitate that's partially soluble in NH₄OH.
For iodide, the same acid treatment gives pungent violet vapours turning starch paper blue:
KI + H₂SO₄ → KHSO₄ + HI
2HI + H₂SO₄ → I₂ + 2H₂O + SO₂
In extract, acidified AgNO₃ gives a yellow precipitate of AgI that stays insoluble even in NH₄OH — this insolubility is the single biggest distinguishing feature between AgI and AgBr/AgCl, and I'd flag it as one of the highest-yield facts in the whole halide section.
7. Chromyl Chloride Test in Detail
This test deserves its own space because it's asked constantly and has several steps students tend to garble. Mix the chloride salt with K₂Cr₂O₇ and heat with conc. H₂SO₄. Orange-red vapours of chromyl chloride (CrO₂Cl₂) are evolved.
4NaCl + K₂Cr₂O₇ + 6H₂SO₄ → 4NaHSO₄ + 2KHSO₄ + 3H₂O + 2CrO₂Cl₂
Pass these vapours through dilute NaOH to get a yellow solution of Na₂CrO₄:
CrO₂Cl₂ + 4NaOH → Na₂CrO₄ + 2NaCl + 2H₂O
Acidify with CH₃COOH and add lead acetate; a yellow precipitate of lead chromate confirms chloride:
Na₂CrO₄ + (CH₃COO)₂Pb → PbCrO₄ (yellow ppt.) + 2CH₃COONa
Why acetic acid specifically, and not any acid? Because PbCrO₄ is soluble in strong alkali, so you neutralise the excess NaOH with a mild acid that won't redissolve your precipitate.
8. Nitrate — Brown Ring Test
Any nitrate salt decomposed by conc. H₂SO₄ gives reddish-brown NO₂ fumes:
NaNO₃ + H₂SO₄ → NaHSO₄ + HNO₃
4HNO₃ → 4NO₂↑ + O₂↑ + 2H₂O
The confirmatory brown ring test is more elegant. Acidify the aqueous extract with dilute H₂SO₄, add freshly prepared FeSO₄ solution, then carefully add conc. H₂SO₄ down the side of the tube. A brown ring forms at the interface between the two layers.
2NaNO₃ + H₂SO₄ → NaHSO₄ + HNO₃
2HNO₃ + 6FeSO₄ + 3H₂SO₄ → 3Fe(SO₄)₃ + 2NO + 4H₂O
FeSO₄ + NO → [Fe(NO)]SO₄ (brown ring)
That brown ring is a nitroso-ferrous sulphate complex, sometimes written as [Fe(H₂O)₅NO]SO₄. One caveat worth internalising: the ring test is unreliable if nitrite, bromide, or iodide ions are also present in the mixture, since these interfere with the redox chemistry involved.
9. Sulphate, Sulphite, Sulphide, Carbonate, Acetate, Nitrite — Confirmatory Round-Up
Sulphate (SO₄²⁻): water extract or sodium extract, acidified with acetic acid, treated with lead acetate solution gives a white precipitate soluble in excess ammonium acetate on warming.
Na₂SO₄ + (CH₃COO)₂Pb → PbSO₄ + 2CH₃COONa
10. Borate and Phosphate Tests
Borate (BO₃³⁻): heat the borate salt with conc. H₂SO₄ and ethyl alcohol. Volatile ethyl borate forms and burns at the mouth of the test tube with a distinctive green-edged flame.
2Na₃BO₃ + 3H₂SO₄ → 3Na₂SO₄ + 2H₃BO₃
H₃BO₃ + 3C₂H₅OH → (C₂H₅O)₃B + 3H₂O
Phosphate (PO₄³⁻): heat with conc. HNO₃, then add excess ammonium molybdate solution. A canary-yellow precipitate of ammonium phosphomolybdate forms.
H₃PO₄ + 12(NH₄)₂MoO₄ + 21HNO₃ → (NH₄)₃PO₄·12MoO₃↓ + NH₄NO₃ + 12H₂O
An alternative confirmatory route: acidify with acetic acid, add magnesia mixture, and after about five minutes a white precipitate of magnesium ammonium phosphate appears.
11. Basic Radical Group Classification (Zero to VI)
Now we move to cations. The whole logic of cation analysis rests on selective precipitation — adding reagents in a specific order so that only certain cations precipitate at each stage, leaving the rest in solution for the next group's reagent.
| Group | Group Reagent | Cations | Precipitate |
|---|---|---|---|
| Zero | NaOH | NH₄⁺ | NH₃ gas evolved |
| I | Dil. HCl | Ag⁺, Pb²⁺, Hg₂²⁺ | White chlorides |
| II | H₂S in presence of dil. HCl | Cu²⁺, Bi³⁺ (and As, Sb, Sn if present) | Coloured sulphides |
| III | NH₄OH in presence of excess NH₄Cl | Fe³⁺, Cr³⁺, Al³⁺ | Hydroxides |
| IV | H₂S in presence of NH₄OH | Co²⁺, Ni²⁺, Zn²⁺, Mn²⁺ | Sulphides |
| V | (NH₄)₂CO₃ in presence of NH₄OH | Ba²⁺, Ca²⁺ | Carbonates |
| VI | Na₂HPO₄ | Mg²⁺ | Mg(NH₄)PO₄ |
Why does Group III use NH₄OH plus excess NH₄Cl rather than NH₄OH alone? Ammonium chloride suppresses the ionisation of NH₄OH through the common ion effect, lowering the OH⁻ concentration just enough that only the least soluble hydroxides (Fe³⁺, Cr³⁺, Al³⁺) precipitate, while Group IV and V cations stay in solution until their own group reagent arrives. This single idea — controlled ionic concentration through common-ion suppression — is really the intellectual heart of the whole classification scheme, and it's worth sitting with until it feels obvious rather than memorised.
12. Test of Ammonium Ion
NH₄⁺ gives a yellow precipitate with sodium cobaltinitrite:
3NH₄Cl + Na₃[Co(NO₂)₆] → (NH₄)₃[Co(NO₂)₆]↓ + 3NaCl
With NaOH, ammonium salts liberate NH₃ gas, testable with Nessler's reagent (an alkaline solution of K₂HgI₄), which gives a reddish-brown precipitate called iodide of Million's base.
13. Group I Cations — Ag⁺, Pb²⁺, Hg₂²⁺
AgCl dissolves in NH₄OH forming a soluble complex:
AgCl + 2NH₄OH → [Ag(NH₃)₂]Cl + 2H₂O
Acidifying this complex with HNO₃ reprecipitates AgCl. Adding KI to the complex gives pale yellow AgI, while adding K₂CrO₄ gives brick-red Ag₂CrO₄ — two distinct colour confirmations from the same starting complex, which is a nice thing to remember as a pair.
Pb²⁺ gives yellow precipitates with both potassium chromate and KI solutions — PbCrO₄ and PbI₂ respectively. If you've ever seen the "golden rain" demonstration, that's PbI₂ crystallising back out of hot solution as it cools.
Mercurous ion (Hg₂²⁺) behaves differently under NH₄OH: it converts Hg₂Cl₂ into a black residue that's actually a mixture of white aminomercuric chloride and finely divided black mercury. This black colour, formed by disproportionation rather than simple precipitation, is a classic test question. Aqua regia dissolves this residue to give soluble HgCl₂, and adding SnCl₂ afterward reduces it back — first to white Hg₂Cl₂, then further to grey metallic Hg. That grey colour progression (white turning grey) is the visual signature examiners test for.
14. Group II Cations — Cu²⁺, Bi³⁺
Cupric ion with excess ammonia gives the deep blue tetraammine copper complex:
Cu(NO₃)₂ + 4NH₄OH → [Cu(NH₃)₄](NO₃)₂ + 4H₂O
Adding potassium ferrocyanide to a cupric salt gives a reddish-brown (chocolate) precipitate of cupric ferrocyanide, insoluble in acetic acid — a detail that matters because it distinguishes this reaction from others that might redissolve under mild acid conditions.
Bi²S₃ dissolves in hot dilute HNO₃. Adding NH₄OH precipitates white Bi(OH)₃, which dissolves in HCl to give BiCl₃. Here's the elegant part: adding water to this BiCl₃ solution causes hydrolysis, giving a white turbidity of bismuth oxychloride (BiOCl) — a reaction driven purely by dilution, without any new reagent. Alkaline sodium stannite reduces bismuth further to a black precipitate of metallic Bi.
15. Group III Cations — Al³⁺, Cr³⁺, Fe³⁺
Al(OH)₃ dissolves in NaOH to form soluble sodium meta-aluminate, and boiling this with NH₄Cl reprecipitates Al(OH)₃ — a reversible pair of reactions that's tested often as a two-step identification sequence.
Cr³⁺ gives dirty green Cr(OH)₃ with NH₄OH.
Fe³⁺ chemistry is the richest part of this whole chapter, honestly, because there are so many distinct colour reactions and they're easy to confuse with each other. Let me lay them out clearly, since precision here is exactly what separates a good score from a great one.
- Fe³⁺ + thiocyanate (NH₄SCN) → blood-red ferric thiocyanate
- Fe²⁺ does NOT respond to the thiocyanate test
- Fe²⁺ + potassium ferricyanide (K₃[Fe(CN)₆]) → deep blue precipitate, called Turnbull's blue
- Fe³⁺ + potassium ferricyanide → brown colour
- Fe³⁺ + potassium ferrocyanide (K₄[Fe(CN)₆]) → deep blue Prussian blue
- Fe²⁺ + potassium ferrocyanide → white precipitate in absence of air, which oxidises to Prussian blue on exposure to air
16. Group IV Cations — Zn²⁺, Mn²⁺, Ni²⁺, Co²⁺
ZnS dissolves in HCl to regenerate ZnCl₂, which forms white Zn(OH)₂ with NaOH — soluble again in excess NaOH as sodium zincate. Passing H₂S through the zincate solution reprecipitates ZnS, completing a neat closed loop of reactions.
Mn²⁺ testing runs through several intermediate colours. MnS dissolves in HCl; NaOH treatment gives brown MnO₂ precipitate; this dissolves in conc. HNO₃; and finally, diluting the solution and adding sodium bismuthate produces a striking purple colour (permanganic acid, HMnO₄). An alternative uses PbO₂ in conc. HNO₃ with boiling — same purple end-point, different oxidising agent.
Ni²⁺ is confirmed almost entirely through one reaction that I'd call the single prettiest test in the chapter: dimethylglyoxime (DMG) added to an ammoniacal solution of Ni²⁺ gives a rosy-red precipitate of nickel dimethylglyoxime, a square-planar chelate complex. This complex's stability owes specifically to hydrogen bonding within the ring plus the chelate effect — not simply to the small size of Ni²⁺, which is a distractor option that appears in MCQs testing this exact point.
Co²⁺ reacts with KCN to give reddish-brown Co(CN)₂, which dissolves in excess KCN to form a yellowish-brown solution of potassium cobaltocyanide, K₄[Co(CN)₆]. On boiling, this oxidises to bright yellow potassium cobalticyanide, K₃[Co(CN)₆].
17. Group V Cation — Ba²⁺
BaCO₃ dissolves in hot dilute acetic acid to give soluble barium acetate. From here, several confirmatory branches open up: K₂CrO₄ gives yellow BaCrO₄ (soluble in conc. HCl, unlike many other chromates); (NH₄)₂SO₄ gives BaSO₄, insoluble even in conc. HNO₃; and ammonium oxalate gives white barium oxalate.
18. Group VI Cation — Mg²⁺
Mg²⁺ gives a white precipitate of magnesium ammonium phosphate with NH₄OH and (NH₄)₂HPO₄, or equivalently with Na₂HPO₄ and NH₄OH. This is the same phosphate chemistry we saw earlier under acidic radical testing, just approached from the cation side.
19. Fusion Mixture Test for Mn and Cr
Fusing a salt with a mixture of Na₂CO₃ and KNO₃ produces distinctive melt colours: green indicates Mn, yellow indicates Cr.
MnSO₄ + 2KNO₃ + 2Na₂CO₃ → Na₂MnO₄ (green) + 2KNO₂ + Na₂SO₄ + 2CO₂
Cr₂(SO₄)₃ + 5Na₂CO₃ + 3KNO₃ → 2Na₂CrO₄ (yellow) + 3KNO₂ + 5CO₂ + 3Na₂SO₄
20. Dry Tests — Flame Test, Borax Bead, Microcosmic Salt Bead, Charcoal Cavity, Cobalt Nitrate Test
Dry tests don't need a solution at all, which makes them fast — and that speed is exactly why examiners love testing the fine print around them.
Flame test: the non-luminous (upper) part of the flame is oxidising and hottest, around 1550°C in a Bunsen flame, and it's used for the actual flame test. The luminous (lower/middle) part contains carbon particles and is used for the charcoal cavity test instead. Characteristic colours worth memorising as a block:
- Pb — pale greenish
- Cu and its salts — blue or green
- Borates — green
- Ba — apple green
- Sr — crimson red
- Ca — brick red
- Na — yellow
- K — pink-violet (lilac)
- As, Sb, Bi — livid blue
Borax bead test: heating borax on platinum wire gives a transparent glassy bead of NaBO₂ + B₂O₃. This bead reacts with metal oxides to give characteristically coloured metaborates, and — this is the part students forget — the colour often differs between the oxidising flame and the reducing flame, and sometimes between hot and cold states of the same bead. Copper, for instance, is green in the oxidising flame but blue when cold in the reducing flame, turning red-brown (copper metal) when hot in the reducing flame. That's three different colours for one element depending on conditions, so read bead-test questions carefully for exactly which flame and which temperature they're asking about.
Microcosmic salt bead test: conceptually parallel to the borax bead, but uses sodium ammonium hydrogen phosphate, which decomposes on heating to give glassy NaPO₃. This reacts with metal oxides to form coloured orthophosphates — cobalt gives blue, chromium gives green, iron gives yellow, and so on.
Charcoal cavity test: the mixture is heated with fusion mixture (Na₂CO₃ + K₂CO₃) in a cavity carved into charcoal, under the reducing flame. The colour of the residue and any metallic bead formed both carry diagnostic value. Cadmium gives a brown residue at both hot and cold with no metallic bead. Lead gives brown-when-hot, yellow-when-cold with a grey bead that marks paper. Copper gives red bead or scales. Silver gives a shining white bead with no characteristic residue.
Cobalt nitrate test: a drop of cobalt nitrate solution added to the white residue left in the charcoal cavity, then heated in oxidising flame, gives colour clues — blue infusible residue indicates Al³⁺, blue fusible residue indicates PO₄³⁻ or BO₃³⁻, greenish indicates Zn²⁺, and pinkish indicates Mg²⁺.
21. Interference of Acid Radicals — The Guidelines You Must Not Skip
This is where a lot of otherwise well-prepared students lose marks, because these "guideline" points aren't reactions to memorise so much as logical consequences of the scheme's structure. I'll go through the ones that matter most.
Certain acid radicals — C₂O₄²⁻, F⁻, BO₃³⁻, and PO₄³⁻ — can interfere in basic radical analysis after Group II. Up through Group II, the medium stays strongly acidic because of the HCl present, so oxalates, fluorides, borates, and phosphates of Group III, IV, V cations, plus Mg²⁺, all remain soluble under those acidic conditions. But once you reach Group III and add NH₄OH to make the medium alkaline, these same anions suddenly form precipitates. The practical consequence: interfering radicals can cause premature precipitation of Group IV, V, and Mg²⁺ cations right there in Group III, instead of waiting for their proper group reagent. If you ever see an odd precipitate showing up "too early" in a systematic scheme question, this interference is very likely the reason being tested.
A few more guidelines worth holding onto:
- HgS resists dissolution in 50% HNO₃, while PbS, Bi₂S₃, CuS, and CdS all dissolve, forming their respective nitrates. This single fact is what lets you separate Hg from the rest of Group II using a controlled nitric acid treatment.
- Cu²⁺ and Cd²⁺ separate cleanly using KCN: both form cyanide complexes (K₃[Cu(CN)₄] and K₂[Cd(CN)₄]), but on passing H₂S gas, only the cadmium complex decomposes to give yellow CdS, while the copper complex stays intact.
- Before testing for acetate with neutral FeCl₃, you must first confirm the absence of CO₃²⁻, PO₄³⁻, SO₃²⁻, and I⁻, since all of these also react with Fe³⁺ and would give a false or confusing colour. Remove them with AgNO₃ first if needed.
- Bismuth and barium carbonates resist decomposition by dilute H₂SO₄ — not because the carbonate itself is unreactive, but because BaSO₄ and Bi₂(SO₄)₃ are themselves insoluble and coat the reacting solid, so dilute HCl is used instead for these two.
22. Solved Examples from the Chapter
Example 1. A mixture gives yellowish-green gas with MnO₂ and conc. H₂SO₄; a gas turning red litmus blue with NaOH; blue precipitate with K₃[Fe(CN)₆] and red colouration with NH₄SCN; and brown precipitate with alkaline K₂HgI₄ solution after boiling with KOH. Working through it: yellowish-green gas with MnO₂/conc. H₂SO₄ signals Cl⁻ (Cl₂ liberated). Gas turning red litmus blue is NH₃, meaning NH₄⁺ is present. Blue precipitate with ferricyanide confirms Fe²⁺ (Turnbull's blue), while the red thiocyanate colour shows some Fe³⁺ has also formed, most likely through partial aerial oxidation of Fe²⁺. Brown precipitate with Nessler's reagent reconfirms NH₄⁺. Put together, the mixture is FeCl₂ and NH₄Cl, with a trace of Fe³⁺ as an impurity from oxidation.
Example 2. A blue CuSO₄ solution treated with H₂S in acidic medium gives black CuS, insoluble in warm KOH. Treated with KI in weakly acidic medium, the blue solution turns yellow and gives a white precipitate. Here, KI reduces Cu²⁺ to Cu⁺, forming unstable CuI₂ that decomposes into white Cu₂I₂ and free I₂ — the free iodine is what accounts for the yellow-brown colour of the solution.
Example 3. Testing a light bluish-green compound: brown precipitate with alkaline K₂HgI₄ confirms NH₄⁺; blue colour with K₃[Fe(CN)₆] confirms Fe²⁺; white precipitate with BaCl₂ in HCl medium confirms SO₄²⁻. The compound is Mohr's salt-type formula: FeSO₄·(NH₄)₂SO₄·6H₂O.
23. Practice MCQs (From the Chapter)
Here's a curated set spanning the full range of question styles this chapter uses — straight single-choice, assertion-reasoning, and comprehension-linked. I've kept the answer key attached to each so you can self-check immediately.
- Which compound does not dissolve in hot dilute HNO₃? (a) HgS (b) PbS (c) CuS (d) CdS
Answer: (a). PbS, CuS, CdS dissolve in hot dilute HNO₃ forming nitrates; HgS resists this treatment. - Cu²⁺ and Cd²⁺ in a mixture are best detected/separated using: (a) conc. HNO₃ and H₂S (b) K₄[Fe(CN)₆] and H₂S (c) KCN and H₂S (d) HCl and H₂S
Answer: (c). - [Ni(DMG)₂], a cherry-red complex, owes its extra stability to: (a) small size of Ni²⁺ (b) 3d⁸ configuration (c) covalent bonding (d) hydrogen bonding and chelation
Answer: (d). - The ion that cannot be precipitated by both HCl and H₂S is: (a) Ag⁺ (b) Cu⁺ (c) Sn²⁺ (d) Pb²⁺
Answer: (c). Sn²⁺ is precipitated by H₂S but not by HCl. - Before adding Group III reagents, the solution is heated with conc. HNO₃ to: (a) increase NO₃⁻ (b) lower pH (c) oxidise Fe²⁺ to Fe³⁺ (d) oxidise Cr³⁺ to Cr₂O₇²⁻
Answer: (c). - Thenard blue is: (a) Cu(NH₃)₄SO₄ (b) CoAl₂O₄ (c) K₂Fe[Fe(CN)₆] (d) Fe₄[Fe(CN)₆]₃
Answer: (b). - Which reagents will precipitate only Al³⁺ from a mixture also containing Ca²⁺, Mg²⁺? NH₄Cl and aqueous NH₃.
Answer: Al³⁺ precipitates; Ca²⁺ and Mg²⁺ remain in solution, since this is the Group III reagent combination. - Assertion: Br⁻ ions do not interfere in the chromyl chloride test for chlorides. Reason: bromide, on oxidation with K₂Cr₂O₇/conc. H₂SO₄, liberates Br₂, which dissolves in NaOH giving a colourless solution.
Answer: Both true, R is correct explanation of A. - Assertion: Ring test fails if NO₃⁻ and Br⁻ ions are present together. Reason: Br⁻ ions are not decomposed by dil. H₂SO₄.
Answer: Both true, but R is NOT the correct explanation — the interference works through a different mechanism (bromide gets oxidised by the nitric acid generated, disrupting the ring formation). - A green substance dissolves in HCl; sodium carbonate extract, neutralised, gives canary-yellow ppt. with ammonium molybdate; the HCl-dissolved residue gives rose-red precipitate with DMG plus excess NH₄OH. The substance is: (a) nickel fluoride (b) nickel phosphate (c) copper phosphate (d) ferrous oxalate
Answer: (b).
24. Additional Practice MCQs (40 Questions With Answers)
These are extra questions I've written specifically to test the same core ideas from different angles — useful once you've worked through the chapter's own question bank and want to check that the concepts have actually settled in, not just the specific wording.
- Which gas, evolved on treating a nitrite with dilute H₂SO₄, gives brown fumes? (a) NO₂ (b) SO₂ (c) CO₂ (d) Cl₂
Answer: (a) - The confirmatory test for sulphate ion uses which reagent after acidifying with acetic acid? (a) BaCl₂ (b) Lead acetate (c) AgNO₃ (d) FeCl₃
Answer: (b) - Which cation gives a black precipitate with H₂S even in strongly acidic (Group II) conditions? (a) Zn²⁺ (b) Mn²⁺ (c) Cu²⁺ (d) Ba²⁺
Answer: (c) - The white precipitate of AgCl dissolves in excess NH₄OH due to formation of: (a) Ag₂O (b) [Ag(NH₃)₂]Cl (c) AgOH (d) Ag₂CO₃
Answer: (b) - Which metal chloride does NOT respond to the chromyl chloride test? (a) NaCl (b) KCl (c) PbCl₂ (d) CaCl₂
Answer: (c) - Fe²⁺ gives which colour with potassium ferricyanide? (a) Brown (b) Blood red (c) Deep blue (Turnbull's blue) (d) Yellow
Answer: (c) - The gas that turns lime water milky and also decolourises on excess passage is: (a) SO₂ (b) CO₂ (c) NH₃ (d) Cl₂
Answer: (b) - Borax bead test is not typically useful for identifying: (a) Cu (b) Cr (c) Na (d) Co
Answer: (c) — Na⁺ has no group reagent and is identified mainly through flame test, not borax bead. - Which sulphide is yellow in colour? (a) CuS (b) PbS (c) CdS (d) NiS
Answer: (c) - Which cation gives greenish colour in aqueous solution and deep green Cr(OH)₃ with excess NH₄OH? (a) Fe³⁺ (b) Cr³⁺ (c) Al³⁺ (d) Mn²⁺
Answer: (b) - Dimethylglyoxime is a specific reagent for: (a) Co²⁺ (b) Ni²⁺ (c) Zn²⁺ (d) Mn²⁺
Answer: (b) - Which flame colour corresponds to strontium? (a) Brick red (b) Crimson red (c) Apple green (d) Lilac
Answer: (b) - Which flame colour corresponds to calcium? (a) Brick red (b) Crimson red (c) Apple green (d) Lilac
Answer: (a) - Nessler's reagent is used to test for: (a) NH₄⁺ (b) Fe³⁺ (c) SO₄²⁻ (d) Cl⁻
Answer: (a) - Which of the following is insoluble in yellow ammonium sulphide? (a) SnS₂ (b) As₂S₃ (c) Sb₂S₃ (d) HgS
Answer: (d) - What causes the "purple colour" test for manganese with sodium bismuthate? (a) Formation of MnO₂ (b) Formation of HMnO₄ (c) Formation of Mn(OH)₂ (d) Formation of MnCl₂
Answer: (b) - Which acid is generally avoided for preparing the original solution in basic radical analysis, since it forms insoluble sulphates with some cations? (a) HCl (b) HNO₃ (c) H₂SO₄ (d) CH₃COOH
Answer: (c) - Which acid is avoided for original solution preparation because nitrate ions interfere with the analysis scheme? (a) HCl (b) HNO₃ (c) H₂SO₄ (d) H₃PO₄
Answer: (b) - A pale yellow precipitate that is insoluble in NH₄OH indicates: (a) AgCl (b) AgBr (c) AgI (d) Ag₂CrO₄
Answer: (c) - A precipitate soluble in excess ammonium acetate on warming, formed with lead acetate, confirms: (a) Chloride (b) Sulphate (c) Carbonate (d) Nitrate
Answer: (b) - Which of these evolves N₂ gas rather than NH₃ on heating? (a) NH₄Cl (b) NH₄NO₃ (c) NH₄NO₂ (d) (NH₄)₂SO₄
Answer: (c) - What colour does copper metaborate show in the oxidising flame when hot? (a) Blue (b) Green (c) Red (d) Colourless
Answer: (b) - Which colour does the same copper metaborate show in the reducing flame when hot? (a) Blue (b) Green (c) Red-brown (d) Colourless
Answer: (c) - Ethyl borate burns with which characteristic flame? (a) Brick red (b) Green-edged (c) Lilac (d) Crimson
Answer: (b) - Zinc metaborate in the borax bead test appears: (a) Blue when hot (b) Colourless/white (c) Deep green (d) Brown
Answer: (b) - Which of these radicals is NOT decomposed by dilute HCl? (a) CO₃²⁻ (b) I⁻ (c) S²⁻ (d) SO₃²⁻
Answer: (b) - Which cation forms a white precipitate with NaOH that is soluble in excess NaOH? (a) Fe³⁺ (b) Zn²⁺ (c) Ba²⁺ (d) Cu²⁺
Answer: (b) - Which group's precipitating medium is deliberately kept alkaline using NH₄OH plus NH₄Cl to control OH⁻ concentration? (a) Group I (b) Group II (c) Group III (d) Group V
Answer: (c) - Which reagent distinguishes sulphite from sulphate in solution? (a) BaCl₂ (b) Acidified K₂Cr₂O₇ (c) AgNO₃ (d) NaOH
Answer: (b) — sulphite reduces and decolourises/turns green the dichromate; sulphate does not react. - Golden yellow PbI₂ dissolves in hot water to give: (a) A yellow solution (b) A colourless solution that recrystallises PbI₂ on cooling (c) A black precipitate (d) No change
Answer: (b) - Which metal's chloride corrodes platinum wire, making the flame test unsuitable? (a) Na (b) K (c) Sb (d) Ca
Answer: (c) - The chocolate-brown precipitate formed with K₄[Fe(CN)₆] is characteristic of: (a) Fe³⁺ (b) Cu²⁺ (c) Zn²⁺ (d) Mn²⁺
Answer: (b) - Which of these gives a black residue turning red on cooling, on strong heating? (a) ZnO (b) PbO (c) CaO (d) MgO
Answer: (b) - Cobalt nitrate charcoal test gives a pink residue for: (a) Al³⁺ (b) Zn²⁺ (c) Mg²⁺ (d) PO₄³⁻
Answer: (c) - Which acid radical interferes in Group III precipitation by forming precipitates of Group IV/V cations prematurely? (a) Cl⁻ (b) NO₃⁻ (c) C₂O₄²⁻ (d) SO₄²⁻
Answer: (c) - The confirmatory test for borate uses ethanol and which acid? (a) HCl (b) HNO₃ (c) Conc. H₂SO₄ (d) Dilute H₂SO₄
Answer: (c) - K₃[Co(CN)₆] (bright yellow) is formed from K₄[Co(CN)₆] through: (a) Reduction (b) Oxidation (c) Hydrolysis (d) Precipitation
Answer: (b) - Ammonium phosphomolybdate precipitate is described as: (a) White (b) Canary yellow (c) Black (d) Blue
Answer: (b) - Which observation confirms bismuth after treatment with alkaline sodium stannite? (a) White turbidity (b) Black precipitate of Bi metal (c) Yellow solution (d) Blue colour
Answer: (b) - The disappearance of milkiness in lime water on passing excess gas confirms which radical? (a) Sulphite (b) Carbonate (c) Sulphide (d) Nitrite
Answer: (b)
That's the full sweep of Principles of Qualitative Analysis — from the first sniff-and-look preliminary tests through to the fine detail of why a blue precipitate from one iron reagent is chemically identical to a blue precipitate from a completely different one. If there's one habit I'd want you to walk away with, it's this: whenever you see a colour change in this chapter, ask yourself what oxidation state changed and why. Almost every "trick" question in this topic is really just that question in disguise.
